View source for Sea surface microlayer

{{Short description|Boundary layer where all exchange occurs between the atmosphere and the ocean}}
[[File:Ocean surface slick (cropped).jpg|thumb|upright=1.2|Sea slicks can provide valuable nurseries for marine larvae]]
{{Ocean habitat topics}}

The '''sea surface microlayer''' ('''SML''') is the boundary interface between the [[atmosphere of Earth|atmosphere]] and [[ocean]], covering about 70% of [[Earth]]'s surface. With an operationally defined thickness between 1 and {{cvt|1000|μm|mm|lk=on}}, the SML has physicochemical and biological properties that are measurably distinct from underlying waters. Recent studies now indicate that the SML covers the ocean to a significant extent, and evidence shows that it is an aggregate-enriched [[biofilm]] environment with distinct [[Microbial consortium|microbial communities]]. Because of its unique position at the air-sea interface, the SML is central to a range of global [[Marine biogeochemical cycles|marine biogeochemical]] and climate-related processes.<ref name="Wurl2017" />

The sea surface microlayer is the boundary layer where all [[Gas exchange|exchange]] occurs between the atmosphere and the ocean.<ref name=Liss2005 /> The chemical, physical, and biological properties of the SML differ greatly from the sub-surface water just a few centimeters beneath.<ref name="zhang">Zhang, Zhengbin et al. (2003). Studies on the sea surface microlayer II. The layer of sudden change of physical and chemical properties. Journal of Colloid and Interface Science. 264, 148-159.</ref>

Despite the huge extent of the ocean's surface, until now relatively little attention has been paid to the sea surface microlayer (SML) as the ultimate interface where [[Heat transfer|heat]], [[Momentum transfer|momentum]] and [[Mass transfer|mass exchange]] between the ocean and the atmosphere takes place. Via the SML, large-scale environmental changes in the ocean such as [[Ocean heat content|warming]], [[Ocean acidification|acidification]], [[Ocean deoxygenation|deoxygenation]], and [[eutrophication]] potentially influence [[cloud formation]], [[precipitation]], and the [[Radiation balance|global radiation balance]]. Due to the deep connectivity between biological, chemical, and physical processes, studies of the SML may reveal multiple sensitivities to global and regional changes.<ref name=Engel2017 />

Understanding the processes at the ocean's surface, in particular involving the SML as an important and determinant interface, could provide an essential contribution to the reduction of uncertainties regarding ocean-climate feedbacks. As of 2017, processes occurring within the SML, as well as the associated rates of material exchange through the SML, remained poorly understood and were rarely represented in marine and atmospheric numerical models.<ref name=Engel2017 />

==Overview==
The sea surface microlayer (SML) is the boundary interface between the atmosphere and ocean, covering about 70% of the Earth's surface. The SML has physicochemical and biological properties that are measurably distinct from underlying waters. Because of its unique position at the air-sea interface, the SML is central to a range of global biogeochemical and climate-related processes. Although known for the last six decades, the SML often has remained in a distinct research niche, primarily as it was not thought to exist under typical oceanic conditions. Recent studies now indicate that the SML covers the ocean to a significant extent,<ref name="Wurl2011h">{{cite journal |doi = 10.5194/bg-8-121-2011|title = Formation and global distribution of sea-surface microlayers|year = 2011|last1 = Wurl|first1 = O.|last2 = Wurl |first2=E. |last3=Miller |first3=L.|last4 = Johnson|first4 = K.|last5 = Vagle|first5 = S.|journal = Biogeosciences|volume = 8|issue = 1|pages = 121–135|bibcode = 2011BGeo....8..121W|doi-access = free}}</ref> highlighting its global relevance as the boundary layer linking two major components of the Earth system – the ocean and the atmosphere.<ref name="Wurl2017" />

In 1983, Sieburth hypothesised that the SML was a [[hydrate]]d [[Gel|gel-like]] layer formed by a complex mixture of [[carbohydrate]]s, [[protein]]s, and [[lipid]]s.<ref name="Air-Sea Exchange of Gases and Parti">{{cite book |doi = 10.1007/978-94-009-7169-1_3|chapter = Microbiological and Organic-Chemical Processes in the Surface and Mixed Layers|title = Air-Sea Exchange of Gases and Particles|year = 1983|author1-link=John McNeill Sieburth|last1 = Sieburth|first1 = John McN.|pages = 121–172|isbn = 978-94-009-7171-4}}</ref> In recent years, his hypothesis has been confirmed, and scientific evidence indicates that the SML is an aggregate-enriched biofilm environment with distinct microbial communities.<ref name="Sea surface microlayers: A unified">{{cite journal |doi = 10.1016/j.pocean.2012.08.004|title = Sea surface microlayers: A unified physicochemical and biological perspective of the air–ocean interface|year = 2013|last1 = Cunliffe|first1 = Michael|last2 = Engel|first2 = Anja|last3 = Frka|first3 = Sanja|last4 = Gašparović|first4 = Blaženka|last5 = Guitart|first5 = Carlos|last6 = Murrell|first6 = J Colin|last7 = Salter|first7 = Matthew|last8 = Stolle|first8 = Christian|last9 = Upstill-Goddard|first9 = Robert|last10 = Wurl|first10 = Oliver|journal = Progress in Oceanography|volume = 109|pages = 104–116|bibcode = 2013PrOce.109..104C}}</ref> In 1999 Ellison et al. estimated that 200 [[Megatonne|Tg]] C yr<sup>−1</sup> (200 million tonnes of carbon per year) accumulates in the SML, similar to [[Rate of sedimentation (geology)|sedimentation rate]]s of carbon to the ocean's seabed, though the accumulated carbon in the SML probably has a very short [[residence time]].<ref>{{cite journal |doi = 10.1029/1999JD900073|title = Atmospheric processing of organic aerosols|year = 1999|last1 = Ellison|first1 = G. Barney|last2 = Tuck|first2 = Adrian F.|last3 = Vaida|first3 = Veronica|journal = Journal of Geophysical Research: Atmospheres|volume = 104|issue = D9|pages = 11633–11641|bibcode = 1999JGR...10411633E|doi-access = free}}</ref> Although the total volume of the microlayer is very small compared to the ocean's volume, Carlson suggested in his seminal 1993 paper that unique interfacial reactions may occur in the SML that may not occur in the underlying water or at a much slower rate there.<ref name="Carlson1993">{{cite book |doi = 10.1007/978-1-4615-2890-6_12|chapter = The Early Diagenesis of Organic Matter: Reaction at the Air-Sea Interface|title = Organic Geochemistry|series = Topics in Geobiology|year = 1993|last1 = Carlson|first1 = David J.|volume = 11|pages = 255–268|isbn = 978-1-4613-6252-4}}</ref> He therefore hypothesised that the SML plays an important role in the [[diagenesis]] of carbon in the upper ocean.<ref name="Carlson1993" /> Biofilm-like properties and highest possible exposure to solar radiation leads to an intuitive assumption that the SML is a biochemical microreactor.<ref>{{cite book | last=Liss | first=P. S. | title=The sea surface and global change | publisher=Cambridge University Press | publication-place=Cambridge New York | year=1997 | chapter=Photochemistry of the sea-surface microlayer | pages=383–424 | isbn=978-0-521-56273-7 | oclc=34933503}}</ref><ref name="Wurl2017" />

[[File:Sea surface microlayer as a biochemical microreactor.png|thumb|upright=1.4|Sea surface microlayer as a biochemical microreactor<ref name="Wurl2017">{{cite journal | last1=Wurl | first1=Oliver | last2=Ekau | first2=Werner | last3=Landing | first3=William M. | last4=Zappa | first4=Christopher J. | editor-last=Deming | editor-first=Jody W. | editor-last2=Bowman | editor-first2=Jeff | title=Sea surface microlayer in a changing ocean – A perspective | journal=Elementa: Science of the Anthropocene | publisher=University of California Press | volume=5 | date=1 January 2017 | issn=2325-1026 | doi=10.1525/elementa.228}}[[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>
{{Ordered list |list_style_type=upper-roman|Unique chemical orientation, reaction and aggregation<ref name="Carlson1993"/>|Distinct microbial communities processing dissolved and particulate organic matter<ref name="Air-Sea Exchange of Gases and Parti"/>|Highest exposure of solar radiation drives photochemical >reactions and formation of radicals<ref>{{cite book |doi = 10.1007/978-1-4684-5215-0_11|chapter = Photochemistry and the Sea-Surface Microlayer: Natural Processes and Potential as a Technique|title = Dynamic Processes in the Chemistry of the Upper Ocean|year = 1986|last1 = Zafiriou|first1 = Oliver C.|pages = 129–135|isbn = 978-1-4684-5217-4}}</ref>}}]]

Historically, the SML has been summarized as being a microhabitat composed of several layers distinguished by their ecological, chemical and physical properties with an operational total thickness of between 1 and 1000&nbsp;μm. In 2005 Hunter defined the SML as a "microscopic portion of the surface ocean which is in contact with the atmosphere and which may have physical, chemical or biological properties that are measurably different from those of adjacent sub-surface waters".<ref>Hunter, K. A. (1977) [https://books.google.com/books?id=HDDqxQEACAAJ&q=%22Chemistry+of+the+sea-surface+microlayer%22 ''Chemistry of the sea-surface microlayer''] University of East Anglia. School of Environmental Sciences.</ref> He avoids a definite range of thickness as it depends strongly on the feature of interest. A thickness of 60&nbsp;μm has been measured based on sudden changes of the pH,<ref>{{cite journal |doi = 10.1360/02yb0192|title = Direct determination of thickness of sea surface microlayer using a pH microelectrode at original location|year = 2003|last1 = Zhang|first1 = Zhengbin|journal = Science in China Series B|volume = 46|issue = 4|page = 339}}</ref> and could be meaningfully used for studying the physicochemical properties of the SML. At such thickness, the SML represents a laminar layer, free of turbulence, and greatly affecting the exchange of gases between the ocean and atmosphere. As a habitat for neuston (surface-dwelling organisms ranging from bacteria to larger siphonophores), the thickness of the SML in some ways depends on the organism or ecological feature of interest. In 2005, Zaitsev described the SML and associated near-[[surface layer]] (down to 5&nbsp;cm) as an incubator or nursery for eggs and larvae for a wide range of aquatic organisms.<ref name="Zaitsev1997">{{cite book | last=Zaitsev | first=Y. | title=The sea surface and global change | publisher=Cambridge University Press | publication-place=Cambridge New York | year=1997 | pages=371–382 | chapter=Neuston of seas and oceans | veditors=Liss PS | isbn=978-0-521-56273-7 | oclc=34933503}}</ref><ref name="Wurl2017" />

Hunter's definition includes all interlinked layers from the laminar layer to the nursery without explicit reference to defined depths.<ref>{{cite book | last=Liss | first=P. S. | title=The sea surface and global change | publisher=Cambridge University Press | publication-place=Cambridge New York | year=1997 | chapter=Chemistry of the sea-surface microlayer | isbn=978-0-511-52502-5 | oclc=34933503}}</ref> In 2017, Wurl ''et al.'' proposed Hunter's definition be validated with a redeveloped SML paradigm that includes its global presence, biofilm-like properties and role as a nursery. The new paradigm pushes the SML into a new and wider context relevant to many ocean and climate sciences.<ref name="Wurl2017" />

According to Wurl ''et al.'', the SML can never be devoid of organics due to the abundance of surface-active substances (e.g., surfactants) in the upper ocean{{hsp}}<ref name="Wurl2011h" /> and the phenomenon of surface tension at air-liquid interfaces.<ref>Levich VG (1962) ''Physicochemical hydrodynamics'', Prentice Hall International.</ref> The SML is analogous to the thermal boundary layer, and [[remote sensing]] of the [[sea surface temperature]] shows ubiquitous anomalies between the sea surface skin and bulk temperature.<ref>{{cite journal |doi = 10.1029/JC095iC08p13341|title = On the bulk-skin temperature difference and its impact on satellite remote sensing of sea surface temperature|year = 1990|last1 = Schluessel |first1=Peter|last2 = Emery|first2 = William J.|last3 = Grassl|first3 = Hartmut|last4 = Mammen|first4 = Theodor|journal = Journal of Geophysical Research|volume = 95|issue = C8|page = 13341|bibcode = 1990JGR....9513341S| hdl=21.11116/0000-0004-BC37-B |hdl-access = free}}</ref> Even so, the differences in both are driven by different processes. Enrichment, defined as concentration ratios of an analyte in the SML to the underlying bulk water, has been used for decades as evidence for the existence of the SML. Consequently, depletions of organics in the SML are debatable; however, the question of enrichment or depletion is likely to be a function of the thickness of the SML (which varies with sea state;<ref>{{cite journal |doi = 10.1016/0304-4203(82)90015-9|title = A field evaluation of plate and screen microlayer sampling techniques|year = 1982|last1 = Carlson|first1 = David J.|journal = Marine Chemistry|volume = 11|issue = 3 |pages=189–208| bibcode=1982MarCh..11..189C }}</ref> including losses via sea spray, the concentrations of organics in the bulk water,<ref name="Wurl2011h" /> and the limitations of sampling techniques to collect thin layers .<ref>Cunliffe M, Wurl O.(2014) [http://www.scor-int.org/Publications/SCOR_GuideSeaSurface_2014.pdf Guide to best practices to study the ocean's surface], Plymouth Occasional Publications of the Marine Biological Association of the United Kingdom.</ref> Enrichment of surfactants, and changes in the sea surface temperature and salinity, serve as universal indicators for the presence of the SML. Organisms are perhaps less suitable as indicators of the SML because they can actively avoid the SML and/or the harsh conditions in the SML may reduce their populations. However, the thickness of the SML remains "operational" in field experiments because the thickness of the collected layer is governed by the sampling method. Advances in SML sampling technology are needed to improve our understanding of how the SML influences air-sea interactions.<ref name="Wurl2017" />

Marine surface habitats sit at the interface between the atmosphere and the ocean. The [[biofilm|biofilm-like]] habitat at the surface of the ocean harbours surface-dwelling microorganisms, commonly referred to as [[neuston]].<ref name=Rahlff2019 /> 
The sea surface microlayer (SML) constitutes the uppermost layer of the ocean, only 1–1000 μm thick, with unique chemical and biological properties that distinguish it from the underlying water (ULW).<ref>{{cite journal |doi = 10.1016/S0021-9797(03)00390-4|title = Studies on the sea surface microlayer|year = 2003|last1 = Zhang|first1 = Zhengbin|last2 = Liu|first2 = Liansheng|last3 = Liu|first3 = Chunying|last4 = Cai|first4 = Weijun|journal = Journal of Colloid and Interface Science|volume = 264|issue = 1|pages = 148–159|pmid = 12885531|bibcode = 2003JCIS..264..148Z}}</ref><ref name=Liss2005>{{cite book | last=Liss | first=P. S. | title=The sea surface and global change | publisher=Cambridge University Press | publication-place=Cambridge New York | year=1997 | isbn=978-0-521-56273-7 | oclc=34933503}}</ref> Due to the location at the air-sea interface, the SML can influence exchange processes across this boundary layer, such as air-sea gas exchange and the formation of sea spray aerosols.<ref name=Liss2005 /><ref name="The gelatinous nature of the sea-su">{{cite journal |doi = 10.1016/j.marchem.2008.02.009|title = The gelatinous nature of the sea-surface microlayer|year = 2008|last1 = Wurl|first1 = Oliver|last2 = Holmes|first2 = Michael|journal = Marine Chemistry|volume = 110|issue = 1–2|pages = 89–97| bibcode=2008MarCh.110...89W }}</ref><ref>{{cite journal |doi = 10.1016/j.atmosenv.2017.01.053|title = Size-resolved characterization of the polysaccharidic and proteinaceous components of sea spray aerosol|year = 2017|last1 = Aller|first1 = Josephine Y.|last2 = Radway|first2 = Joann C.|last3 = Kilthau|first3 = Wendy P.|last4 = Bothe|first4 = Dylan W.|last5 = Wilson|first5 = Theodore W.|last6 = Vaillancourt|first6 = Robert D.|last7 = Quinn|first7 = Patricia K.|last8 = Coffman|first8 = Derek J.|last9 = Murray|first9 = Benjamin J.|last10 = Knopf|first10 = Daniel A.|journal = Atmospheric Environment|volume = 154|pages = 331–347|bibcode = 2017AtmEn.154..331A|doi-access = free}}</ref><ref name=Engel2017 /><ref name="Zäncker2018">{{cite journal | last1=Zäncker | first1=Birthe | last2=Cunliffe | first2=Michael | last3=Engel | first3=Anja | title=Bacterial Community Composition in the Sea Surface Microlayer Off the Peruvian Coast | journal=Frontiers in Microbiology | publisher=Frontiers Media SA | volume=9 | date=15 November 2018 | page=2699 | issn=1664-302X | doi=10.3389/fmicb.2018.02699| pmid=30498480 | pmc=6249803 | doi-access=free }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

Due to its exclusive position between the atmosphere and the hydrosphere and by spanning about 70% of the Earth's surface, the sea-surface microlayer (sea-SML) is regarded as a fundamental component in air–sea exchange processes and in biogeochemical cycling.<ref name="Sea surface microlayers: A unified"/> Although having a minor thickness of <1000&nbsp;μm,<ref name=Liss2005 /> the elusive SML is long known for its distinct physicochemical characteristics compared to the underlying water,<ref name=Hardy1982>{{cite journal |doi = 10.1016/0079-6611(82)90001-5|title = The sea surface microlayer: Biology, chemistry and anthropogenic enrichment|year = 1982|last1 = Hardy|first1 = J.T.|journal = Progress in Oceanography|volume = 11|issue = 4|pages = 307–328|bibcode = 1982PrOce..11..307H}}</ref> e.g., by featuring the accumulation of dissolved and particulate organic matter,<ref name=Hardy1982 /><ref name=Sieburth1976>{{cite journal |doi = 10.1126/science.194.4272.1415|title = Dissolved Organic Matter and Heterotrophic Microneuston in the Surface Microlayers of the North Atlantic|year = 1976|last1 = Sieburth|first1 = John McN.|last2 = Willis|first2 = Paula-Jean|last3 = Johnson|first3 = Kenneth M.|last4 = Burney|first4 = Curtis M.|last5 = Lavoie|first5 = Dennis M.|last6 = Hinga|first6 = Kenneth R.|last7 = Caron|first7 = David A.|last8 = French|first8 = Frederick W.|last9 = Johnson|first9 = Paul W.|last10 = Davis|first10 = Paul G.|journal = Science|volume = 194|issue = 4272|pages = 1415–1418| pmid=17819279 |bibcode = 1976Sci...194.1415M|s2cid = 24058391}}</ref> transparent exopolymer particles (TEP), and surface-active molecules.<ref>{{cite journal |doi = 10.1016/j.marchem.2009.04.007|title = The distribution and fate of surface-active substances in the sea-surface microlayer and water column|year = 2009|last1 = Wurl|first1 = Oliver|last2 = Miller|first2 = Lisa|last3 = Röttgers|first3 = Rüdiger|last4 = Vagle|first4 = Svein|journal = Marine Chemistry|volume = 115|issue = 1–2|pages = 1–9| bibcode=2009MarCh.115....1W }}</ref><ref name="The gelatinous nature of the sea-su"/> Therefore, the SML is a gelatinous biofilm,<ref>{{cite journal |doi = 10.1038/ismej.2009.69|title = The sea-surface microlayer is a gelatinous biofilm|year = 2009|last1 = Cunliffe|first1 = Michael|last2 = Murrell|first2 = J Colin|journal = The ISME Journal|volume = 3|issue = 9|pages = 1001–1003|pmid = 19554040|s2cid = 32923256|doi-access = free}}</ref> maintaining physical stability through surface tension forces.<ref name=Franklin2005>{{cite journal |doi = 10.1111/j.1462-2920.2004.00736.x|title = Bacterial diversity in the bacterioneuston (Sea surface microlayer): The bacterioneuston through the looking glass|year = 2005|last1 = Franklin|first1 = Mark P.|last2 = McDonald|first2 = Ian R.|last3 = Bourne|first3 = David G.|last4 = Owens|first4 = Nicholas J. P.|last5 = Upstill-Goddard|first5 = Robert C.|last6 = Murrell|first6 = J. Colin|journal = Environmental Microbiology|volume = 7|issue = 5|pages = 723–736|pmid = 15819854}}</ref> It also forms a vast habitat for different organisms, collectively termed as neuston{{hsp}}<ref name="Franklin2005"/> with a recent global estimate of 2 × 1023 microbial cells for the sea-SML.<ref>{{cite journal |doi = 10.1038/s41579-019-0158-9|title = Bacteria and archaea on Earth and their abundance in biofilms|year = 2019|last1 = Flemming|first1 = Hans-Curt|last2 = Wuertz|first2 = Stefan|journal = Nature Reviews Microbiology|volume = 17|issue = 4|pages = 247–260|pmid = 30760902|s2cid = 61155774}}</ref><ref name=Rahlff2019 />

Life at air–water interfaces has never been considered easy, mainly because of the harsh environmental conditions that influence the SML.<ref>{{cite book | last=Ford | first=Timothy | title=Aquatic microbiology : an ecological approach | publisher=Blackwell Scientific Publications | publication-place=Boston | year=1993 | isbn=978-0-86542-225-4 | oclc=25915881}}</ref> However, high abundances of microorganisms, especially of bacteria and picophytoplankton, accumulating in the SML compared to the underlying water were frequently reported,<ref name=Sieburth1976 /><ref name=Tsyban1971>{{cite journal |doi = 10.1007/BF02109331|title = Marine bacterioneuston|year = 1971|last1 = Tsyban|first1 = A. V.|journal = Journal of the Oceanographical Society of Japan|volume = 27|issue = 2|pages = 56–66|s2cid = 198202161}}</ref><ref>{{cite journal |doi = 10.1016/j.pocean.2016.03.002|title = Biofilm-like properties of the sea surface and predicted effects on air–sea {{CO2}} exchange|year = 2016|last1 = Wurl|first1 = Oliver|last2 = Stolle|first2 = Christian|last3 = Van Thuoc|first3 = Chu|last4 = The Thu|first4 = Pham|last5 = Mari|first5 = Xavier|journal = Progress in Oceanography|volume = 144|pages = 15–24|bibcode = 2016PrOce.144...15W}}</ref> accompanied by a predominant heterotrophic activity.<ref>{{cite journal |doi = 10.3354/ame039293|title = Enhanced heterotrophic activity in the surface microlayer of the Mediterranean Sea|year = 2005|last1 = Obernosterer|first1 = I.|last2 = Catala|first2 = P.|last3 = Reinthaler|first3 = T.|last4 = Herndl|first4 = GJ|last5 = Lebaron|first5 = P.|journal = Aquatic Microbial Ecology|volume = 39|pages = 293–302|doi-access = free}}</ref><ref>{{cite journal |doi = 10.4319/lo.2008.53.1.0122|title = Dissolved organic matter and bacterial production and respiration in the sea-surface microlayer of the open Atlantic and the western Mediterranean Sea|year = 2008|last1 = Reinthaler|first1 = Thomas|last2 = Sintes|first2 = Eva|last3 = Herndl|first3 = Gerhard J.|journal = Limnology and Oceanography|volume = 53|issue = 1|pages = 122–136|bibcode = 2008LimOc..53..122R| s2cid=17499707 |doi-access = free}}</ref><ref>{{cite journal |doi = 10.3390/jmse5040046|doi-access = free|title = SISI: A New Device for in Situ Incubations at the Ocean Surface|year = 2017|last1 = Rahlff|first1 = Janina|last2 = Stolle|first2 = Christian|last3 = Wurl|first3 = Oliver|journal = Journal of Marine Science and Engineering|volume = 5|issue = 4|page = 46}}</ref> This is because primary production at the immediate air–water interface is often hindered by photoinhibition.<ref>{{cite journal |doi = 10.1016/0304-4203(86)90033-2|title = Chemical and microbiological studies of sea-surface films in the Southern Gulf of California and off the West Coast of Baja California|year = 1986|last1 = Williams|first1 = P.M|last2 = Carlucci|first2 = A.F|last3 = Henrichs|first3 = S.M|last4 = Van Vleet|first4 = E.S|last5 = Horrigan|first5 = S.G|last6 = Reid|first6 = F.M.H|last7 = Robertson|first7 = K.J|journal = Marine Chemistry|volume = 19| issue=1 |pages = 17–98| bibcode=1986MarCh..19...17W }}</ref><ref>{{cite journal |doi = 10.1139/m80-063|title = Photosynthetic activities of phytoneuston and phytoplankton|year = 1980|last1 = Albright|first1 = Lawrence J.|journal = Canadian Journal of Microbiology|volume = 26|issue = 3|pages = 389–392|pmid = 7407715}}</ref> However, some exceptions of photosynthetic organisms, e.g., Trichodesmium, Synechococcus, or Sargassum, show more tolerance towards high light intensities and, hence, can become enriched in the SML.<ref name=Sieburth1976 /><ref name=Yue2018>{{cite journal |doi = 10.1371/journal.pone.0198735|doi-access = free|title = Effect of temperature on the accumulation of marine biogenic gels in the surface microlayer near the outlet of nuclear power plants and adjacent areas in the Daya Bay, China|year = 2018|last1 = Yue|first1 = Wei-Zhong|last2 = Sun|first2 = Cui-ci|last3 = Shi|first3 = Ping|last4 = Engel|first4 = Anja|last5 = Wang|first5 = You-Shao|last6 = He|first6 = Wei-Hong|journal = PLOS ONE|volume = 13|issue = 6|pages = e0198735|pmid = 29889860|pmc = 5995428|bibcode = 2018PLoSO..1398735Y}}</ref><ref name=Hardy1988>{{cite journal |doi = 10.1139/f88-099|title = A Hydrophobic Large-Volume Sampler for Collecting Aquatic Surface Microlayers: Characterization and comparison with the Glass Plate Method|year = 1988|last1 = Hardy|first1 = John T.|last2 = Coley|first2 = James A.|last3 = Antrim|first3 = Liam D.|last4 = Kiesser|first4 = Steven L.|journal = Canadian Journal of Fisheries and Aquatic Sciences|volume = 45|issue = 5|pages = 822–826}}</ref> Previous research has provided evidence that neustonic organisms can cope with wind and wave energy,<ref name=Tsyban1971 /><ref name=Stolle2011>{{cite journal |doi = 10.1128/AEM.00042-11|title = Bacterioneuston Community Structure in the Southern Baltic Sea and Its Dependence on Meteorological Conditions|year = 2011|last1 = Stolle|first1 = Christian|last2 = Labrenz|first2 = Matthias|last3 = Meeske|first3 = Christian|last4 = Jürgens|first4 = Klaus|journal = Applied and Environmental Microbiology|volume = 77|issue = 11|pages = 3726–3733|pmid = 21478321|pmc = 3127628|bibcode = 2011ApEnM..77.3726S}}</ref><ref name="Rahlff2017"/> solar and ultraviolet (UV) radiation,<ref>{{cite journal |doi = 10.1111/j.1472-765X.2011.03011.x|title = Diversity in UV sensitivity and recovery potential among bacterioneuston and bacterioplankton isolates|year = 2011|last1 = Santos|first1 = A.L.|last2 = Lopes|first2 = S.|last3 = Baptista|first3 = I.|last4 = Henriques|first4 = I.|last5 = Gomes|first5 = N.C.M.|last6 = Almeida|first6 = A.|last7 = Correia|first7 = A.|last8 = Cunha|first8 = Â.|journal = Letters in Applied Microbiology|volume = 52|issue = 4|pages = 360–366| pmid=21255057 |s2cid = 20013700|hdl = 10773/11080|hdl-access = free}}</ref><ref name="Resistance of Marine Bacterioneusto">{{cite journal |doi = 10.1128/AEM.71.9.5282-5289.2005|title = Resistance of Marine Bacterioneuston to Solar Radiation|year = 2005|last1 = Agogué|first1 = Hélène|last2 = Joux|first2 = Fabien|last3 = Obernosterer|first3 = Ingrid|last4 = Lebaron|first4 = Philippe|journal = Applied and Environmental Microbiology|volume = 71|issue = 9|pages = 5282–5289|pmid = 16151115|pmc = 1214640|bibcode = 2005ApEnM..71.5282A}}</ref><ref>{{cite journal |doi = 10.1128/AEM.06344-11|title = Effects of UV-B Radiation on the Structural and Physiological Diversity of Bacterioneuston and Bacterioplankton|year = 2012|last1 = Santos|first1 = Ana L.|last2 = Oliveira|first2 = Vanessa|last3 = Baptista|first3 = Inês|last4 = Henriques|first4 = Isabel|last5 = Gomes|first5 = Newton C. M.|last6 = Almeida|first6 = Adelaide|last7 = Correia|first7 = António|last8 = Cunha|first8 = Angela|journal = Applied and Environmental Microbiology|volume = 78|issue = 6|pages = 2066–2069|pmid = 22247171|pmc = 3298163|bibcode = 2012ApEnM..78.2066S}}</ref> fluctuations in temperature and salinity,<ref>{{cite journal |doi = 10.4319/lo.1973.18.4.0525|title = Phytoneuston Ecology of a Temperate Marine Lagoon1|year = 1973|last1 = Hardy|first1 = John T.|journal = Limnology and Oceanography|volume = 18|issue = 4|pages = 525–533|bibcode = 1973LimOc..18..525H}}</ref><ref>{{cite book |vauthors=Norkrans B | veditors= Marshall K | title=Advances in microbial ecology | chapter=Surface microlayers in aquatic environments | pages=51–85 | publisher=Plenum Press | publication-place=New York, N.Y | year=1985 | isbn=978-1-4615-9412-3 | oclc=884920706}}</ref> and a higher potential predation risk by the zooneuston.<ref>{{cite journal |doi = 10.1038/s41598-018-29869-7|title = Blue pigmentation of neustonic copepods benefits exploitation of a prey-rich niche at the air-sea boundary|year = 2018|last1 = Rahlff|first1 = Janina|last2 = Ribas-Ribas|first2 = Mariana|last3 = Brown|first3 = Scott M.|last4 = Mustaffa|first4 = Nur Ili Hamizah|last5 = Renz|first5 = Jasmin|last6 = Peck|first6 = Myron A.|last7 = Bird|first7 = Kimberley|last8 = Cunliffe|first8 = Michael|last9 = Melkonian|first9 = Katharina|last10 = Zappa|first10 = Christopher J.|journal = Scientific Reports|volume = 8|issue = 1|page = 11510|pmid = 30065353|pmc = 6068160|bibcode = 2018NatSR...811510R}}</ref> Furthermore, wind action promoting sea spray formation and bubbles rising from deeper water and bursting at the surface release SML-associated microbes into the atmosphere.<ref>{{cite journal |doi = 10.2307/1351816|jstor = 1351816|title = The Ejection of Drops from the Sea and Their Enrichment with Bacteria and Other Materials: A Review|last1 = Blanchard|first1 = Duncan C.|journal = Estuaries|year = 1989|volume = 12|issue = 3|pages = 127–137|s2cid = 84640631}}</ref> In addition to being more concentrated compared to planktonic counterparts, the bacterioneuston, algae, and protists display distinctive community compositions compared to the underlying water, in both marine{{hsp}}<ref name=Franklin2005 /><ref name=Yue2018 /><ref name=Hardy1988 /><ref name=Stolle2011 /><ref name="Cunliffe2009c"/><ref>{{cite journal |doi = 10.1038/ismej.2009.133|title = Eukarya 18S rRNA gene diversity in the sea surface microlayer: Implications for the structure of the neustonic microbial loop|year = 2010|last1 = Cunliffe|first1 = Michael|last2 = Murrell|first2 = J Colin|journal = The ISME Journal|volume = 4|issue = 3|pages = 455–458|pmid = 20010632|s2cid = 21434175|doi-access = free}}</ref> and freshwater habitats.<ref>{{cite journal |doi = 10.3354/ame01825|title = Unique and highly variable bacterial communities inhabiting the surface microlayer of an oligotrophic lake|year = 2017|last1 = Hugoni|first1 = M.|last2 = Vellet|first2 = A.|last3 = Debroas|first3 = D.|journal = Aquatic Microbial Ecology|volume = 79|issue = 2|pages = 115–125| s2cid=90090547 }}</ref><ref>{{cite journal |doi = 10.1111/j.1574-6941.2010.00904.x|title = The bacterial community composition of the surface microlayer in a high mountain lake|year = 2010|last1 = Hörtnagl|first1 = Paul|last2 = Pérez|first2 = Maria Teresa|last3 = Zeder|first3 = Michael|last4 = Sommaruga|first4 = Ruben|journal = FEMS Microbiology Ecology|volume = 73|issue = 3|pages = 458–467|pmid = 20528985|pmc = 2955963}}</ref> Furthermore, the bacterial community composition was often dependent on the SML sampling device being used.<ref>{{cite journal |doi = 10.3354/ame01351|title = Bacterial activity in the sea-surface microlayer: In situ investigations in the Baltic Sea and the influence of sampling devices|year = 2009|last1 = Stolle|first1 = C.|last2 = Nagel|first2 = K.|last3 = Labrenz|first3 = M.|last4 = Jürgens|first4 = K.|journal = Aquatic Microbial Ecology|volume = 58|pages = 67–78|doi-access = free}}</ref><ref>{{cite journal |doi = 10.4319/lom.2004.2.213|title = Comparison of samplers for the biological characterization of the sea surface microlayer|year = 2004|last1 = Agogué|first1 = Hélène|last2 = Casamayor|first2 = Emilio O.|last3 = Joux|first3 = Fabien|last4 = Obernosterer|first4 = Ingrid|last5 = Dupuy|first5 = Christine|last6 = Lantoine|first6 = François|last7 = Catala|first7 = Philippe|last8 = Weinbauer|first8 = Markus G.|last9 = Reinthaler|first9 = Thomas|last10 = Herndl|first10 = Gerhard J.|last11 = Lebaron|first11 = Philippe|journal = Limnology and Oceanography: Methods|volume = 2|issue = 7|pages = 213–225| s2cid=11605343 |url = https://hal.archives-ouvertes.fr/hal-01102867/file/Agogu%C3%A9%2CLOm.pdf}}</ref><ref name="Comparison and validation of sampli">{{cite journal |doi = 10.3354/ame01330|title = Comparison and validation of sampling strategies for the molecular microbial analysis of surface microlayers|year = 2009|last1 = Cunliffe|first1 = M.|last2 = Harrison|first2 = E.|last3 = Salter|first3 = M.|last4 = Schäfer|first4 = H.|last5 = Upstill-Goddard|first5 = RC|last6 = Murrell|first6 = JC|journal = Aquatic Microbial Ecology|volume = 57|pages = 69–77|doi-access = free}}</ref> While being well defined with respect to bacterial community composition, little is known about viruses in the SML, i.e., the virioneuston. This review has its focus on virus–bacterium dynamics at air–water interfaces, even if viruses likely interact with other SML microbes, including archaea and the phytoneuston, as can be deduced from viral interference with their planktonic counterparts.<ref>{{cite journal |doi = 10.1111/j.1550-7408.2004.tb00537.x|title = Viral Control of Phytoplankton Populations-a Review1|year = 2004|last1 = Brussaard|first1 = Corina P. D.|journal = The Journal of Eukaryotic Microbiology|volume = 51|issue = 2|pages = 125–138|pmid = 15134247|s2cid = 21017882}}</ref><ref>{{cite journal |doi = 10.1038/nature08060|title = Viruses manipulate the marine environment|year = 2009|last1 = Rohwer|first1 = Forest|last2 = Thurber|first2 = Rebecca Vega|journal = Nature|volume = 459|issue = 7244|pages = 207–212|pmid = 19444207|bibcode = 2009Natur.459..207R|s2cid = 4397295}}</ref> Although viruses were briefly mentioned as pivotal SML components in a recent review on this unique habitat,<ref name=Engel2017>{{cite journal |doi = 10.3389/fmars.2017.00165|doi-access = free|title = The Ocean's Vital Skin: Toward an Integrated Understanding of the Sea Surface Microlayer|year = 2017|last1 = Engel|first1 = Anja|last2 = Bange|first2 = Hermann W.|last3 = Cunliffe|first3 = Michael|last4 = Burrows|first4 = Susannah M.|last5 = Friedrichs|first5 = Gernot|last6 = Galgani|first6 = Luisa|last7 = Herrmann|first7 = Hartmut|last8 = Hertkorn|first8 = Norbert|last9 = Johnson|first9 = Martin|last10 = Liss|first10 = Peter S.|last11 = Quinn|first11 = Patricia K.|last12 = Schartau|first12 = Markus|last13 = Soloviev|first13 = Alexander|last14 = Stolle|first14 = Christian|last15 = Upstill-Goddard|first15 = Robert C.|last16 = Van Pinxteren|first16 = Manuela|last17 = Zäncker|first17 = Birthe|journal = Frontiers in Marine Science|volume = 4 |display-authors = 4}} Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> a synopsis of the emerging knowledge and the major research gaps regarding bacteriophages at air–water interfaces is still missing in the literature.<ref name=Rahlff2019 />

== Properties ==
Organic compounds such as [[amino acids]], [[carbohydrates]], [[fatty acids]], and [[phenols]] are highly enriched in the SML interface.  Most of these come from [[biota (ecology)|biota]] in the sub-surface waters, which decay and become transported to the surface,<ref name="aller">Aller, J., Kuznetsova, M., Jahns, C., Kemp, P. (2005) The sea surface microlayer as a source of viral and bacterial enrichment in marine aerosols. Journal of aerosol science. Vol. 36, pp. 801-812.</ref><ref name="carlson-1983">Carlson, David J. (1983). Dissolved Organic Materials in Surface Microlayers: Temporal and Spatial Variability and Relation to Sea State. Limnology and Oceanography, 28.3. 415-431</ref> though other sources exist also such as [[atmospheric deposition]], coastal [[Surface runoff|runoff]], and anthropogenic nutrification.<ref name=Liss2005 />  The relative concentration of these compounds is dependent on the nutrient sources as well as [[climate]] conditions such as [[wind speed]] and [[Precipitation (meteorology)|precipitation]].<ref name="carlson-1983"/> These [[organic compounds]] on the surface create a "film," referred to as a "slick" when visible,<ref name="zhang"/> which affects the physical and optical properties of the interface. These films occur because of the [[hydrophobic]] tendencies of many organic compounds, which causes them to protrude into the air-interface.<ref name=Liss2005 /><ref name="carlson-1982">Carlson, David J. (1982). Surface microlayer phenolic enrichments indicate sea surface slicks. Nature. 296.1. 426-429.</ref>  The existence of organic [[surfactants]] on the ocean surface impedes wave formation for low wind speeds.  For increasing concentrations of surfactant there is an increasing critical wind speed necessary to create ocean waves.<ref name=Liss2005 /><ref name="zhang"/>  Increased levels of organic compounds at the surface also hinders air-sea gas exchange at low wind speeds.<ref name="woodcock">Woodcock, A. (1953). Salt nuclei in marine air as a function of altitude and wind force. Journal of Meteorology, 10, 362–371.</ref>  One way in which particulates and organic compounds on the surface are transported into the atmosphere is the process called "bubble bursting".<ref name=Liss2005 /><ref name="wallace">Wallace Jr., G.T., Duce, R.A., 1978. Transport of particulate organic matter by bubbles in marine waters. Limnol. Oceanogr. 23 Ž6., 1155–1167.</ref>  Bubbles generate the major portion of marine [[aerosols]].<ref name="woodcock"/><ref name="gustafsson">Gustafsson, M. E. R., & Franzen, L. G. (2000). Inland transport of marine aerosols in southern Sweden. Atmospheric Environments, 34, 313–325.</ref><ref name="grammatika">Grammatika, M., & Zimmerman, W. B. (2001). Microhydrodynamics offloatation process in the sea surface layer. Dynamics of Atmospheres and Ocean, 34, 327–348.</ref> They can be dispersed to heights of several meters, picking up whatever particles latch on to their surface. However, the major supplier of materials comes from the SML.<ref name="aller"/>

{{clear}}

==Processes==
[[File:Transport processes across the sea surface microlayer.jpg|thumb|upright=1.4|Transport processes across the sea surface microlayer<ref name=Engel2017 />]]
Surfaces and interfaces are critical zones where major physical, chemical, and biological exchanges occur. As the ocean covers 362 million km<sup>2</sup>, about 71% of the Earth's surface, the ocean-atmosphere interface is plausibly one of the largest and most important interfaces on the planet. Every substance entering or leaving the ocean from or to the atmosphere passes through this interface, which on the water-side -and to a lesser extent on the air-side- shows distinct physical, chemical, and biological properties. On the water side the uppermost 1 to 1000 μm of this interface are referred to as the sea surface microlayer (SML).<ref name=Hunter1980>{{cite journal | last=Hunter | first=K.A. | title=Processes affecting particulate trace metals in the sea surface microlayer | journal=Marine Chemistry | publisher=Elsevier BV | volume=9 | issue=1 | year=1980 | issn=0304-4203 | doi=10.1016/0304-4203(80)90006-7 | pages=49–70| bibcode=1980MarCh...9...49H }}</ref> Like a skin, the SML is expected to control the rates of exchange of energy and matter between air and sea, thereby potentially exerting both short-term and long-term impacts on various Earth system processes, including biogeochemical cycling, production and uptake of radiately active gases like {{CO2}} or DMS,<ref>{{cite journal |last1=Briand |first1=F. |last2=Fowler |first2=Scott W. |last3=Hardy |first3=Jack T. |last4=Herndl |first4=Gerhard J. |last5=Liss |first5=Peter S. |date=2000 |title=Sea surface microlayer - An overview |journal=CIESM Workshop Series |volume=9 |pages=7–15}}[https://www.researchgate.net/publication/365899728]</ref> thus ultimately climate regulation.<ref name="Cunliffe 2013 104–116">{{cite journal | last1=Cunliffe | first1=Michael | last2=Engel | first2=Anja | last3=Frka | first3=Sanja | last4=Gašparović | first4=Blaženka | last5=Guitart | first5=Carlos | last6=Murrell | first6=J Colin | last7=Salter | first7=Matthew | last8=Stolle | first8=Christian | last9=Upstill-Goddard | first9=Robert | last10=Wurl | first10=Oliver | title=Sea surface microlayers: A unified physicochemical and biological perspective of the air–ocean interface | journal=Progress in Oceanography | publisher=Elsevier BV | volume=109 | year=2013 | issn=0079-6611 | doi=10.1016/j.pocean.2012.08.004 | pages=104–116| bibcode=2013PrOce.109..104C }}</ref> As of 2017, processes occurring within the SML, as well as the associated rates of material exchange through the SML, remained poorly understood and were rarely represented in marine and atmospheric numerical models.<ref name=Engel2017 />

An improved understanding of the biological, chemical, and physical processes at the ocean's upper surface could provide an essential contribution to the reduction of uncertainties regarding ocean-climate feedbacks. Due to its positioning between atmosphere and ocean, the SML is the first to be exposed to climate changes including temperature, climate relevant trace gases, wind speed, and precipitation as well as to pollution by human waste, including nutrients, toxins, nanomaterials, and plastic debris.<ref name=Engel2017 />

==Bacterioneuston==
{{see also|Marine prokaryotes}}

The term [[neuston]] describes the organisms in the SML and was first suggested by Naumann in 1917.<ref>Naumann, E. (1917)."Über das Neuston des Süsswassers". ''Biol. Cent'', '''37'''(2): 98–106.</ref> As in other marine ecosystems, bacterioneuston communities have important roles in SML functioning.<ref name="Microbiology of aquatic surface mic">{{cite journal |doi = 10.1111/j.1574-6976.2010.00246.x|title = Microbiology of aquatic surface microlayers|year = 2011|last1 = Cunliffe|first1 = Michael|last2 = Upstill-Goddard|first2 = Robert C.|last3 = Murrell|first3 = J. Colin|journal = FEMS Microbiology Reviews|volume = 35|issue = 2|pages = 233–246|pmid = 20726895}}</ref> Bacterioneuston community composition of the SML has been analysed and compared to the underlying water in different habitats with varying results, and has primarily focused on coastal waters and shelf seas, with limited study of the open ocean .<ref name="Agogué2005a">{{cite journal |doi = 10.1016/j.femsec.2005.04.002|title = A survey on bacteria inhabiting the sea surface microlayer of coastal ecosystems|year = 2005|last1 = Agoguã©|first1 = HÃ©lÃ¨ne|last2 = Casamayor|first2 = Emilio O.|last3 = Bourrain|first3 = Muriel|last4 = Obernosterer|first4 = Ingrid|last5 = Joux|first5 = Fabien|last6 = Herndl|first6 = Gerhard J.|last7 = Lebaron|first7 = Philippe|journal = FEMS Microbiology Ecology|volume = 54|issue = 2|pages = 269–280|pmid = 16332325| s2cid=42716308 |doi-access = free}}</ref><ref name=Franklin2005/><ref name=Cunliffe2009c>{{cite journal |doi = 10.1128/AEM.01374-09|title = Comparison of Bacterioneuston and Bacterioplankton Dynamics during a Phytoplankton Bloom in a Fjord Mesocosm|year = 2009|last1 = Cunliffe|first1 = Michael|last2 = Whiteley|first2 = Andrew S.|last3 = Newbold|first3 = Lindsay|last4 = Oliver|first4 = Anna|last5 = Schäfer|first5 = Hendrik|last6 = Murrell|first6 = J. Colin|journal = Applied and Environmental Microbiology|volume = 75|issue = 22|pages = 7173–7181|pmid = 19783743|pmc = 2786535|bibcode = 2009ApEnM..75.7173C}}</ref> In the North Sea, a distinct bacterial community was found in the SML with Vibrio spp. and Pseudoalteromonas spp. dominating the bacterioneuston.<ref name=Franklin2005 /> During an artificially induced phytoplankton bloom in a fjord [[mesocosm]] experiment, the most dominant [[denaturing gradient gel electrophoresis]] (DGGE) bands of the bacterioneuston consisted of two bacterial families: [[Flavobacteriaceae]] and [[Alteromonadaceae]].<ref name=Cunliffe2009c /> Other studies have however, found little or no differences in the bacterial community composition of the SML and the ULW.<ref name="Agogué2005a" /><ref>{{cite journal |doi = 10.5194/bg-5-693-2008|title = Biochemical characteristics and bacterial community structure of the sea surface microlayer in the South Pacific Ocean|year = 2008|last1 = Obernosterer|first1 = I.|last2 = Catala|first2 = P.|last3 = Lami|first3 = R.|last4 = Caparros|first4 = J.|last5 = Ras|first5 = J.|last6 = Bricaud|first6 = A.|last7 = Dupuy|first7 = C.|last8 = Van Wambeke|first8 = F.|last9 = Lebaron|first9 = P.|journal = Biogeosciences|volume = 5|issue = 3|pages = 693–705|bibcode = 2008BGeo....5..693O|doi-access = free}}</ref> Difficulties in direct comparisons between studies can arise because of the different methods used to sample the SML, which result in varied sampling depths.<ref>{{cite journal |doi = 10.4319/lom.2004.2.213|title = Comparison of samplers for the biological characterization of the sea surface microlayer|year = 2004|last1 = Agogué|first1 = Hélène|last2 = Casamayor|first2 = Emilio O.|last3 = Joux|first3 = Fabien|last4 = Obernosterer|first4 = Ingrid|last5 = Dupuy|first5 = Christine|last6 = Lantoine|first6 = François|last7 = Catala|first7 = Philippe|last8 = Weinbauer|first8 = Markus G.|last9 = Reinthaler|first9 = Thomas|last10 = Herndl|first10 = Gerhard J.|last11 = Lebaron|first11 = Philippe|journal = Limnology and Oceanography: Methods|volume = 2|issue = 7|pages = 213–225| s2cid=11605343 |url = https://hal.archives-ouvertes.fr/hal-01102867/file/Agogu%C3%A9%2CLOm.pdf}}</ref><ref name="Comparison and validation of sampli"/><ref name="Microbiology of aquatic surface mic"/><ref name="Zäncker2018" />

Even less is known about the community control mechanisms in the SML and how the bacterial community assembles at the air-sea interface. The bacterioneuston community could be altered by differing wind conditions and radiation levels,<ref name="Resistance of Marine Bacterioneusto"/><ref name=Joux2006>{{cite journal |doi = 10.3354/ame042091|title = Microbial community structure in the sea surface microlayer at two contrasting coastal sites in the northwestern Mediterranean Sea|year = 2006|last1 = Joux|first1 = F.|last2 = Agogué|first2 = H.|last3 = Obernosterer|first3 = I.|last4 = Dupuy|first4 = C.|last5 = Reinthaler|first5 = T.|last6 = Herndl|first6 = GJ|last7 = Lebaron|first7 = P.|journal = Aquatic Microbial Ecology|volume = 42|pages = 91–104|doi-access = free}}</ref><ref name=Stolle2011/><ref name=Rahlff2017>{{cite journal |doi = 10.1093/femsec/fix041|title = High wind speeds prevent formation of a distinct bacterioneuston community in the sea-surface microlayer|year = 2017|last1 = Rahlff|first1 = Janina|last2 = Stolle|first2 = Christian|last3 = Giebel|first3 = Helge-Ansgar|last4 = Brinkhoff|first4 = Thorsten|last5 = Ribas-Ribas|first5 = Mariana|last6 = Hodapp|first6 = Dorothee|last7 = Wurl|first7 = Oliver|journal = FEMS Microbiology Ecology|volume = 93|issue = 5|pmid = 28369320|pmc = 5812515}}</ref> with high wind speeds inhibiting the formation of a distinct bacterioneuston community.<ref name=Stolle2011 /><ref name=Rahlff2017 /> Wind speed and radiation levels refer to external controls, however, bacterioneuston community composition might also be influenced by internal factors such as nutrient availability and [[organic matter]] (OM) produced either in the SML or in the ULW.<ref>{{cite journal |doi = 10.1007/BF00166806|title = Bacteria-organic matter coupling and its significance for oceanic carbon cycling|year = 1994|last1 = Azam|first1 = F.|last2 = Smith|first2 = D. C.|last3 = Steward|first3 = G. F.|last4=Hagström|first4=Å.|journal = Microbial Ecology|volume = 28|issue = 2|pages = 167–179|pmid = 24186443|s2cid = 19627720}}</ref><ref>{{cite journal |doi = 10.1890/0012-9658(2006)87[2068:VIDOMC]2.0.CO;2|issn = 0012-9658|year = 2006|volume = 87|title = Variation in Dissolved Organic Matter Controls Bacterial Production and Community Composition|last1 = Judd|first1 = Kristin E.|last2 = Crump|first2 = Byron C.|last3 = Kling|first3 = George W.|journal = Ecology|issue = 8|pages = 2068–2079|pmid = 16937646| hdl=2027.42/117103 |hdl-access = free}}</ref><ref>{{cite journal |doi = 10.3354/meps10481|title = Enrichment of microbial abundance in the sea-surface microlayer over a coral reef: Implications for biogeochemical cycles in reef ecosystems|year = 2013|last1 = Nakajima|first1 = R.|last2 = Tsuchiya|first2 = K.|last3 = Nakatomi|first3 = N.|last4 = Yoshida|first4 = T.|last5 = Tada|first5 = Y.|last6 = Konno|first6 = F.|last7 = Toda|first7 = T.|last8 = Kuwahara|first8 = VS|last9 = Hamasaki|first9 = K.|last10 = Othman|first10 = BHR|last11 = Segaran|first11 = TC|last12 = Effendy|first12 = AWM|journal = Marine Ecology Progress Series|volume = 490|pages = 11–22|bibcode = 2013MEPS..490...11N|doi-access = free}}</ref><ref name="Zäncker2018" />

One of the principal OM components consistently enriched in the SML are [[transparent exopolymer particles]] (TEP),<ref>{{cite journal |doi = 10.1111/j.1574-6968.2009.01751.x|title = Dissolved organic carbon and bacterial populations in the gelatinous surface microlayer of a Norwegian fjord mesocosm|year = 2009|last1 = Cunliffe|first1 = Michael|last2 = Salter|first2 = Matthew|last3 = Mann|first3 = Paul J.|last4 = Whiteley|first4 = Andrew S.|last5 = Upstill-Goddard|first5 = Robert C.|last6 = Murrell|first6 = J. Colin|journal = FEMS Microbiology Letters|volume = 299|issue = 2|pages = 248–254|pmid = 19732151| s2cid=205794933 |doi-access = free}}</ref><ref name="Wurl2011">{{cite journal |doi = 10.1029/2011JC007342|title = Production and fate of transparent exopolymer particles in the ocean|year = 2011|last1 = Wurl|first1 = Oliver|last2 = Miller|first2 = Lisa|last3 = Vagle|first3 = Svein|journal = Journal of Geophysical Research: Oceans|volume = 116|issue = C7|bibcode = 2011JGRC..116.0H13W|doi-access = free}}</ref><ref>{{cite journal |doi = 10.5194/bg-13-989-2016|title = The organic sea-surface microlayer in the upwelling region off the coast of Peru and potential implications for air–sea exchange processes|year = 2016|last1 = Engel|first1 = Anja|last2 = Galgani|first2 = Luisa|journal = Biogeosciences|volume = 13|issue = 4|pages = 989–1007|bibcode = 2016BGeo...13..989E|doi-access = free}}</ref> which are rich in carbohydrates and form by the aggregation of dissolved precursors excreted by phytoplankton in the [[euphotic zone]].<ref>{{cite journal |doi = 10.1038/35345|title = Spontaneous assembly of marine dissolved organic matter into polymer gels|year = 1998|last1 = Chin|first1 = Wei-Chun|last2 = Orellana|first2 = Mónica V.|last3 = Verdugo|first3 = Pedro|journal = Nature|volume = 391|issue = 6667|pages = 568–572|bibcode = 1998Natur.391..568C|s2cid = 4423082}}</ref><ref>{{cite journal |doi = 10.3354/meps192001|title = Formation of transparent exopolymer particles, TEP, from dissolved precursor material|year = 2000|last1 = Passow|first1 = U.|journal = Marine Ecology Progress Series|volume = 192|pages = 1–11|bibcode = 2000MEPS..192....1P|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1016/S0278-4343(00)00101-1|title = The origin of transparent exopolymer particles (TEP) and their role in the sedimentation of particulate matter|year = 2001|last1 = Passow|first1 = U.|last2 = Shipe|first2 = R.F|last3 = Murray|first3 = A.|last4 = Pak|first4 = D.K|last5 = Brzezinski|first5 = M.A|last6 = Alldredge|first6 = A.L|journal = Continental Shelf Research|volume = 21|issue = 4|pages = 327–346|bibcode = 2001CSR....21..327P| url=https://epic.awi.de/id/eprint/6316/1/Pas2001c.pdf }}</ref><ref>{{cite journal |doi = 10.1038/nature02453|title = Polysaccharide aggregation as a potential sink of marine dissolved organic carbon|year = 2004|last1 = Engel|first1 = Anja|last2 = Thoms|first2 = Silke|last3 = Riebesell|first3 = Ulf|last4 = Rochelle-Newall|first4 = Emma|last5 = Zondervan|first5 = Ingrid|journal = Nature|volume = 428|issue = 6986|pages = 929–932|pmid = 15118723|bibcode = 2004Natur.428..929E|s2cid = 3213755|url = https://epic.awi.de/id/eprint/10127/1/Eng2004b.pdf}}</ref> Higher TEP formation rates in the SML, facilitated through wind shear and dilation of the surface water, have been proposed as one explanation for the observed enrichment in TEP.<ref name="Wurl2011" /><ref>{{cite journal |doi =  10.5194/bg-15-3577-2018|title = Effect of wind speed on the size distribution of gel particles in the sea surface microlayer: Insights from a wind–wave channel experiment|year = 2018|last1 = Sun|first1 = Cui-Ci|last2 = Sperling|first2 = Martin|last3 = Engel|first3 = Anja|journal = Biogeosciences|volume = 15|issue = 11|pages = 3577–3589 |bibcode=2018BGeo...15.3577S |s2cid=54865841|doi-access = free}}</ref> Also, due to their natural positive buoyancy, when not ballasted by other particles sticking to them, TEP ascend through the water column and ultimately end up at the SML .<ref name="Azetsu-Scott2004">{{cite journal |doi = 10.4319/lo.2004.49.3.0741|title = Ascending marine particles: Significance of transparent exopolymer particles (TEP) in the upper ocean|year = 2004|last1 = Azetsu-Scott|first1 = Kumiko|last2 = Passow|first2 = Uta|journal = Limnology and Oceanography|volume = 49|issue = 3|pages = 741–748|bibcode = 2004LimOc..49..741A| s2cid=32205017 |url = https://epic.awi.de/id/eprint/10032/1/Aze2004a.pdf}}</ref> A second possible pathway of TEP from the water column to the SML is by bubble scavenging.<ref>{{cite journal |doi = 10.4319/lo.1998.43.8.1860|title = The role of surface-active carbohydrates in the formation of transparent exopolymer particles by bubble adsorption of seawater|year = 1998|last1 = Zhou|first1 = Jian|last2 = Mopper|first2 = Kenneth|last3 = Passow|first3 = Uta|journal = Limnology and Oceanography|volume = 43|issue = 8|pages = 1860–1871|bibcode = 1998LimOc..43.1860Z| s2cid=56385229 }}</ref><ref name="Zäncker2018" />

Next to rising bubbles, another potential transport mechanism for bacteria from the ULW to the SML could be ascending particles{{hsp}}<ref name="Agogué2005a" /><ref name=Joux2006 /> or more specifically TEP.<ref name="Azetsu-Scott2004" /> Bacteria readily attach to TEP in the [[water column]].<ref>{{cite journal |doi = 10.3354/meps124227|title = Formation and significance of transparent exopolymeric particles in the northern Adriatic Sea|year = 1995|last1 = Schuster|first1 = S.|last2 = Herndl|first2 = GJ|journal = Marine Ecology Progress Series|volume = 124|pages = 227–236|bibcode = 1995MEPS..124..227S|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1093/plankt/18.6.969|title = Abundance, size distribution and bacterial colonization of transparent exopolymeric particles (TEP) during spring in the Kattegat|year = 1996|last1 = Mari|first1 = Xavier|last2 = Kiørboe|first2 = Thomas|journal = Journal of Plankton Research|volume = 18|issue = 6|pages = 969–986|doi-access = free}}</ref><ref>{{cite journal |doi = 10.3389/fmars.2017.00166|doi-access = free|title = Bacterial Colonization and Vertical Distribution of Marine Gel Particles (TEP and CSP) in the Arctic Fram Strait|year = 2017|last1 = Busch|first1 = Kathrin|last2 = Endres|first2 = Sonja|last3 = Iversen|first3 = Morten H.|last4 = Michels|first4 = Jan|last5 = Nöthig|first5 = Eva-Maria|last6 = Engel|first6 = Anja|journal = Frontiers in Marine Science|volume = 4}}</ref> TEP can serve as microbial hotspots and can be used directly as a substrate for bacterial degradation,<ref>{{cite journal |doi = 10.1016/S0079-6611(02)00138-6|title = Transparent exopolymer particles (TEP) in aquatic environments|year = 2002|last1 = Passow|first1 = U.|journal = Progress in Oceanography|volume = 55|issue = 3–4|pages = 287–333|bibcode = 2002PrOce..55..287P| s2cid=31747785 |url = https://epic.awi.de/id/eprint/6307/1/Pas2002b.pdf}}</ref><ref>{{cite journal |doi = 10.3354/ame01214|title = Exopolymer particles: Microbial hotspots of enhanced bacterial activity in Arctic fast ice (Chukchi Sea)|year = 2008|last1 = Meiners|first1 = K.|last2 = Krembs|first2 = C.|last3 = Gradinger|first3 = R.|journal = Aquatic Microbial Ecology|volume = 52|pages = 195–207|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1111/1758-2229.12513|title = Coastal bacterioplankton community response to diatom-derived polysaccharide microgels|year = 2017|last1 = Taylor|first1 = Joe D.|last2 = Cunliffe|first2 = Michael|journal = Environmental Microbiology Reports|volume = 9|issue = 2|pages = 151–157|pmid = 27943607|hdl = 10026.1/16045|s2cid = 32970231|hdl-access = free}}</ref> and as grazing protection for attached bacteria, e.g., by acting as an alternate food source for zooplankton.<ref>{{cite journal |doi = 10.3354/meps096033|title = Inhibition of copepod grazing by diatom exudates: A factor in the development of mucus aggregates?|year = 1993|last1 = Malej|first1 = A.|last2 = Harris|first2 = RP|journal = Marine Ecology Progress Series|volume = 96|pages = 33–42|bibcode = 1993MEPS...96...33M|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1093/plankt/21.11.2203|title = Do transparent exopolymer particles (TEP) inhibit grazing by the euphausiid Euphausia pacifica?|year = 1999|last1 = Passow|first1 = U.|journal = Journal of Plankton Research|volume = 21|issue = 11|pages = 2203–2217|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1016/j.hal.2004.12.003|title = Inhibition of copepod feeding by exudates and transparent exopolymer particles (TEP) derived from a Phaeocystis globosa dominated phytoplankton community|year = 2005|last1 = Dutz|first1 = J.|last2 = Klein Breteler|first2 = W.C.M.|last3 = Kramer|first3 = G.|journal = Harmful Algae|volume = 4|issue = 5|pages = 929–940}}</ref> TEP have also been suggested to serve as light protection for microorganisms in environments with high irradiation.<ref>{{cite journal |doi = 10.5194/bg-6-3071-2009|title = Effects of ultraviolet B radiation on (Not so) transparent exopolymer particles|year = 2009|last1 = Ortega-Retuerta|first1 = E.|last2 = Passow|first2 = U.|last3 = Duarte|first3 = C. M.|last4 = Reche|first4 = I.|journal = Biogeosciences|volume = 6|issue = 12|pages = 3071–3080|bibcode = 2009BGeo....6.3071O|doi-access = free|hdl = 10481/32275|hdl-access = free}}</ref><ref name="Zäncker2018" />

==Virioneuston==
{{see also|Marine viruses}}

[[File:Viral–bacterial dynamics in the ocean surface.png|thumb|upright=1.4|Viral–bacterial dynamics in the surface microlayer (SML) of the ocean and beyond. DOM = [[dissolved organic matter]], UV = [[ultraviolet]].<ref name=Rahlff2019>{{cite journal | doi = 10.3390/v11020191 | volume=11 | title=The Virioneuston: A Review on Viral–Bacterial Associations at Air–Water Interfaces | journal=Viruses | page=191 | last1 = Rahlff | first1 = Janina| year=2019 | issue=2 | pmid=30813345 | pmc=6410083 | doi-access=free }}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]]

Viruses in the sea surface microlayer, the so-called ''virioneuston'', have recently become of interest to researchers as enigmatic biological entities in the boundary [[surface layer]]s with potentially important ecological impacts. Given this vast air–water interface sits at the intersection of major air–water exchange processes spanning more than 70% of the global surface area, it is likely to have profound implications for [[marine biogeochemical cycles]], on the [[microbial loop]] and gas exchange, as well as the [[marine food web]] structure, the global dispersal of airborne viruses originating from the sea surface microlayer, and human health.<ref name=Rahlff2019 />

Viruses are the most abundant biological entities in the water column of the world's oceans.<ref>{{cite journal |doi = 10.1038/nature04160|title = Viruses in the sea|year = 2005|last1 = Suttle|first1 = Curtis A.|journal = Nature|volume = 437|issue = 7057|pages = 356–361|pmid = 16163346|bibcode = 2005Natur.437..356S|s2cid = 4370363}}</ref> In the free water column, the virioplankton typically outnumbers the bacterioplankton by one order of magnitude reaching typical bulk water concentrations of 10<sup>7</sup> viruses mL<sup>−1</sup>.<ref name= Fuhrman1999>{{cite journal |doi = 10.1038/21119|title = Marine viruses and their biogeochemical and ecological effects|year = 1999|last1 = Fuhrman|first1 = Jed A.|journal = Nature|volume = 399|issue = 6736|pages = 541–548|pmid = 10376593|bibcode = 1999Natur.399..541F|s2cid = 1260399}}</ref> Moreover, they are known as integral parts of global biogeochemical cycles<ref name= Fuhrman1999 /> to shape and drive microbial diversity{{hsp}}<ref>{{cite journal |doi = 10.1046/j.1462-2920.2003.00539.x|title = Are viruses driving microbial diversification and diversity?|year = 2003|last1 = Weinbauer|first1 = Markus G.|last2 = Rassoulzadegan|first2 = Fereidoun|journal = Environmental Microbiology|volume = 6|issue = 1|pages = 1–11|pmid = 14686936}}</ref> and to structure trophic networks.<ref name= Thingstad1993>{{cite journal |doi = 10.1016/0169-5347(93)90101-T|title = Are viruses important partners in pelagic fend webs?|year = 1993|last1 = Thingstad|first1 = T.F.|last2 = Heldal|first2 = M.|last3 = Bratbak|first3 = G.|last4 = Dundas|first4 = I.|journal = Trends in Ecology & Evolution|volume = 8|issue = 6|pages = 209–213|pmid = 21236150}}</ref> Like other neuston members, the virioneuston likely originates from the bulk seawater. For instance, in 1977 Baylor et al. postulated adsorption of viruses onto air bubbles as they rise to the surface,<ref>{{cite journal |doi = 10.1126/science.918656|title = Virus Transfer from Surf to Wind|year = 1977|last1 = Baylor|first1 = E. R.|last2 = Baylor|first2 = M. B.|last3 = Blanchard|first3 = Duncan C.|last4 = Syzdek|first4 = Lawrence D.|last5 = Appel|first5 = Curtis|journal = Science|volume = 198|issue = 4317|pages = 575–580|pmid = 918656|bibcode = 1977Sci...198..575B}}</ref> or viruses can stick to organic particles{{hsp}}<ref>{{cite journal |doi = 10.1128/AEM.00762-07|title = Virus Attachment to Transparent Exopolymeric Particles along Trophic Gradients in the Southwestern Lagoon of New Caledonia|year = 2007|last1 = Mari|first1 = Xavier|last2 = Kerros|first2 = Marie-Emmanuelle|last3 = Weinbauer|first3 = Markus G.|journal = Applied and Environmental Microbiology|volume = 73|issue = 16|pages = 5245–5252|pmid = 17586679|pmc = 1950989|bibcode = 2007ApEnM..73.5245M}}</ref> also being transported to the SML via bubble scavenging.<ref>{{cite journal |doi = 10.1038/nature14986|title = A marine biogenic source of atmospheric ice-nucleating particles|year = 2015|last1 = Wilson|first1 = Theodore W.|last2 = Ladino|first2 = Luis A.|last3 = Alpert|first3 = Peter A.|last4 = Breckels|first4 = Mark N.|last5 = Brooks|first5 = Ian M.|last6 = Browse|first6 = Jo|last7 = Burrows|first7 = Susannah M.|last8 = Carslaw|first8 = Kenneth S.|last9 = Huffman|first9 = J. Alex|last10 = Judd|first10 = Christopher|last11 = Kilthau|first11 = Wendy P.|last12 = Mason|first12 = Ryan H.|last13 = McFiggans|first13 = Gordon|last14 = Miller|first14 = Lisa A.|last15 = Nájera|first15 = Juan J.|last16 = Polishchuk|first16 = Elena|last17 = Rae|first17 = Stuart|last18 = Schiller|first18 = Corinne L.|last19 = Si|first19 = Meng|last20 = Temprado|first20 = Jesús Vergara|last21 = Whale|first21 = Thomas F.|last22 = Wong|first22 = Jenny P. S.|last23 = Wurl|first23 = Oliver|last24 = Yakobi-Hancock|first24 = Jacqueline D.|last25 = Abbatt|first25 = Jonathan P. D.|last26 = Aller|first26 = Josephine Y.|last27 = Bertram|first27 = Allan K.|last28 = Knopf|first28 = Daniel A.|last29 = Murray|first29 = Benjamin J.|journal = Nature|volume = 525|issue = 7568|pages = 234–238|pmid = 26354482|bibcode = 2015Natur.525..234W|s2cid = 4405246|url = http://repository.essex.ac.uk/17251/1/Marine%20biogenic%20source%20INPs_Combined_FINAL_changes_accepted_no%20mendeley_updated_with%20editorial%20changes_accepted_merged.pdf}}</ref><ref name=Rahlff2019 />

Within the SML, viruses interacting with the bacterioneuston will probably induce the viral shunt, a phenomenon that is well known for marine pelagic systems. The term viral shunt describes the release of organic carbon and other nutritious compounds from the virus-mediated lysis of host cells, and its addition to the local dissolved organic matter (DOM) pool.<ref>{{cite journal |doi = 10.2307/1313569|jstor = 1313569|title = Viruses and Nutrient Cycles in the Sea|journal = BioScience|date = October 1999|volume = 49|issue = 10|pages = 781–788|last1 = Wilhelm|first1 = Steven W.|last2 = Suttle|first2 = Curtis A.|doi-access = free}}</ref> The enriched and densely packed bacterioneuston forms an excellent target for viruses compared to the bacterioplankton populating the subsurface. This is because high host-cell numbers will increase the probability of host–virus encounters. The viral shunt might effectively contribute to the SML's already high DOM content enhancing bacterial production as previously suggested for pelagic ecosystems{{hsp}}<ref name= Thingstad1993 /> and in turn replenishing host cells for viral infections. By affecting the DOM pool, viruses in the SML might directly interfere with the microbial loop being initiated when DOM is microbially recycled, converted into biomass, and passed along the food web. In addition, the release of DOM from lysed host cells by viruses contributes to organic particle generation.<ref>{{cite journal |doi = 10.1016/0304-4203(92)90045-C|title = Marine submicron particles|year = 1992|last1 = Wells|first1 = Mark L.|last2 = Goldberg|first2 = Edward D.|journal = Marine Chemistry|volume = 40|issue = 1–2|pages = 5–18| bibcode=1992MarCh..40....5W }}</ref> However, the role of the virioneuston for the microbial loop has never been investigated.<ref name=Rahlff2019 />

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== Measurement ==
Devices used to sample the concentrations of particulates and compounds of the SML include a glass fabric, metal mesh screens, and other hydrophobic surfaces.  These are placed on a rotating cylinder which collects surface samples as it rotates on top of the ocean surface.<ref name="harvey">Harvey, George W. (1966). Microlayer Collection from the Sea Surface: A New Method and Initial Results. Limnology and Oceanography, 11.4. 608-613</ref>

The glass plate sampler is commonly used.<ref name=Anderson2018>{{cite journal | last1=Anderson | first1=Zachary T. | last2=Cundy | first2=Andrew B. | last3=Croudace | first3=Ian W. | last4=Warwick | first4=Phillip E. | last5=Celis-Hernandez | first5=Omar | last6=Stead | first6=Jessica L. | title=A rapid method for assessing the accumulation of microplastics in the sea surface microlayer (SML) of estuarine systems | journal=Scientific Reports | publisher=Springer Science and Business Media LLC | volume=8 | issue=1 | date=21 June 2018 | page=9428 | issn=2045-2322 | doi=10.1038/s41598-018-27612-w| pmid=29930338 | pmc=6013445 | bibcode=2018NatSR...8.9428A }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> It was first described in 1972 by Harvey and Burzell as a simple but effective method of collecting small sea surface microlayer samples.{{hsp}}<ref name=Harvey1972>{{cite journal | last1=HARVEY | first1=GEORGE W. | last2=BURZELL | first2=LINDEN A. | title=A Simple Microlayer Method for Small Samples1 | journal=Limnology and Oceanography | publisher=Wiley | volume=17 | issue=1 | year=1972 | issn=0024-3590 | doi=10.4319/lo.1972.17.1.0156 | pages=156–157| bibcode=1972LimOc..17..156H | doi-access=free }}</ref> A clean glass plate is immersed vertically into the water and then withdrawn in a controlled manner. Harvey and Burzell used a plate  which was 20&nbsp;cm square and 4&nbsp;mm thick. They withdrew it from the sea at the rate of 20&nbsp;cm per second.<ref name=Harvey1972 /> Typically the uppermost 20–150&nbsp;μm of the surface microlayer adheres to the plate as it is withdrawn.<ref name="Cunliffe 2013 104–116"/> The sample is then wiped from both sides of the plate into a sampling vial.<ref name=Cunliffe2014>Cunliffe, M. and Wurl, O. (2014) ''Guide to best practices to study the ocean's surface''. Occasional publication of the ''Marine Biological Association of the United Kingdom''. {{doi|10.25607/OBP-1512}}</ref>

<gallery mode="packed" style="float:left" heights="220px" caption="Glass plate sampling of the sea surface microlayer">
File:Glass plate sampling of the sea surface microlayer.webp
</gallery>

<gallery mode="packed" style="float:right" heights="220px">
File:Sampling the sea surface microlayer 2.jpg
</gallery>

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For a plate of the size used by Harvey and Burzel, the resulting sample volumes are between about 3 and 12 cubic centimetres. The sampled SML thickness ''h'' in micrometres is given by:

:<math>\mathrm{h} = \frac{104 V}{A N}</math>

where ''V'' is the sample volume in cm<sup>3</sup>, ''A'' is the total immersed plate area of both sides in cm<sup>2</sup>, and ''N'' is the number of times the sample was dipped.<ref name=Cunliffe2014 />

==Remote sensing==
[[File:Bacteria, sea slicks and satellite remote sensing.webp|thumb|upright=1.4|Bacteria, sea slicks and satellite remote sensing. Surfactants are capable of dampening the short capillary ocean surface waves and smoothing the sea surface. [[Synthetic aperture radar]] (SAR) satellite remote sensing can detect areas with concentrated surfactants or sea slicks, which appear as dark areas on the SAR images.<ref name=Kurata2016>Kurata, N., Vella, K., Hamilton, B., Shivji, M., Soloviev, A., Matt, S., Tartar, A. and Perrie, W. (2016) "Surfactant-associated bacteria in the near-surface layer of the ocean". ''Scientific Reports'', '''6'''(1): 1–8. {{doi|10.1038/srep19123}}. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]]

Ocean surface habitats sit at the interface between the ocean and the atmosphere. The [[biofilm|biofilm-like]] habitat at the surface of the ocean harbours surface-dwelling microorganisms, commonly referred to as [[neuston]]. This vast air–water interface sits at the intersection of major air–water exchange processes spanning more than 70% of the global surface area . Bacteria in the surface microlayer of the ocean,  called ''bacterioneuston'', are of interest due to practical applications such as air-sea gas exchange of greenhouse gases, production of climate-active marine aerosols, and remote sensing of the ocean.<ref name=Kurata2016 /> Of specific interest is the production and degradation of [[surfactant]]s (surface active materials) via microbial biochemical processes. Major sources of surfactants in the open ocean include phytoplankton,<ref>Ẑutić, V., Ćosović, B., Marčenko, E., Bihari, N. and Kršinić, F. (1981) "Surfactant production by marine phytoplankton". ''Marine Chemistry'', '''10'''(6): 505–520. {{doi|10.1016/0304-4203(81)90004-9}}.</ref> terrestrial runoff, and deposition from the atmosphere.<ref name=Kurata2016 />

Unlike coloured algal blooms, surfactant-associated bacteria may not be visible in ocean colour imagery. Having the ability to detect these "invisible" surfactant-associated bacteria using [[synthetic aperture radar]] has immense benefits in all-weather conditions, regardless of cloud, fog, or daylight.<ref name=Kurata2016 /> This is particularly important in very high winds, because these are the conditions when the most intense air-sea gas exchanges and marine aerosol production take place. Therefore, in addition to colour satellite imagery, SAR satellite imagery may provide additional insights into a global picture of biophysical processes at the boundary between the ocean and atmosphere, air-sea greenhouse gas exchanges and production of climate-active marine aerosols.<ref name=Kurata2016 />

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==Aeroplankton==
{{main|Aeroplankton}}
[[File:Ocean mist and spray 2.jpg|thumb|[[Sea spray]] containing [[marine microorganisms]] can be swept high into the atmosphere and may travel the globe as [[aeroplankton]] before falling back to earth.]]

A stream of airborne microorganisms, including [[marine virus]]es, [[Marine protist|bacteria]] and [[Marine protist|protists]], circles the planet above weather systems but below commercial air lanes.<ref>[https://www.smithsonianmag.com/science-nature/living-bacteria-are-riding-earths-air-currents-180957734/ Living Bacteria Are Riding Earth's Air Currents] ''Smithsonian Magazine'', 11 January 2016.</ref> Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in [[sea spray]]. In 2018, scientists reported that hundreds of millions of these viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.<ref>{{cite news |last=Robbins |first=Jim |title=Trillions Upon Trillions of Viruses Fall From the Sky Each Day |url=https://www.nytimes.com/2018/04/13/science/virosphere-evolution.html |date=13 April 2018 |work=[[The New York Times]] |access-date=14 April 2018}}</ref><ref name="ISME-2018">{{cite journal |last1=Reche |first1=Isabel |last2=D’Orta |first2=Gaetano |last3=Mladenov |first3=Natalie |last4=Winget |first4= Danielle M |last5=Suttle |first5= Curtis A |title=Deposition rates of viruses and bacteria above the atmospheric boundary layer |journal=ISME Journal |volume=12 |issue=4 |pages=1154–1162 |date=29 January 2018 |doi=10.1038/s41396-017-0042-4 |pmid=29379178 |pmc=5864199}}</ref>

Compared to the sub-surface waters, the sea surface microlayer contains elevated concentration of [[bacteria]] and [[viruses]], as well as [[toxic metals]] and organic pollutants.<ref name=Liss2005 /><ref name="blanchard">Blanchard, D.C., 1983. The production, distribution and bacterial enrichment of the sea-salt aerosol. In: Liss, P.S., Slinn, W.G.N. ŽEds.., Air–Sea Exchange of Gases and Particles. D. Reidel Publishing Co., Dordrecht, Netherlands, pp. 407-444.</ref><ref name="hoffmann">Hoffmann, G.L., Duce, R.A., Walsh, P.R., Hoffmann, E.J., Ray, B.J., 1974. Residence time of some particulate trace metals in the oceanic surface microlayer: significance of atmospheric deposition. J. Rech. Atmos. 8, 745–759.</ref><ref name="hunter">Hunter, K.A., 1980. Process affecting particulate trace metals in the sea surface microlayer. Mar. Chem. 9, 49–70.</ref><ref name="hardy">Hardy, J.T., Word, J., 1986. Contamination of the water surface of Puget Sound. Puget Sound Notes, U.S. EPA. Region 10 Seattle, WA, pp. 3–6.</ref> These materials can be transferred from the sea-surface to the atmosphere in the form of wind-generated aqueous aerosols due to their high vapor tension and a process known as [[volatilisation]].<ref name="wallace"/> When airborne, these [[microbes]] can be transported long distances to coastal regions. If they hit land they can have detrimental effects on animals, vegetation and human health.<ref name="WHO">WHO, 1998. Draft guidelines for safe recreational water environments: coastal and fresh waters, draft for consultation. World Health Organization, Geneva, EOSrDRAFTr98 14, pp. 207–299.</ref> Marine aerosols that contain viruses can travel hundreds of kilometers from their source and remain in liquid form as long as the humidity is high enough (over 70%).<ref name="klassen">Klassen, R. D., & Roberge, P. R. (1999). Aerosol transport modeling as an aid to understanding atmospheric corrosivity patterns. Materials & Design, 20, 159–168.</ref><ref name="moorthy">Moorthy, K. K., Satheesh, S. K., & Krishna Murthy, B.V. (1998). Characteristics ofspectral optical depths and size distributions of aerosols over tropical oceanic regions. Journal of Atmospheric and Solar–Terrestrial Physics, 60, 981–992.
</ref><ref>Chow, J. C., Watson, J. G., Green, M. C., Lowenthal, D. H., Bates, B., Oslund, W., & Torre, G. (2000). Cross-border transport and spatial variability of suspended particles in Mexicali and California's Imperial Valley. Atmospheric Environment, 34, 1833–1843.</ref> These aerosols are able to remain suspended in the atmosphere for about 31 days.<ref name="aller"/>  Evidence suggests that bacteria can remain viable after being transported inland through aerosols. Some reached as far as 200 meters at 30 meters above sea level.<ref name="marks"/> It was also noted that the process which transfers this material to the atmosphere causes further enrichment in both bacteria and viruses in comparison to either the SML or sub-surface waters (up to three orders of magnitude in some locations).<ref name="cincinelli">Cincinelli A.; Stortini A.M.; Perugini M.; Checchini L.; Lepri L., 2001. Organic Pollutants in sea-surface microlayer and aerosol in the coastal environment Of Leghorn- (Tyrrhenian Sea). Marine Chemistry, Volume 76, Number 1, pp. 77-98(22)</ref><ref name="marks">Marks, R., Kruczalak, K., Jankowska, K., & Michalska, M. (2001). Bacteria and fungi in air over the GulfofGdansk and Baltic sea. Journal of Aerosol Science, 32, 237–250.</ref>

==Mathematical modeling==

[[File:Stagnant film model C. DeBlois.png|thumb|The ''stagnant film model'' showing gas exchange between a mixed atmosphere and mixed ocean with a film in-between with thickness Z]]

The ''stagnant film model'' is a [[mathematical model]] used to simulate the sea surface microlayer. It is a [[Kinematics|kinematic]] model which can be used to describe how gas exchange from the ocean's surface and the atmosphere reaches [[Chemical equilibrium|equilibrium]].<ref name="Broecker and Peng 1974">Broecker, W. S., & Peng, T. H. (1974). [https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.2153-3490.1974.tb01948.x Gas exchange rates between air and sea.] Tellus, 26(1-2), 21-35.</ref><ref name="Murry and James 2000">Murray, James W. "The oceans." Jacobson, MC Charlson, RJ; Rodhe, H.; Orians, GH Earth System Science: From Biogeochemical Cycles to Global Changes, Oxford, UK: Elsevier (2000): 230-278.</ref> The model assumes both the ocean and atmosphere are composed mostly of well-mixed, constantly moving fluid layers with the sea surface microlayer present as a permanent thin-film layer in the middle. Gas exchange occurs by [[molecular diffusion]]  between the two fluid layers through the sea surface microlayer.<ref name="Broecker and Peng 1974" /><ref name="Crump et al. 2022">Crump, Byron C., Jeremy M. Testa, and Kenneth H. Dunton, eds. Estuarine ecology. John Wiley & Sons, 2022. 42-43.</ref>

==See also==
* [[Ocean surface ecosystem]]
* [[Ocean surface topography]]
* [[Planetary boundary layer]]
* [[Surface layer]]

== References ==
{{reflist|30em}}

{{physical oceanography|expanded=other}}

[[Category:Surface science]]
[[Category:Marine biology]]