View source for Sulfur-reducing bacteria

{{Short description|Microorganisms able to reduce elemental sulfur to hydrogen sulfide}}
[[File:Biocostruzioni di solfobatteri.jpg|thumb|On the left: Bioconstructions created by sulfur bacteria in a sulfurous cave at a depth of about 30 mt in Santa Cesarea Terme, Lecce, Italy. On the right: The same bioconstructions suspended in water. These structures are extremely fragile, and even a small air bubble emitted by a diver can disperse them in the water.]]
'''Sulfur-reducing bacteria''' are microorganisms able to [[Redox reaction|reduce]] elemental [[sulfur]] (S<sup>0</sup>) to [[hydrogen sulfide]] (H<sub><big>2</big></sub>S).<ref name=":4" /> These microbes use inorganic sulfur compounds as electron acceptors to sustain several activities such as respiration, conserving energy and growth, in absence of oxygen.<ref name=":7">{{cite book | vauthors = Florentino AP, Weijma J, Stams AJ, Sánchez-Andrea I | title = Biotechnology of Extremophiles| chapter = Ecophysiology and Application of Acidophilic Sulfur-Reducing Microorganisms|date=2016 |pages=141–175| veditors = Rampelotto PH |series=Grand Challenges in Biology and Biotechnology| volume = 1|place=Cham|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-13521-2_5|isbn=978-3-319-13521-2 }}</ref> The final product of these processes, sulfide, has a considerable influence on the chemistry of the environment and, in addition, is used as [[electron donor]] for a large variety of microbial metabolisms.<ref>{{cite book | vauthors = Rabus R, Hansen TA, Widdel F |chapter=Dissimilatory Sulfate- and Sulfur-Reducing Prokaryotes|date=2006 |title=The Prokaryotes|veditors=Dworkin M, Falkow S, Rosenberg E, Schleifer KH, Stackebrandt E|pages=659–768|place=New York, NY|publisher=Springer New York|isbn=978-0-387-25492-0 |doi=10.1007/0-387-30742-7_22}}</ref> Several types of [[bacteria]] and many non-[[methanogen]]ic [[archaea]] can reduce sulfur. Microbial sulfur reduction was already shown in early studies, which highlighted the first proof of S<sup>0</sup> reduction in a vibrioid bacterium from mud, with sulfur as electron acceptor and {{chem|H|2}} as electron donor.<ref>{{Cite journal| vauthors = Parker CT, Taylor D, Garrity GM | veditors = Parker CT, Garrity GM |date=2003-01-01|title=Exemplar Abstract for Chlorobium vibrioforme Pelsh 1936 (Approved Lists 1980) and Prosthecochloris vibrioformis (Pelsh 1936) Imhoff 2003. |website=The NamesforLife Abstracts|doi=10.1601/ex.789}}</ref> The first pure cultured species of sulfur-reducing bacteria, ''[[Desulfuromonas acetoxidans]]'', was discovered in 1976 and described by Pfennig Norbert and Biebel Hanno as an anaerobic sulfur-reducing and acetate-oxidizing bacterium, not able to reduce sulfate.<ref>{{cite journal | vauthors = Pfennig N, Biebl H | title = Desulfuromonas acetoxidans gen. nov. and sp. nov., a new anaerobic, sulfur-reducing, acetate-oxidizing bacterium | journal = Archives of Microbiology | volume = 110 | issue = 1 | pages = 3–12 | date = October 1976 | pmid = 1015937 | doi = 10.1007/BF00416962 | s2cid = 9330789 }}</ref> Only few [[Taxon|taxa]] are true sulfur-reducing bacteria, using sulfur reduction as the only or main catabolic reaction.<ref name=":6">{{cite book | vauthors = Stetter KO, Zillig W |title = Chapter 2: Thermoplasma and the Thermophilic Sulfur-Dependent Archaebacteria|date=1985-01-01 |work=Archabacteria|pages=85–170| veditors = Woese CR, Wolfe RS |publisher=Academic Press|language=en|doi=10.1016/b978-0-12-307208-5.50008-8|isbn=978-0-12-307208-5 }}</ref> Normally, they couple this reaction with the oxidation of [[acetate]], [[succinate]] or other [[organic compounds]]. In general, [[Sulfate-reducing microorganism|sulfate-reducing bacteria]] are able to use both sulfate and elemental sulfur as electron acceptors. Thanks to its abundancy and thermodynamic stability, sulfate is the most studied electron acceptor for anaerobic respiration that involves sulfur compounds. Elemental sulfur, however, is very abundant and important, especially in deep-sea hydrothermal vents, hot springs and other extreme environments, making its isolation more difficult.<ref name=":7" /> Some bacteria{{spaced ndash}}such as ''[[Proteus (bacterium)|Proteus]]'', ''[[Campylobacter]]'', ''[[Pseudomonas]]'' and ''[[Salmonella]]''{{spaced ndash}}have the ability to reduce sulfur, but can also use oxygen and other terminal electron acceptors.

== Taxonomy ==
Sulfur reducers are known to cover about 74 genera within the Bacteria domain.<ref name=":7" /><ref name=":17">{{Cite web|title=GTDB – Tree|url=https://gtdb.ecogenomic.org/tree|access-date=2020-12-11|website=gtdb.ecogenomic.org}}</ref><ref name=":18">{{Cite web|title=Home – Taxonomy – NCBI|url=https://www.ncbi.nlm.nih.gov/taxonomy|access-date=2020-12-11|website=www.ncbi.nlm.nih.gov}}</ref><ref name=":1" /><ref name=":0" /><ref name=":4" /> Several types of sulfur-reducing bacteria have been discovered in different habitats like deep and shallow sea hydrothermal vents, freshwater, volcanic acidic hot springs and others.<ref>{{Cite book| vauthors = Munn CB |url=https://www.worldcat.org/oclc/671701780|title=Marine microbiology : ecology and applications|date=2011|publisher=Garland Science|isbn=978-0-8153-6517-4|edition=2nd|location=New York|oclc=671701780}}</ref> Many sulfur reducers belong to the phylum [[Thermodesulfobacteriota]] (''Desulfuromonas'', ''Pelobacter'', ''Desulfurella'', ''Geobacter''), the class [[Gammaproteobacteria]], and the phylum [[Campylobacterota]]<ref name=":2">{{cite journal | vauthors = Waite DW, Vanwonterghem I, Rinke C, Parks DH, Zhang Y, Takai K, Sievert SM, Simon J, Campbell BJ, Hanson TE, Woyke T, Klotz MG, Hugenholtz P | display-authors = 6 | title = Epsilonproteobacteria and Proposed Reclassification to Epsilonbacteraeota (phyl. nov.) | language = en | journal = Frontiers in Microbiology | volume = 8 | pages = 682 | date = 2017 | pmid = 28484436 | doi = 10.3389/fmicb.2017.00682 | pmc = 5401914 | doi-access = free }}</ref><ref name=":3">{{cite journal | vauthors = Waite DW, Vanwonterghem I, Rinke C, Parks DH, Zhang Y, Takai K, Sievert SM, Simon J, Campbell BJ, Hanson TE, Woyke T, Klotz MG, Hugenholtz P | display-authors = 6 | title = Epsilonproteobacteria and Proposed Reclassification to Epsilonbacteraeota (phyl. nov.) | language = en | journal = Frontiers in Microbiology | volume = 9 | pages = 772 | date = 2018 | pmid = 29720974 | doi = 10.3389/fmicb.2018.00772 | pmc = 5915535 | doi-access = free }}</ref> according to [https://gtdb.ecogenomic.org/tree GTDB classification]. Other phyla that present sulfur-reducing bacteria are: [[Bacillota]] (''Desulfitobacterium'', ''[[Ammonifex]]'', and ''Carboxydothermus''), [[Aquificota]] (''Desulfurobacterium'' and ''Aquifex''), [[Synergistota]] (''Dethiosulfovibrio''), [[Deferribacterota]] (''Geovibrio''), [[Thermodesulfobacteriota]], [[Spirochaetota]], and [[Chrysiogenota]].<ref name=":4">{{Cite book| vauthors = Madigan MT, Martinko JM, Bender KS, Buckley DH, Stahl DA |title=Brock Biology of Microorganisms|publisher=Benjamin-Cummings Pub Co|year=2014|isbn=978-0321897398|pages=448–449}}</ref><ref name=":7" />
{| class="wikitable"
|+

 Table 1. Genera of sulfur-reducing bacteria (NCBI classification).<ref name=":4" /><ref name=":7" /><ref name=":18" /><ref name=":1" /><ref name=":0" /><ref name = "Janssen_1992">{{cite journal | vauthors = Janssen PH, Morgan HW | title = Heterotrophic sulfur reduction by Thermotoga sp. strain FjSS3.B1 | journal = FEMS Microbiology Letters | volume = 75 | issue = 2–3 | pages = 213–7 | date = September 1992 | pmid = 1398039 | doi = 10.1111/j.1574-6968.1992.tb05419.x | doi-access = free }}</ref>
!Phylum
!Class
!Genus (74) (and species)
|-
|'''Aquificota'''
| Aquificia
|''Aquifex (pyrophilus), Balnearium (lithotrophicum), Desulfurobacterium (crinifex, pacificum, thermolithotrophum),'' 
''Persephonella (guaimasensis, marina), Thermocrinis (ruber), Thermosulfidibacter (takaii),''

''Thermovibrio (ammonificans, guaymasensis, ruber)''
|-
|'''Bacteroidota'''
|Bacteroidia
|''Petrimonas (sulfuriphila)''
|-
|'''Caldisericota'''
|Caldisericia
|''Caldisericum (exile)''
|-
|'''Calditrichota'''
|Calditrichae
|''Caldithrix (abyssi)''
|-
|'''Chrysiogenota'''
|Chrysiogenetes
|''Desulfurispirillum (alkaliphilum)''
|-
|'''Coprothermobacterota'''
|Coprothermobacteria
|''Coprothermobacter (proteoliticus)''
|-
|'''Deferribacterota'''
|Deferribacteres
|''Deferribacter (desulfuricans), Geovibrio (thiophilus)''
|-
|'''Deinococcota'''
|Deinococci 
|''Oceanithermus (desulfurans)''
|-
|'''Bacillota'''
|Clostridia 
|''Ammonifex (degensii), Carboxydothermus (pertinax), Clostridium (thiosulfatireducens, tunisiense, sulfidigenes), Dethiobacter (alkaliphilus),'' 
''Desulfitibacter (alkalitolerans), Desulfitispora (alkaliphila), Desulfitobacterium (hafniense, chlororespirans, dehalogenans, metallireducens),''

''Desulfosporosinus (acididurans, acidiphilus, orientis, meridiei, auripigmenti), Desulfotomaculum (thermosubterraneus, salinum,''

''geothermicum, reducens, intricatum), Ercella (succinogenes), Halanaerobium (congolense), Halarsenatibacter (silvermanii),''

''Sporanaerobacter (acetigenes), Thermoanaerobacter (sulfurophilus)''
|-
|'''Pseudomonadota'''
|Gammaproteobacteria 
|''Acidithiobacillus (ferrooxidans), Pseudomonas (mendocina), Shewanella (putrefaciens)''
|-
|'''Thermodesulfobacteriota'''
|
|''Desulfobacter (postgatei), Desulfobacterium, Desulfobotulus (alkaliphilus), Desulfobulbus (propionicus), Desulfomicrobium (baculatum),'' 
''Desulfomonile (tiedjei), Desulfonatronovibrio (thiodismutans), Desulfonatronum (thioautotrophicum), Desulfovermiculus (halophilus),''

''Desulfovibrio, Desulfurella,'' ''Desulfurivibrio (alkaliphilus), Desulfuromonas,'' ''Desulfuromusa, Geoalkalibacter (subterraneus), Geobacter,''

''Hippea'' ''(maritima), Pelobacter''

''Caldimicrobium (exile), Thermodesulfobacterium (geofontis)''
|-
|'''Campylobacterota'''
|
|''Caminibacter, Hydrogenimonas, Lebetimonas, Nautilia,'' ''Nitratiruptor, Sulfurimonas, Sulfurospirillum, Sulfurovum,''
''Thioreductor'' (''incertae sedis'')'', Wolinella (succinogenes)''
|-
|'''Spirochaetota'''
|Spirochaetia
|''Spirochaeta (perfilievii, smaragdinae)''
|-
|'''Synergistota'''
|Synergistia
|''Anaerobaculum (mobile, thermoterrenum), Dethiosulfovibrio (acidaminovorans, marinus, peptidovorans, russensis),'' 
''Thermanaerovibrio (acidaminovorans, velox), Thermovirga (lienii),''
|-
|'''Thermotogota'''
|Thermotogae
|''Fervidobacterium (changbaicum, islandicum, nodosum, riparium, ), Geotoga (petraea, subterranea), Marinitoga (camini, hydrogenitolerans, okinawensis, piezophila),'' 
''Mesotoga (infera, prima), Petrotoga (mexicana, miotherma, mobilis), Thermosipho (aficanus), Thermotoga (lettingae, maritima, naphthophila, neapolitana),''
|}
{| class="wikitable"

|+Table 2. Genera of sulfur-reducing bacteria (GTDB classification) <ref name=":7" /><ref name=":17" /><ref name=":1" /><ref name=":0" /><ref name=":4" />
!Phylum
!Class
!Genus (74)
|-
|'''Aquificota'''
| Aquificia
|''Aquifex, Persephonella, Thermocrinis'' 
|-
|'''Thermodesulfobacteriota'''
|
|''Balnearium,'' ''Desulfurobacterium, Thermovibrio''
|-
|
|Desulfobacteria
|''Desulfobacter, Desulfobacterium, Desulfobotulus''
|-
|
|Desulfobulbia
|''Desulfobulbus, Desulfurivibrio'' 
|-
|
|Desulfovibrionia
|''Desulfomicrobium, Desulfonatronovibrio, Desulfonatronum, Desulfovermiculus,'' ''Desulfovibrio,''  
|-
|
|Desulfuromonadia;
|''Desulfuromonas, Desulfuromusa, Geoalkalibacter, Geobacter,'' 
''Pelobacter''
|-
|
|Desulfomonilia
|''Desulfomonile''
|-
|
|Thermodesulfobacteria
|''Caldimicrobium, Thermodesulfobacterium,'' 
|-
|
|Thermosulfidibacteria
|''Thermosulfidibacter'' 
|-
|'''Bacteroidota'''
|Bacteroidia
|''Petrimonas''
|-
|'''Caldisericota'''
|Caldisericia
|''Caldisericum''
|-
|'''Calditrichota''' 
|Calditrichia
|''Caldithrix'' 
|-
|'''Campylobacterota'''
|Campylobacteria
|''Caminibacter, Hydrogenimonas, Lebetimonas,''
''Nautilia, Nitratiruptor, Sulfurimonas, Sulfurospirillum,''

''Sulfurovum, Wolinella'' 
|-
|
|Desulfurellia
|''Desulfurella, Hippea''
|-
|'''Chrysiogenota'''
|Chrysiogenetes
|''Desulfurispirillum''
|-
|'''Coprothermobacterota'''
|Coprothermobacteria
|''Coprothermobacter''
|-
|'''Deferribacterota'''
|Deferribacteres
|''Deferribacter, Geovibrio''
|-
|'''Deinococcota'''
|Deinococci
|''Oceanithermus''
|-
|'''Bacillota_A'''
|Clostridia
|''Clostridium, Sporanaerobacter'' 
|-
|
|Thermoanaerobacteria
|''Thermoanaerobacter'' 
|-
|'''Bacillota_B'''
|Desulfitobacteria
|''Desulfitobacterium, Desulfosporosinus''
|-
|
|Desulfotomaculia
|''Ammonifex, Carboxydothermus, Desulfotomaculum''
|-
|
|Moorellia
|''Desulfitibacter'' 
|-
|'''Bacillota_D'''
|Dethiobacteria
|''Dethiobacter''
|-
|'''Bacillota_F'''
|Halanaerobia
|''Halanaerobium, Halarsenatibacter'' 
|-
|'''Pseudomonadota'''
|Gammaproteobacteria
|''Acidithiobacillus, Pseudomonas, Shewanella''
|-
|'''Spirochaetota'''
|Spirochaetia
|''Sediminispirochaeta''
|-
|'''Synergistota'''
|Synergistia
|''Anaerobaculum,'' ''Dethiosulfovibrio, Thermanaerovibrio,'' 
|-
|'''Thermotogota'''
|Thermotogae
|''Fervidobacterium, Geotoga, Marinitoga, Mesotoga, Petrotoga, Thermosipho, Thermotoga''
|-
|'''Unclassified''' (from NCBI)
|
|''Desulfitispora (alkaliphila), Ercella (succinogenes), Thermovirga, Thioreductor''
|}

== Metabolism ==
Sulfur reduction [[metabolism]] is an ancient process, found in the deep branches of the [[phylogenetic tree]].<ref>{{cite encyclopedia | vauthors = Loka Bharathi PA |title=Sulfur Cycle|date=2008 | encyclopedia = Encyclopedia of Ecology|pages=3424–3431|publisher=Elsevier|doi=10.1016/b978-008045405-4.00761-8|isbn=978-0-08-045405-4 }}</ref> Sulfur reduction uses elemental sulfur (S<sup>0</sup>) and generates [[hydrogen sulfide]] (H<sub>2</sub>S) as the main end product. This metabolism is largely present in extreme environments where, especially in recent years, many microorganisms have been isolated, bringing new and important data on the subject.<ref name=":7" />

Many sulfur-reducing bacteria are able to produce ATP through [[lithotroph]]ic sulfur respiration, using zero-valence sulfur as electron acceptor, for instance the genera ''[[Wolinella]], [[Ammonifex]], [[Desulfuromonas]]'' and ''Desulfurobacterium''. On the other side, there are obligate fermenters able to reduce elemental sulfur, for example ''[[Thermotoga]], Thermosipho'' and ''Fervidobacterium''. Among these fermenters there are species, such as ''[[Thermotoga maritima|Thermotoga maritina]]'', that are not dependent on sulfur reduction, and utilize it as a supplementary electron sink.<ref name=":0">{{Cite journal| vauthors = Hedderich R, Klimmek O, Kröger A, Dirmeier R, Keller M, Stetter KO |date=1998-12-01|title=Anaerobic respiration with elemental sulfur and with disulfides |journal=FEMS Microbiology Reviews|volume=22|issue=5|pages=353–381|doi=10.1111/j.1574-6976.1998.tb00376.x|issn=1574-6976|doi-access=free}}</ref> Some researches<ref name=":0" /><ref>{{Cite journal| vauthors = Schauder R, Müller E |date=1993-11-01|title=Polysulfide as a possible substrate for sulfur-reducing bacteria |journal=Archives of Microbiology|language=en|volume=160|issue=5|pages=377–382|doi=10.1007/BF00252224|s2cid=25603061|issn=1432-072X}}</ref><ref>{{cite journal | vauthors = Zöphel A, Kennedy MC, Beinert H, Kroneck PM | title = Investigations on microbial sulfur respiration. Isolation, purification, and characterization of cellular components from Spirillum 5175 | journal = European Journal of Biochemistry | volume = 195 | issue = 3 | pages = 849–56 | date = February 1991 | pmid = 1847872 | doi = 10.1111/j.1432-1033.1991.tb15774.x | doi-access =  }}</ref> propose the hypothesis that polysulfide could be an intermediate of sulfur respiration, due to the conversion of elemental sulfur into polysulfide that occurs in sulfide solutions, performing this reaction:

<math display="inline">nS^0+HS^-\Longrightarrow S^{2-}_{n+1} +H^+
</math> <ref name=":0" />

=== Pseudomonadota ===
The ''[[Pseudomonadota ]]''are a  major phylum of [[gram-negative bacteria]]. There is a wide range of metabolisms. Most members are facultative or obligately anaerobic, chemoautotrophs and heterotrophs. Many are able to move using flagella, others are nonmotile.<ref>{{Cite web|title=Proteobacteria {{!}} Boundless Microbiology|url=https://courses.lumenlearning.com/boundless-microbiology/chapter/proteobacteria/|access-date=2020-12-04|website=courses.lumenlearning.com}}</ref> They are currently divided into several classes, referred to by Greek letters, based on rRNA sequences: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Zetaproteobacteria'', etc.<ref name=":18" />''<ref>{{Cite journal| vauthors = Stackebrandt E, Murray RG, Trüper HG |date=1988|title=Proteobacteria classis nov., a Name for the Phylogenetic Taxon That Includes the "Purple Bacteria and Their Relatives" |journal=International Journal of Systematic and Evolutionary Microbiology|volume=38|issue=3|pages=321–325|doi=10.1099/00207713-38-3-321|issn=1466-5026|doi-access=free}}</ref>

==== Class Gammaproteobacteria ====
The [[Gammaproteobacteria]] class include several medically, ecologically and scientifically important groups of bacteria. They are major organisms in diverse marine ecosystems and even extreme environments. This class contains a huge variety of taxonomic and metabolic diversity, including aerobic and anaerobic species, chemolitoauthotrophic, chemoorganotrophic and phototrophic species and also free living, biofilms formers, commensal and symbionts.<ref>{{cite journal | vauthors = Williams KP, Gillespie JJ, Sobral BW, Nordberg EK, Snyder EE, Shallom JM, Dickerman AW | title = Phylogeny of gammaproteobacteria | journal = Journal of Bacteriology | volume = 192 | issue = 9 | pages = 2305–14 | date = May 2010 | pmid = 20207755 | doi = 10.1128/JB.01480-09 | pmc = 2863478 }}</ref>

===== ''Acidithiobacillus'' spp. =====
''Acidithiobacillus'' are chemolithoautrophics, Gram-negative road-shaped bacteria, using energy from the oxidation of iron and sulfur containing minerals for growth. They are able to live at extremely low pH (pH 1–2) and fixes both carbon and nitrogen from the atmosphere. It solubilizes copper and other metals from rocks and plays an important role in nutrient and metal biogeochemical cycling in acid environments.<ref>{{cite journal | vauthors = Zhan Y, Yang M, Zhang S, Zhao D, Duan J, Wang W, Yan L | title = Iron and sulfur oxidation pathways of Acidithiobacillus ferrooxidans | journal = World Journal of Microbiology & Biotechnology | volume = 35 | issue = 4 | pages = 60 | date = March 2019 | pmid = 30919119 | doi = 10.1007/s11274-019-2632-y | s2cid = 85544603 }}</ref> Acidithiobacillus ferrooxidans is abundant in natural environments associated with pyritic ore bodies, coal deposits, and their acidified drainages. It obtain energy by the oxidation of reduced sulfur compounds and it can also reduce ferric ion and elemental sulfur, thus promoting the recycling of iron and sulfur compounds under anaerobic conditions. It can also fix {{CO2}} and nitrogen and be a primary producer of carbon and nitrogen in acidic environments.<ref>{{cite journal | vauthors = Valdés J, Pedroso I, Quatrini R, Dodson RJ, Tettelin H, Blake R, Eisen JA, Holmes DS | display-authors = 6 | title = Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications | journal = BMC Genomics | volume = 9 | issue = 1 | pages = 597 | date = December 2008 | pmid = 19077236 | pmc = 2621215 | doi = 10.1186/1471-2164-9-597 | doi-access = free }}</ref>

===== ''Shewanella'' spp. =====
''Shewanella'' are Gram-negative, motile bacilli. The first description of the species was provided in 1931, ''Shewanella'' ''putrefaciens,'' a non-fermentative bacilli with a single polar flagellum which grow well on conventional solid media. This species is pathogenic for humans, even if infections are rare and reported especially in the geographic area characterized by warm climates.<ref>{{cite journal | vauthors = Holt HM, Gahrn-Hansen B, Bruun B | title = Shewanella algae and Shewanella putrefaciens: clinical and microbiological characteristics | journal = Clinical Microbiology and Infection | volume = 11 | issue = 5 | pages = 347–52 | date = May 2005 | pmid = 15819859 | doi = 10.1111/j.1469-0691.2005.01108.x | doi-access = free }}</ref>

===== ''Pseudomonas'' spp. =====
''[[Pseudomonas]]'' are Gram-negative chemoorganotrophic Gammaproteobacteria, straight or slightly curved rod-shaped. They are able to move thanks to one or several polar flagella; rarely nonmotile. Aerobic, having a strictly respiratory type of metabolism with oxygen as the terminal electron acceptor; in some cases, allowing growth anaerobically, nitrate can be used as an alternate electron acceptor. Almost all the species fail to grow under acid conditions (pH 4.5 or lower). Pseudomonas are widely distributed in nature. Some species are pathogenic for humans, animals, or plants.<ref name = "Palleroni_2015">{{cite book | vauthors = Palleroni NJ |chapter=Pseudomonas|date=2015|title=Bergey's Manual of Systematics of Archaea and Bacteria|veditors = Trujillo ME, Dedysh S, DeVos P, Hedlund B, Kämpfer P, Rainey FA, Whitman WB|pages=1|publisher=American Cancer Society |doi= 10.1002/9781118960608.gbm01210 |isbn=978-1-118-96060-8 }}</ref> Type species: ''Pseudomonas mendocina.''

=== Phylum Thermodesulfobacteriota ===
The ''Thermodesulfobacteriota'' phylum comprises several morphologically different bacterial groups, Gram-negative, non-sporeforming that exhibit either anaerobic or aerobic growth. They are ubiquitous in [[marine sediment]]s and contains most of the known sulfur reducing bacteria (e.g. ''Desulfuromonas'' spp.). The aerobic representatives are able to digest other bacteria and several of these members are important constituents of the microflora in soil and waters.<ref>{{Cite book | vauthors = Garrity GM, Holt JG |title=Bergey's Manual of Systematics of Archaea and Bacteria|date=2015-04-17|publisher=Wiley|doi=10.1002/9781118960608|isbn=978-1-118-96060-8| veditors = Trujillo ME, Dedysh S, DeVos P, Hedlund B, Kämpfer P, Rainey FA, Whitman WB }}</ref>

==== ''Desulfuromusa'' spp. ====
''Desulfuromusa'' genus includes bacteria obligately anaerobic that use sulfur as an electron acceptor and short-chain fatty acids, dicarboxylic acids, and amino acids, as electron donors that are oxidized completely to {{CO2}}. They are gram negative and complete oxidizer bacteria; their cells are motile and slightly curved or rod shaped. Three sulfur reducing species are known, ''Desulfromusa kysingii, Desulfuromusa bakii'' and ''Desulfuromusa succinoxidans''.<ref>{{Cite journal| vauthors = Liesack W, Finster K |title=Phylogenetic Analysis of Five Strains of Gram-Negative, Obligately Anaerobic, Sulfur-Reducing Bacteria and Description of Desulfuromusa gen. nov., Including Desulfuromusa kysingii sp. nov., Desulfuromusa bakii sp. nov., and Desulfuromusa succinoxidans sp. nov.|journal=International Journal of Systematic Bacteriology|year=1994 |volume=44|issue=4|pages=753–758|doi=10.1099/00207713-44-4-753|doi-access=free}}</ref>

==== ''Desulfurella'' spp. ====
''Desulfurella'' are short rod-shaped, gram-negative cells, motile thanks to a single polar flagellum or nonmotile, non-sporeforming. Obligately anaerobic, moderate thermophilic, they generally occur in warm sediments and in thermally heated cyanobacterial or bacterial communities that are rich in organic compounds and elemental sulfur. Type species: ''Desulfurella acetivorans.''<ref>{{cite book | vauthors = Rainey FA, Hollen B |title=Desulfurella|chapter=''Desulfurella'' |date=2015 |work=Bergey's Manual of Systematics of Archaea and Bacteria|pages=1–4|publisher=American Cancer Society|language=en|doi=10.1002/9781118960608.gbm01037|isbn=978-1-118-96060-8 }}</ref>

==== ''Hippea'' spp. ====
''Hippea'' species are moderate thermophiles neutrophiles to moderate acidophiles, obligate anaerobes sulfur-reducing bacteria with gram-negative rod-shaped cells. They are able to grow lithotrophically with hydrogen and sulfur, and oxidize completely volatile fatty acids, fatty acids and alcohols. They inhabit submarine hot vents. The type species is ''Hippea maritima''.<ref>{{cite journal | vauthors = Miroshnichenko ML, Rainey FA, Rhode M, Bonch-Osmolovskaya EA | title = Hippea maritima gen. nov., sp. nov., a new genus of thermophilic, sulfur-reducing bacterium from submarine hot vents | journal = International Journal of Systematic Bacteriology | volume = 49 Pt 3 | issue = 3 | pages = 1033–8 | date = July 1999 | pmid = 10425760 | doi = 10.1099/00207713-49-3-1033 | doi-access = free }}</ref>

==== ''Desulfuromonas'' spp. ====
''[[Desulfuromonas]]'' species are gram-negative, mesophilic, obligately anaerobic and complete oxidizers <ref name=":4" /> sulfur-reducing bacteria. They are able to grow on acetate as sole organic substrate and reduce elemental sulfur or polysulfide to sulfide.<ref>{{Cite journal| vauthors = Finster K, Bak F, Pfennig N |date=1994-04-01|title=Desulfuromonas acetexigens sp. nov., a dissimilatory sulfur-reducing eubacterium from anoxic freshwater sediments |journal=Archives of Microbiology |volume=161|issue=4|pages=328–33 2|doi=10.1007/BF00303588|s2cid=24380675|issn=1432-072X}}</ref> Currently known species of the genus Desulfuromonas are ''[[Desulfuromonas acetoxidans]]'', ''Desulfuromonas acetexigens'', the marine organism ''Desulfuromonas palmitates'' and ''Desulfuromonas thiophila''.
* ''Desulfiromonas thiophila'' is an obligate anaerobic bacteria, that uses sulfur as only electron acceptor. Multiplies by binary fission and cells are motile thanks to polar flagella. They live in anoxic mud of freshwater sulfur springs, at a temperature from 26 to 30&nbsp;°C and pH 6.9 to 7.9.<ref>{{cite journal | vauthors = Finster K, Coates JD, Liesack W, Pfennig N | title = Desulfuromonas thiophila sp. nov., a new obligately sulfur-reducing bacterium from anoxic freshwater sediment | journal = International Journal of Systematic Bacteriology | volume = 47 | issue = 3 | pages = 754–8 | date = July 1997 | pmid = 9226908 | doi = 10.1099/00207713-47-3-754  | doi-access = free }}</ref>

==== ''Geobacter'' spp. ====
[[Geobacter]] species have a respiratory metabolism with Fe(III) serving as the common terminal electron acceptor in all species.
* ''[[Geobacter sulfurreducens]]'' was isolated from a drainage ditch in Norman, Okla. It is rod-shaped, gram-negative, non-motile and non-spore forming. The optimum temperature range is 30 to 35°. About the metabolism, is strict anaerobic chemoorganotroph which oxidizes acetate with Fe(III), S, Co(III), fumarate, or malate as the electron acceptor. Hydrogen is also used as an electron donor for Fe(III) reduction, whereas other carboxylic acids, sugars, alcohols, amino acids, yeast extract, phenol, and benzoate are not. C-type cytochromes was found in cells.<ref>{{cite journal | vauthors = Caccavo F, Lonergan DJ, Lovley DR, Davis M, Stolz JF, McInerney MJ | title = Geobacter sulfurreducens sp. nov., a hydrogen- and acetate-oxidizing dissimilatory metal-reducing microorganism | journal = Applied and Environmental Microbiology | volume = 60 | issue = 10 | pages = 3752–9 | date = October 1994 | pmid = 7527204 | doi = 10.1128/AEM.60.10.3752-3759.1994 | pmc = 201883 | bibcode = 1994ApEnM..60.3752C | doi-access = free }}</ref>

==== ''Pelobacter'' spp. ====
''[[Pelobacter]]'' is unique group of fermentative microorganisms belonging to the phylum Thermodesulfobacteriota. They consume fermentatively alcohols such as 2,3-butanediol, acetoin and ethanol, but not sugars, with acetate plus ethanol and/or hydrogen as the end products.<ref name=":16">{{cite journal | vauthors = Aklujkar M, Haveman SA, DiDonato R, Chertkov O, Han CS, Land ML, Brown P, Lovley DR | display-authors = 6 | title = The genome of Pelobacter carbinolicus reveals surprising metabolic capabilities and physiological features | journal = BMC Genomics | volume = 13 | pages = 690 | date = December 2012 | pmid = 23227809 | pmc = 3543383 | doi = 10.1186/1471-2164-13-690 | doi-access = free }}</ref>
* ''Paleobacter carbinolcus'', isolated from anoxic mud, it belongs to the family Desulfuromonadaceae. This bacterial species grow by fermentation,  syntrophic hydrogen/formate transfer, or electron transfer to sulfur from short-chain alcohols, hydrogen or formate but they don't oxidize acetate. There is no recent information about sugar fermentation or autotrophic growth. The sequencing analysis of genome demonstrated the expression of c-type cytochromes and the utilization of Fe (III) as a terminal acceptor with the indirect reduction of elemental sulfur that acts as a shuttle for electron transfer to Fe (III). Recent study thought that this electron transfer involves two periplasmic [[thioredoxin]]s (Pcar_0426, Pcar_0427), an outer membrane protein (Pcar_0428), and a cytoplasmic [[oxidoreductase]] (Pcar_0429) encoded by the most highly upregulated genes.<ref name=":16" />

=== Thermodesulfobacteriota ===
[[Thermodesulfobacteriota]] are Gram- negative, rod-shaped cells, occur singly, in pairs, or in chains in young cultures. Do not form spores. Usually nonmotile, but motility might be observed in some species. Thermophilic, strictly anaerobic, chemoheterotrophs.<ref>{{cite book | vauthors = Garrity GM, Holt JG, Hatchikian EC, Ollivier B, Garcia JL |chapter=Phylum BIII. Thermodesulfobacteria phy. Nov. |title=Bergey's Manual® of Systematic Bacteriology|date=2001 |work=Bergey's Manual® of Systematic Bacteriology: Volume One : The Archaea and the Deeply Branching and Phototrophic Bacteria|pages=389–393| veditors = Boone DR, Castenholz RW, Garrity GM |place=New York, NY|publisher=Springer |doi=10.1007/978-0-387-21609-6_20|isbn=978-0-387-21609-6 }}</ref>

=== Phylum Campylobacterota ===
The phylum [[Campylobacterota]] presents many sulfur-oxidizing known species, that have been recently recognized as able to reduce elemental sulfur, in some cases also preferring this pathway, coupled with hydrogen oxidation.<ref>{{Cite journal| vauthors = Sievert SM, Vetriani C |date=2012|title=Chemoautotrophy at Deep-Sea Vents: Past, Present, and Future |journal=Oceanography|volume=25|issue=1|pages=218–233|doi=10.5670/oceanog.2012.21|jstor=24861161|issn=1042-8275|doi-access=free|hdl=1912/5172|hdl-access=free}}</ref> Here's a list of the species able to reduce elemental sulfur. The mechanism used to reduce sulfur is still unclear for some of these species.<ref name=":1">{{cite journal | vauthors = Wang S, Jiang L, Hu Q, Liu X, Yang S, Shao Z | title = Elemental sulfur reduction by a deep-sea hydrothermal vent Campylobacterium Sulfurimonas sp. NW10 | journal = Environmental Microbiology | date = September 2020 | volume = 23 | issue = 2 | pages = 965–979 | pmid = 32974951 | doi = 10.1111/1462-2920.15247 | s2cid = 221917122 }}</ref>
{| class="wikitable"
|+Table 3. Sulfur-reducing bacteria among Epsiloproteobacteria/Campylobacteriota<ref name=":4" /><ref name=":0" /><ref name=":1" />
!
!Species
|-
|From Hydrothermal vents
|''Caminibacter'' spp. ''(C. hydrogeniphilus, C. mediatlanticus, C. profundus)''
|-
|
|''Hydrogenimonas thermophila''
|-
|
|''Lebetimonas acidiphila''
|-
|
|''Nautilia'' spp. ''(N. abyssi, N. lithotrophica, N. nitratireducens, N. profundicola)''
|-
|
|''Nitratiruptor tergarcus''
|-
|
|''Sulfurimonas'' spp.
|-
|
|''Sulfurospirillum'' sp. Am-N 
|-
|
|''Sulfurovum'' sp. NCB37-1 
|-
|
|''Thioreductor micantisoli'' 
|-
|From cattle rumen
|''Wolinella succinogenes''
|}

==== ''Wolinella'' ====
''Wolinella'' is a sulfur reducing genus of bacteria and incomplete oxidizer that cannot use acetate as an electron donor.<ref name=":4" /> Publicly known is one species, ''Wolinella succinogenens.''<ref>{{cite journal | vauthors = Wolin MJ, Wolin EA, Jacobs NJ | title = Cytochrome-producing anaerobic Vibrio succinogenes, sp. n | journal = Journal of Bacteriology | volume = 81 | issue = 6 | pages = 911–7 | date = June 1961 | pmid = 13786398 | pmc = 314759 | doi = 10.1128/JB.81.6.911-917.1961 }}</ref>
* ''[[Wolinella]] succinogenens'' is a well known non-vent sulfur-reducing bacteria, found in cattle rumen, that utilizes a [Ni-fe] hydrogenase to oxidize hydrogen and a single periplasmatic polysulfide reductase (PsrABC) bounded to the inner membrane to reduce elemental sulfur.<ref name=":0" /> PsrA is responsible for polysulfide reduction to {{H2S}}, at a molybdopterin active site, PsrB is an [FeS] electron transfer protein and PsrC is a quinone-containing membrane anchor.<ref name=":10" />

==== ''Sulfurospirillum'' ====
''Sulfurospirillum'' species are sulfur reducing bacteria and incomplete oxidizer that use either {{chem|H|2}} or formate as electron donor but not acetate.<ref name=":4" />

==== ''Sulfurovum'' ====
* ''Sulfurovum'' sp. NCB37-1 has been given the hypothesis in which a polysulfide reductase (PsrABC) is involved in its sulfur reduction.<ref name=":1" /><ref>{{cite journal | vauthors = Yamamoto M, Nakagawa S, Shimamura S, Takai K, Horikoshi K | title = Molecular characterization of inorganic sulfur-compound metabolism in the deep-sea epsilonproteobacterium Sulfurovum sp. NBC37-1 | journal = Environmental Microbiology | volume = 12 | issue = 5 | pages = 1144–53 | date = May 2010 | pmid = 20132283 | doi = 10.1111/j.1462-2920.2010.02155.x }}</ref>

==== ''Sulfurimonas'' ====
''[[Sulfurimonas]]'' species were previously considered to be chemolithoautotrophic [[Sulfur oxidizing bacteria|sulfur-oxidizing bacteria]] (SOB), and there were only genetic evidences supporting a possible sulfur-reducing metabolism, but now it has been shown that sulfur reduction also occurs in this genus. The mechanism and the enzymes involved in this process have also been deduced, using ''Sulfurimonas'' sp. NW10 as a representative. In particular the presence of both a cytoplasmic and a periplasmic [[polysulfide reductase]]s has been detected, in order to reduce [[cyclooctasulfur]], which is the most common form of elemental sulfur in vent environments.<ref name=":1" />
* ''Sulfurimonas'' sp. NW10 shows an over-expression of the gene clusters (<math>psrA_1B_1CDE</math> and <math>psrA_2B_2</math>) coding for the two reductases while reducing sulfur. These clusters were also found in other ''Sulfurimonas'' species isolated from hydrothermal vents, meaning that sulfur reduction is common in ''[[Sulfurimonas]]'' spp.<ref name=":1" />

Further genetic analysis revealed that the polysulfide reductases from ''Sulfurimonas'' sp.NW10 share less than 40% sequence similarity with the one from ''W. succinogenes''. This means that through time there has been a significant genetic differentiation between the two bacteria, most likely due to their different environments. Furthermore, the cytoplasmic sulfur-reduction performed by ''Sulfurimonas'' sp. NW10 is nowadays considered unique, being the only example among all the mesophilic sulfur-reducing bacteria.<ref name=":1" /> Before this discovery, only two hyperthermophilic bacteria were known to be able to do cytoplasmic sulfur-reduction, ''Aquifex aeolicus''<ref>{{cite journal | vauthors = Guiral M, Tron P, Aubert C, Gloter A, Iobbi-Nivol C, Giudici-Orticoni MT | title = A membrane-bound multienzyme, hydrogen-oxidizing, and sulfur-reducing complex from the hyperthermophilic bacterium Aquifex aeolicus | journal = The Journal of Biological Chemistry | volume = 280 | issue = 51 | pages = 42004–15 | date = December 2005 | pmid = 16236714 | doi = 10.1074/jbc.M508034200 | s2cid = 21987444 | url = http://www.jbc.org/content/280/51/42004 | doi-access = free }}</ref> and ''Thermovibrio ammonificans''.<ref name=":10">{{cite journal | vauthors = Jelen B, Giovannelli D, Falkowski PG, Vetriani C | title = Elemental sulfur reduction in the deep-sea vent thermophile, Thermovibrio ammonificans | journal = Environmental Microbiology | volume = 20 | issue = 6 | pages = 2301–2316 | date = June 2018 | pmid = 29799164 | doi = 10.1111/1462-2920.14280 | s2cid = 44134055 | doi-access =  }}</ref>

==== ''Nautilia'' ====
''[[Nautilia]]'' species are anaerobic, neutrophile, thermophilic sulfur-reducing bacteria, first discovered and isolated from a [[polychaete]] worm inhabiting deep sea hydrothermal vents, ''[[Alvinella pompejana]]''. They are very short, gram-negative, motile and rod-shaped cells with a single polar flagellum.<ref name=":12">{{cite journal | vauthors = Miroshnichenko ML, Kostrikina NA, L'Haridon S, Jeanthon C, Hippe H, Stackebrandt E, Bonch-Osmolovskaya EA | title = Nautilia lithotrophica gen. nov., sp. nov., a thermophilic sulfur-reducing epsilon-proteobacterium isolated from a deep-sea hydrothermal vent | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 52 | issue = Pt 4 | pages = 1299–1304 | date = July 2002 | pmid = 12148643 | doi = 10.1099/00207713-52-4-1299 | doi-access = free }}</ref> They grow chemolithoautotrophically on molecular hydrogen, elemental sulfur and {{CO2}}.<ref>{{cite journal | vauthors = Smith JL, Campbell BJ, Hanson TE, Zhang CL, Cary SC | title = Nautilia profundicola sp. nov., a thermophilic, sulfur-reducing epsilonproteobacterium from deep-sea hydrothermal vents | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 58 | issue = Pt 7 | pages = 1598–602 | date = July 2008 | pmid = 18599701 | doi = 10.1099/ijs.0.65435-0 | s2cid = 12751566 }}</ref> The use of sugars, peptides, organic acids or alcohols are not required in either the absence nor presence of sulfur. They rarely use sulfite and colloidal sulfur as electron acceptors. Sulfate, thiosulfate, nitrate, fumarate and ferric iron are not used. Four species have been found: ''Nautilia lithotrophica'', ''Nautilia profundicola'', ''Nautilia nitratireducens'' and ''Nautilia abyssi''. The type species is ''[[Nautilia lithotrophica]]''.<ref name=":12" />
*'' Nautilia abyssi'' is a gram-negative sulfur-reducing bacterium that lives in anaerobic conditions at hydrothermal vents. It was first discovered living on a chimney at a depth of 2620 meters on the East Pacific Rise. It grows at temperatures between 33° and 65°C, and within a pH range of 5.0–8.0.  Under optimal conditions (60°C, pH 6.0–6.5), the generation time is 120 minutes. Like other species in this genus, cells move by means of a single polar flagellum. For metabolism ''N. abyssi'' use {{chem|H|2}} as an electron donor, elemental sulfur as an electron acceptor and {{CO2}} as the carbon source.<ref name="Nautilia abyssi sp. nov., a thermop">{{cite journal | vauthors = Alain K, Callac N, Guégan M, Lesongeur F, Crassous P, Cambon-Bonavita MA, Querellou J, Prieur D | display-authors = 6 | title = Nautilia abyssi sp. nov., a thermophilic, chemolithoautotrophic, sulfur-reducing bacterium isolated from an East Pacific Rise hydrothermal vent | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 59 | issue = Pt 6 | pages = 1310–5 | date = June 2009 | pmid = 19502307 | doi = 10.1099/ijs.0.005454-0 | s2cid = 39110522 | url = https://archimer.ifremer.fr/doc/2009/publication-6585.pdf }}</ref>

==== ''Caminibacter'' ====
* ''[[Caminibacter mediatlanticus]]'' was first isolated from a deep-sea hydrothermal vent on the Middle Atlantic Ridge.<ref name=":15">{{cite journal | vauthors = Voordeckers JW, Starovoytov V, Vetriani C | title = Caminibacter mediatlanticus sp. nov., a thermophilic, chemolithoautotrophic, nitrate-ammonifying bacterium isolated from a deep-sea hydrothermal vent on the Mid-Atlantic Ridge | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 55 | issue = Pt 2 | pages = 773–779 | date = March 2005 | pmid = 15774661 | doi = 10.1099/ijs.0.63430-0 | doi-access = free }}</ref> It is a thermophilic chemolithoautotroph, {{chem|H|2}}-oxidizing marine bacteria, that uses nitrate or elemental sulfur as electron acceptors, producing ammonia or hydrogen sulfide and it cannot use oxygen, thiosulfate, sulfite, selenate and arsenate. Its growth optimum is at 55&nbsp;°C, and it seems to be inhibited by acetate, formate, lactate and peptone.<ref name=":15" />

=== Aquificota ===
''[[Aquificota]]'' phylum comprises rod-shaped, motile cells. Includes chemoorganotrophs and some of them are able to reduce elemental sulfur. Growth has been observed between pH 6.0 and 8.0.<ref name = "Garrity_2001">{{cite book | vauthors = Garrity GM, Holt JG, Castenholz RW, Pierson BK, Keppen OI, Gorlenko WM |chapter=Phylum BVI. Chloroflexi phy. Nov. |title=Bergey's Manual® of Systematic Bacteriology|date=2001 |work=Bergey's Manual of Systematic Bacteriology|pages=427–446|place=New York, NY|publisher=Springer New York|isbn=978-1-4419-3159-7 |doi=10.1007/978-0-387-21609-6_23}}</ref>

==== ''Aquifex'' ====
''Aquifex'' are rod-shaped, Gram-negative, nonsporulating cells with rounded ends. Wedge-shaped refractile areas in the cells are formed during growth. Type species: ''[[Aquifex pyrophilus]].''<ref name = "Garrity_2001" />

==== ''Desulfurobacterium'' ====
''Desulfurobacterium'' are rod-shaped, Gram-negative cells. Type species: ''[[Desulfurobacterium thermolithotrophum]]''.<ref name = "Garrity_2001" />

==== ''Thermovibrio ammonificans'' ====
''Thermovibrio ammonificans''<ref name=":10" /> is a gram-negative sulfur reducing bacteria, found in deep sea hydrothermal vent chimney. It is a chemolithoautotroph that grows in the presence of {{chem|H|2}} and {{CO2}}, using nitrate or elemental sulfur as electron acceptors with concomitant formation of ammonium or hydrogen sulfide, respectively. Thiosulfate, sulfite and oxygen are not used as electron acceptors. Cells are short rods shape and motile thanks to polar flagellation. Their growth range temperature is from 60&nbsp;°C to 80&nbsp;°C and pH 5–7.<ref name=":9">{{cite journal | vauthors = Vetriani C, Speck MD, Ellor SV, Lutz RA, Starovoytov V | title = Thermovibrio ammonificans sp. nov., a thermophilic, chemolithotrophic, nitrate-ammonifying bacterium from deep-sea hydrothermal vents | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 54 | issue = Pt 1 | pages = 175–181 | date = January 2004 | pmid = 14742477 | doi = 10.1099/ijs.0.02781-0 | doi-access = free }}</ref>

==== ''Thermosulfidibacter'' spp. ====
''Thermosulfidibacter'' are gram-negative, anaerobic, thermophilic and neutrophilic bacteria. Strictly chemolithoautotrophic. The type species is ''Thermosulfidibacter takaii''.

* ''Thermosulfidibacter takaii'' are motile rods with a polar flagellum. Strictly anaerobic. Growth occurs at 55–78&nbsp;°C (optimum, 70&nbsp;°C), pH 5.0–7.5 (optimum, pH 5.5–6.0). They are sulfur-reducers.<ref>{{cite journal | vauthors = Nunoura T, Oida H, Miyazaki M, Suzuki Y | title = Thermosulfidibacter takaii gen. nov., sp. nov., a thermophilic, hydrogen-oxidizing, sulfur-reducing chemolithoautotroph isolated from a deep-sea hydrothermal field in the Southern Okinawa Trough | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 58 | issue = Pt 3 | pages = 659–65 | date = March 2008 | pmid = 18319474 | doi = 10.1099/ijs.0.65349-0 | doi-access = free }}</ref>

=== Bacillota ===
''[[Bacillota]]'' are mostly Gram-positive bacteria with some Gram-negative exceptions.<ref>{{cite book | vauthors = Schleifer KH |chapter=Phylum XIII. Firmicutes Gibbons and Murray 1978, 5 (Firmacutes &#91;sic&#93; Gibbons and Murray 1978, 5) |title=Systematic Bacteriology|date=2009 |work=Bergey's Manual® of Systematic Bacteriology: Volume Three The Firmicutes|pages=19–1317| veditors = De Vos P, Garrity GM, Jones D, Krieg NR |place=New York, NY|publisher=Springer|language=en|doi=10.1007/978-0-387-68489-5_3|isbn=978-0-387-68489-5 }}</ref>

==== ''Ammonifex'' ====
These bacteria are Gram-negative, extremely thermophilic, strictly anaerobic, faculatative chemolithoautotrophic. Type species: ''Ammonifex degensii.''<ref>{{cite book | vauthors = Huber R | chapter = Ammonifex|date=2015 | title = Bergey's Manual of Systematics of Archaea and Bacteria|pages=1–4|publisher=American Cancer Society |doi=10.1002/9781118960608.gbm00744|isbn=978-1-118-96060-8 }}</ref><ref>{{cite journal | vauthors = Huber R, Rossnagel P, Woese CR, Rachel R, Langworthy TA, Stetter KO | title = Formation of ammonium from nitrate during chemolithoautotrophic growth of the extremely thermophilic bacterium ammonifex degensii gen. nov. sp. nov | journal = Systematic and Applied Microbiology | volume = 19 | issue = 1 | pages = 40–9 | date = March 1996 | pmid = 11539844 | doi = 10.1016/S0723-2020(96)80007-5 }}</ref>

==== ''Carboxydothermus'' ====

* ''Carboxydothermus pertinax'' differs from other members of his genus by its ability to grow chemolithoautotrophically with reduction of elemental sulfur or thiosulfate coupled to CO oxidation. The other electron acceptor is ferric citrate, amorphous iron (III) oxide, 9,10-anthraquinone 2,6-disulfonate. Hydrogen is used as energy source and {{CO2}} as carbon source. Cells are rod-shaped with peritrichous flagella and grow at 65&nbsp;°C temperature.<ref>{{cite journal | vauthors = Yoneda Y, Yoshida T, Kawaichi S, Daifuku T, Takabe K, Sako Y | title = Carboxydothermus pertinax sp. nov., a thermophilic, hydrogenogenic, Fe(III)-reducing, sulfur-reducing carboxydotrophic bacterium from an acidic hot spring | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 62 | issue = Pt 7 | pages = 1692–1697 | date = July 2012 | pmid = 21908679 | doi = 10.1099/ijs.0.031583-0 }}</ref>

=== Chrysiogenota ===
[[Chrysiogenota]] are Gram-negative bacteria, motile thanks to a single polar flagellum, curved, rod-shaped cells. They are mesophilic, exhibiting anaerobic respiration in which arsenate serves as the electron acceptor. Strictly anaerobic, these bacteria are grown at 25–30&nbsp;°C.<ref>{{cite book | vauthors = Garrity GM, Holt JG, Macy JM, Krafft T, Sly LI | title = Bergey's Manual® of Systematic Bacteriology| chapter = Phylum BV. Chrysiogenetes phy. Nov.|date=2001 |pages=421–425| veditors = Boone DR, Castenholz RW, Garrity GM |place=New York, NY|publisher=Springer|language=en|doi=10.1007/978-0-387-21609-6_22|isbn=978-0-387-21609-6 }}</ref>

==== ''Desulfurispirillum'' spp. ====
''Desulfurispirillum'' species are gram-negative, motile [[spirilla]], obligately anaerobic with respiratory metabolism. Use elemental sulfur and nitrate as electron acceptors, and short-chain fatty acids and hydrogen as electron donors. Alkaliphilic and slightly halophilic.<ref name=":13" />

* ''Desulfurispirillum alkaliphilum''<ref name=":13">{{cite journal | vauthors = Sorokin DY, Foti M, Tindall BJ, Muyzer G | title = Desulfurispirillum alkaliphilum gen. nov. sp. nov., a novel obligately anaerobic sulfur- and dissimilatory nitrate-reducing bacterium from a full-scale sulfide-removing bioreactor | journal = Extremophiles | volume = 11 | issue = 2 | pages = 363–70 | date = March 2007 | pmid = 17242870 | doi = 10.1007/s00792-006-0048-8 | s2cid = 20745693 | url = http://resolver.tudelft.nl/uuid:0447d188-38c3-497a-97de-26e6170b45f8 }}</ref> is obligate anaerobic and heterotrophic bacteria, motile by single bipolar flagella. It uses elemental sulfur, polysulfide, nitrate and fumarate as electron acceptors. The final products are sulfide and ammonium. Utilizes short-chain fatty acids and H<sub>2</sub> as electron donor and carbon as source. It is moderate alkaliphilic with a pH range for growth between 8.0 and 10.2 and an optimum at pH 9.0 and slightly halophilic with a salt range from 0.1 to 2.5 M Na+. Mesophilic with a maximum temperature for growth at 45 and an optimum at 35&nbsp;°C.<ref name=":13" />

=== Spirochaetota ===
''[[Spirochaetota|Spirochaete]]s'' are free-living, gram-negative, helical-shaped and motile bacteria, often protist or animal-associated. They are obligate and facultative anaerobes.<ref>{{Cite journal| vauthors = Olsen I, Paster B, Dewhirst FE |date=2000-02-01|title=Taxonomy of spirochetes |journal=Anaerobe|volume=6|issue=1|pages=39–57|doi=10.1006/anae.1999.0319|issn=1075-9964}}</ref> Among this phylum, two species are recognized as sulfur-reducing bacteria, ''Spirochaeta perfilievii'' and ''Spirochaeta smaragdinae.''

* ''Spirochaeta perfilievii'' are gram-negative, helical bacteria. Their size range varies from 10 to 200 μm. The shortest cells are those grown in extremely anaerobic environments. They are mesophilic with a temperature range 4–32&nbsp;°C (optimum at 28–30&nbsp;°C). Grows at pH 6.5–8.5 (optimum pH 7.0–7.5). Obligate, moderate halophile. Under anaerobic conditions, sulfur and thiosulfate are reduced to sulfide.<ref>{{cite journal | vauthors = Dubinina G, Grabovich M, Leshcheva N, Rainey FA, Gavrish E | title = Spirochaeta perfilievii sp. nov., an oxygen-tolerant, sulfide-oxidizing, sulfur- and thiosulfate-reducing spirochaete isolated from a saline spring | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 61 | issue = Pt 1 | pages = 110–117 | date = January 2011 | pmid = 20173011 | doi = 10.1099/ijs.0.018333-0 | doi-access = free }}</ref>
* ''Spirochaeta smaragdinae'' are gram-negative, chemoorganotrophic, obligately anaerobic and halophilic bacteria. They are able to reduce sulfur to sulfide. Their temperature range is from 20 to 40&nbsp;°C (optimum 37&nbsp;°C), their pH range varies from 5.5 to 8.0 (optimum 7.0).<ref>{{cite journal | vauthors = Magot M, Fardeau ML, Arnauld O, Lanau C, Ollivier B, Thomas P, Patel BK | title = Spirochaeta smaragdinae sp. nov., a new mesophilic strictly anaerobic spirochete from an oil field | journal = FEMS Microbiology Letters | volume = 155 | issue = 2 | pages = 185–91 | date = October 1997 | pmid = 9351200 | doi = 10.1111/j.1574-6968.1997.tb13876.x | doi-access = free }}</ref>

=== Synergistota ===

==== ''Dethiosulfovibrio'' spp. ====
''Dethiosulfovibrio'' are a gram negative sulfur reducing bacteria that was isolated from "Thiodendron", bacterial sulfur mats obtained from different saline environments. Cells are curved or fibroid-like rods and motile thanks to flagella located on the concave side of the cells.
The temperature range is from 15° to 40&nbsp;°C and at pH values between 5±5 and 8±0. About their metabolism, they ferments proteins, peptides, some organic acids and amino acids like serine, histidine, lysine, arginine, cysteine and threonine. Only in the presence of sulfur or thiosulfate can use alanine, glutamate, isoleucine, leucine and valine, moreover the presence of sulfur or thiosulfate increases the cell yield and the growth rate. They are obligately anaerobic and slightly halophilic. In the presence of fermentable substrates they are able to reduce elemental sulfur and thiosulfate but not sulfate or sulfite to sulfide. Growth did not occur with {{chem|H|2}} as electron donor and carbon dioxide or acetate as carbon sources in the presence of thiosulfate or elemental sulfur as electron acceptor. Unable to utilize carbohydrates, alcohols and some organic acids like acetate or succinate. Four species were found, ''Dethiosulfovibrio russensis'', ''Dethiosulfovibrio marinus,'' ''[[Dethiosulfovibrio peptidovorans]]'' and ''Dethiosulfovibrio acidaminovorans''.<ref name=":21">{{cite journal | vauthors = Surkov AV, Dubinina GA, Lysenko AM, Glöckner FO, Kuever J | title = Dethiosulfovibrio russensis sp. nov., Dethosulfovibrio marinus sp. nov. and Dethosulfovibrio acidaminovorans sp. nov., novel anaerobic, thiosulfate- and sulfur-reducing bacteria isolated from 'Thiodendron' sulfur mats in different saline environments | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 51 | issue = Pt 2 | pages = 327–37 | date = March 2001 | pmid = 11321077 | doi = 10.1099/00207713-51-2-327 | doi-access = free }}</ref>

==== ''Thermanaerovibrio'' spp. ====
Thermophilic and neutrophilic Gram-negative bacteria. Motile thanks to lateral flagella, located on the concave side of the cell. Non-spore-forming. Multiplication occurs by binary fission. Strictly anaerobic with chemo-organotrophic growth on fermentable substrates or lithoheterotrophic growth with molecular hydrogen and elemental sulfur, reducing the sulfur to {{H2S}}. Inhabits the granular methanogenic sludge and neutral hot springs. The type species is ''[[Thermanaerovibrio acidaminovorans]]''.<ref name=":20">{{cite journal | vauthors = Zavarzina DG, Zhilina TN, Tourova TP, Kuznetsov BB, Kostrikina NA, Bonch-Osmolovskaya EA | title = Thermanaerovibrio velox sp. nov., a new anaerobic, thermophilic, organotrophic bacterium that reduces elemental sulfur, and emended description of the genus Thermanaerovibrio | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 50 Pt 3 | issue = 3 | pages = 1287–1295 | date = May 2000 | pmid = 10843074 | doi = 10.1099/00207713-50-3-1287 | doi-access = free }}</ref>
* ''[[Thermanaerovibrio velox]]'' is gram-negative bacteria that was isolated from a thermophilic cyanobacterial mat from caldera Uzon, Kamchatka, Russia. The reproduction occurs by binary-fission and they do not form spore. Growth temperature goes from 45° to 70°, and pH range from 4 to 8.<ref name=":20" />

=== Thermotogota ===
''[[Thermotoga]]'' spp. are gram-negative, rod-shaped, non-spore forming, hyperthermophilic microorganisms, given their name by the presence of a sheathlike envelope called “toga”. They are strictly anaerobes and fermenters, catabolizing sugars or starch and producing lactate, acetate, {{CO2}}, and {{chem|H|2}} as products,<ref name=":4" /> and can grow in a range temperature of 48–90&nbsp;°C.<ref>{{cite book | vauthors = Huber R, Hannig M | chapter = Thermotogales|date=2006 | title = The Prokaryotes: Volume 7: Proteobacteria: Delta, Epsilon Subclass|pages=899–922| veditors = Dworkin M, Falkow S, Rosenberg E, Schleifer KH |place=New York, NY|publisher=Springer|language=en|doi=10.1007/0-387-30747-8_38|isbn=978-0-387-30747-3 }}</ref> High levels of {{chem|H|2}} inhibit their growth, and they share many genetic similarities with Archaea, caused by horizontal gene transfer<ref>{{cite journal | vauthors = Nelson KE, Clayton RA, Gill SR, Gwinn ML, Dodson RJ, Haft DH, Hickey EK, Peterson JD, Nelson WC, Ketchum KA, McDonald L, Utterback TR, Malek JA, Linher KD, Garrett MM, Stewart AM, Cotton MD, Pratt MS, Phillips CA, Richardson D, Heidelberg J, Sutton GG, Fleischmann RD, Eisen JA, White O, Salzberg SL, Smith HO, Venter JC, Fraser CM | display-authors = 6 | title = Evidence for lateral gene transfer between Archaea and bacteria from genome sequence of Thermotoga maritima | journal = Nature | volume = 399 | issue = 6734 | pages = 323–9 | date = May 1999 | pmid = 10360571 | doi = 10.1038/20601 | bibcode = 1999Natur.399..323N | s2cid = 4420157 }}</ref> They are also able to perform anaerobic respiration using {{chem|H|2}} as electron donor and usually Fe(III) as electron acceptor.  Species belonging to the genus ''Thermotoga'' were found in terrestrial hot springs and marine hydrothermal vents. The species able to reduce sulfur do not show an alteration of growth yield and stoichiometry of organic products, and no ATP production occurs. Furthermore, toleration to {{chem|H|2}} increases during sulfur reduction, thus they produce {{H2S}} to overcome growth inhibition.<ref name = "Janssen_1992" /> The genome of ''Thermotoga'' spp. is widely used as a model for studying adaptation to high temperatures, microbial evolution and biotechnological opportunities, such as biohydrogen production and biocatalysis.<ref>{{cite journal | vauthors = Frock AD, Notey JS, Kelly RM | title = The genus Thermotoga: recent developments | journal = Environmental Technology | volume = 31 | issue = 10 | pages = 1169–81 | date = September 2010 | pmid = 20718299 | pmc = 3752655 | doi = 10.1080/09593330.2010.484076 }}</ref>
* ''[[Thermotoga maritima]]'' is the type species for the genus ''Thermotoga,'' growth is observed between 55&nbsp;°C and 90&nbsp;°C, the optimum is at 80&nbsp;°C. Each cell presents a unique sheath- like structure and monotrichous flagellum. It was firstly isolated from a geothermally heated, shallow marine sediment at Vulcano, in Italy.<ref>{{Cite journal| vauthors = Huber R, Langworthy TA, König H, Thomm M, Woese CR, Sleytr UB, Stetter KO |date=1986-05-01|title=Thermotoga maritima sp. nov. represents a new genus of unique extremely thermophilic eubacteria growing up to 90°C |journal=Archives of Microbiology|language=en|volume=144|issue=4|pages=324–333|doi=10.1007/BF00409880|s2cid=12709437|issn=1432-072X}}</ref>
*''[[Thermotoga neapolitana]]'' is the second species isolated belonging to the genus ''Thermotoga''. It was firstly found in a submarine thermal vent at Lucrino, near Naples, Italy, and has its optimum growth at 77&nbsp;°C<ref name=":19" /><ref name = "Jannasch_1998">{{Cite journal| vauthors = Jannasch HW, Huber R, Belkin S, Stetter KO |date=1988-05-01|title=Thermotoga neapolitana sp. nov. of the extremely thermophilic, eubacterial genus Thermotoga |journal=Archives of Microbiology|language=en|volume=150|issue=1|pages=103–104|doi=10.1007/BF00409725|s2cid=22417048|issn=1432-072X}}</ref>

== Ecology ==
Sulfur-reducing bacteria are mostly [[Mesophile|mesophilic]] and [[Thermophile|thermophilic]].<ref name=":0" /> Growth has been observed between a temperature range 37-95&nbsp;°C, however the optimum is different depending on the species (i.e. ''Thermotoga neapolitana'' optimum 77&nbsp;°C, ''Nautilia lithotrophica'' optimum 53&nbsp;°C).<ref name=":19">{{cite journal | vauthors = Belkin S, Wirsen CO, Jannasch HW | title = A new sulfur-reducing, extremely thermophilic eubacterium from a submarine thermal vent | journal = Applied and Environmental Microbiology | volume = 51 | issue = 6 | pages = 1180–5 | date = June 1986 | pmid = 16347075 | doi = 10.1128/AEM.51.6.1180-1185.1986 | pmc = 239042 | bibcode = 1986ApEnM..51.1180B | url = }}</ref><ref name=":12"/><ref name = "Jannasch_1998" /> They have been reported in many different environments, such as anoxic marine sediments, brackish and freshwater sediments, anoxic muds, bovine rumen, hot waters from solfataras and volcanic areas.<ref name=":0" /> Many of these bacteria are found in hot vents, where elemental sulfur is an abundant sulfur species. This happens due to volcanic activities, in which hot vapours and elemental sulfur are released together through the fractures of Earth's crust.<ref name=":11">{{Cite journal| vauthors = Bonch-Osmolovskaya EA |date=1994-09-01|title=Bacterial sulfur reduction in hot vents |journal=FEMS Microbiology Reviews|language=en|volume=15|issue=1|pages=65–77|doi=10.1111/j.1574-6976.1994.tb00122.x|issn=0168-6445|doi-access=free}}</ref> The ability of using zero valence sulfur as both an electron donor or acceptor, allows ''Sulfurimonas'' spp. to spread widely among different habitats, from highly reducing to more oxidizing deep-sea environments.<ref name=":1" /> In some communities found in [[hydrothermal vent]]s, their proliferation is enhanced thanks to the reactions carried out by thermophilic photo- or chemoautotrophs, in which there is simultaneously production of elemental sulfur and organic matter, respectively electron acceptor and energy source for sulfur-reducing bacteria.<ref name=":11" /> Sulfur reducers of hydrothermal vents can be free-living organisms, or endosymbionts of animals such as shrimps and tube worms.<ref name="Nautilia abyssi sp. nov., a thermop"/>

=== Symbiosis ===
''Thiodendron latens'' is a [[Symbiosis|symbiotic]] association of aerotolerant spirochaetes and anaerobic sulfidogenes. The [[spirochaete]] species are the main structural and functional component of these mats and they may accumulate elemental sulfur in the intracellular space. This association of micro-organisms inhabits sulfide-rich habitats, where the chemical oxidation of sulfide by oxygen, manganese or ferric iron or by the activity of sulfide-oxidizing bacteria results in the formation of thiosulfate or elemental sulfur. The partly oxidized sulfur compounds can be either completely oxidized to sulfate by sulfur-oxidizing bacteria, if enough oxygen is present, or reduced to sulfide by sulfidogenic bacteria. In such places oxygen limitation is frequent, as indicated by micro-profile measurements from such habitats. This relationship may represent an effective shortcut in the sulfur cycle.<ref name=":21" />

=== Synthophy ===
''Desulfuromonas acetooxidans'' is able to grow in cocultures with green sulfur bacteria such as ''[[Chlorobium]] (vibrioforme'' and ''phaeovibroides''). The electron donor for the sulfur-reducing bacterium is acetate, coupled with elemental sulfur reduction to sulfide. The green sulfur bacterium produces {{chem|H|2}} re-oxidizing the {{H2S}} previously produced, in presence of light. During these cocultures experiments no elemental sulfur appears in the medium because it is immediately reduced.<ref>{{Cite journal| vauthors = Warthmann R, Cypionka H, Pfennig N |date=1992-04-01|title=Photoproduction of {{chem|H|2}} from acetate by syntrophic cocultures of green sulfur bacteria and sulfur-reducing bacteria |journal=Archives of Microbiology|language=en|volume=157|issue=4|pages=343–348|doi=10.1007/BF00248679|s2cid=25411079|issn=1432-072X}}</ref>

=== Sulfur cycle ===

The [[sulfur cycle]] is one of the major [[Biogeochemical cycle|biogeochemical processes]].<ref>{{cite journal | vauthors = Anantharaman K, Hausmann B, Jungbluth SP, Kantor RS, Lavy A, Warren LA, Rappé MS, Pester M, Loy A, Thomas BC, Banfield JF | display-authors = 6 | title = Expanded diversity of microbial groups that shape the dissimilatory sulfur cycle | journal = The ISME Journal | volume = 12 | issue = 7 | pages = 1715–1728 | date = June 2018 | pmid = 29467397 | pmc = 6018805 | doi = 10.1038/s41396-018-0078-0 }}</ref> The majority of sulfur on Earth is present in sediments and rocks, but its quantity in the oceans represents the primary reservoir of sulfate of the entire biosphere. Human activities such as burning fossil fuels, also contribute to the cycle by entering a significant amount of sulfur dioxide in the atmosphere.<ref name=":5">{{Cite book |title=Biology of microorganisms |vauthors=Brock TD, Martinko JM, Parker J |publisher=Prentice Hall |year=1994 |isbn=1-4058-5345-X |location=Upper Saddle River |pages=694–95}}</ref> The earliest life forms on Earth were sustained by sulfur metabolism, and the enormous diversity of present microorganisms is still supported by the sulfur cycle.<ref name=":5" /> It also interacts with numerous biogeochemical cycles of other elements such as carbon, oxygen, nitrogen and iron.<ref name=":8">{{cite book | vauthors = Canfield DE, Farquhar J |chapter=The Global Sulfur Cycle|date=2012-03-30 |title=Fundamentals of Geobiology|pages=49–64| veditors = Knoll AH, Canfield DE, Konhauser KO |place=Chichester, UK|publisher=John Wiley & Sons, Ltd|doi=10.1002/9781118280874.ch5|isbn=978-1-118-28087-4 }}</ref><ref name=":5" /> Sulfur has diverse [[oxidation state]]s ranging from +6 to −2, which permit to sulfur compounds to be used as electron donors and electron acceptors in numerous microbial metabolisms, which transform organic and inorganic sulfur compounds, contributing to physical, biological and chemical components of the biosphere.<ref name=":7" /><ref name=":8" />

The sulfur cycle follows several linked pathways.

'''Sulfate Reduction'''[[File:Sulfur cycle - English.jpg|thumb|533x533px|The sulfur cycle]]Under [[Anaerobic respiration|anaerobic]] conditions, sulfate is reduced to sulfide by sulfate reducing bacteria, such as ''Desulfovibrio'' and ''Desulfobacter''.

{{chem2|SO4(2-) + 4H2 -> H2S + 2H2O + 2OH−}}

'''Sulfide Oxidation'''

Under aerobic conditions, sulfide is oxidized to sulfur and then sulfate by sulfur oxidizing bacteria, such as ''Thiobacillus'', ''Beggiatoa'' and many others. Under anaerobic conditions, sulfide can be oxidized to sulfur and then sulfate by Purple and Green sulfur bacteria.

{{chem2|H2S -> S^{0} -> SO4(2-)}}

'''Sulfur Oxidation'''

Sulfur can also be oxidized to [[sulfuric acid]] by chemolithotrophic bacteria, such as ''Thiobacillus'' and ''Acidithiobacillus''

{{chem2|S^{0} + 2O2 -> H2SO4}}

'''Sulfur Reduction'''

Some bacteria are capable to reduce sulfur to sulfide enacting a sort of anaerobic respiration. This process can be carried out by both sulfate reducing bacteria and sulfur reducing bacteria. Although they thrive in the same habitats, sulfur reducing bacteria are incapable of sulfate reduction. Bacteria like ''Desulfuromonas acetoxidans'' are able to reduce sulfur at the cost of acetate. Some iron reducing bacteria reduce sulfur to generate ATP.<ref>{{cite book | vauthors = Loka Bharathi PA | chapter = Sulfur Cycle|date=2008 |title = Encyclopedia of Ecology|pages=3424–3431|publisher=Elsevier|doi=10.1016/b978-008045405-4.00761-8|isbn=978-0-08-045405-4 }}</ref>

{{chem2|S^{0} + H2 -> H2S}}

These are the main inorganic processes involved in the sulfur cycle but organic compounds can contribute as well to the cycle. The most abundant in nature is [[dimethyl sulfide]] ({{chem2|CH3\sS\sCH3}}) produced by the degradation of [[dimethylsulfoniopropionate]]. Many other organic S compounds affect the global sulfur cycle, including [[methanethiol]], [[dimethyl disulfide]], and [[carbon disulfide]].<ref name=":5" />

== Uses ==
Microorganisms that have sulfur-based metabolism represent a great opportunity for industrial processes, in particular the ones that execute sulfidogenesis (production of sulfide). For example, these type of bacteria can be used in to generate [[hydrogen sulfide]] in order to obtain the selective precipitation and recovery of heavy metals in metallurgical and mining industries.<ref name=":7" />

=== Flue gases treatment ===
According to innovative Chinese research, the SCDD process used to desulfurize [[flue gas]]es can be lowered in costs and environmental impact, using biological reduction of elemental sulfur to {{H2S}}, which represents the reducing agent in this process. The electron donors would be organics from wastewater, such as acetate and glucose. The SCDD process revisited in this way would take three steps at determinate conditions of pH, temperature and reagents concentration. The first in which biological sulfur reduction occurs, the second through which dissolved {{H2S}} in wastewaters is stripped into hydrogen sulfide gas, and the third consists in the treatment of flue gases, removing over 90% of {{SO2}} and NO, according to this study. Furthermore, the 88% of the sulfur input would be recovered as octasulfur and then reutilized, representing both a chemical-saving and a profitable solution.<ref>{{cite journal | vauthors = Sun J, Li L, Zhou G, Wang X, Zhang L, Liu Y, Yang J, Lü X, Jiang F | display-authors = 6 | title = 2 and NO and Sulfur Recovery from Flue Gas | journal = Environmental Science & Technology | volume = 52 | issue = 8 | pages = 4754–4762 | date = April 2018 | pmid = 29547691 | doi = 10.1021/acs.est.7b06551 }}</ref>

=== Treatment of arsenic-contaminated waters ===
Sulfur reducing bacteria are used to remove [[Arsenite]] from the arsenic-contaminated waters, like [[acid mine drainage]] (AMD), metallurgy industry effluents, soils, surface and ground waters. The sulfidogenic process driven by sulfur reducing bacteria (''Desulfurella'') take place under acid condition and produce sulfide with which arsenite precipitates. Microbial sulfur reduction also produces protons that lower the pH in arsenic-contaminated water and prevent the formation of thioarsenite by-production with sulfide.<ref name="Sun_2019">{{cite journal | vauthors = Sun J, Hong Y, Guo J, Yang J, Huang D, Lin Z, Jiang F | title = Arsenite removal without thioarsenite formation in a sulfidogenic system driven by sulfur reducing bacteria under acidic conditions | journal = Water Research | volume = 151 | pages = 362–370 | date = March 2019 | pmid = 30616048 | doi = 10.1016/j.watres.2018.12.027 | bibcode = 2019WatRe.151..362S | s2cid = 58657337 }}</ref>

<math>CH3COOH+4S^0 + 2H2O\longrightarrow   4HS^- + 2CO2 + 4H^+</math> <ref name="Sun_2019" />

=== Treatment of mercury-contaminated waters ===
Wastewater deriving from industries that work on chloralkali and battery production, contains high levels of mercury ions, threatening aquatic ecosystems.<ref>{{Cite journal| vauthors = Huang Y, Du JR, Zhang Y, Lawless D, Feng X |date=2015-11-01|title=Removal of mercury (II) from wastewater by polyvinylamine-enhanced ultrafiltration |journal=Separation and Purification Technology|volume=154|pages=1–10|doi=10.1016/j.seppur.2015.09.003|issn=1383-5866}}</ref> Recent studies demonstrate that sulfidogenic process by sulfur reducing bacteria can be a good technology in the treatment of mercury-contaminate waters.<ref>{{cite journal | vauthors = Wang JT, Zhang L, Kang Y, Chen G, Jiang F | title = Long-Term Feeding of Elemental Sulfur Alters Microbial Community Structure and Eliminates Mercury Methylation Potential in Sulfate-Reducing Bacteria Abundant Activated Sludge | journal = Environmental Science & Technology | volume = 52 | issue = 8 | pages = 4746–4753 | date = April 2018 | pmid = 29617126 | doi = 10.1021/acs.est.7b06399 | bibcode = 2018EnST...52.4746W }}</ref>

== References ==
{{Reflist}}

[[Category:Phototrophic bacteria]]
[[Category:Hydrogen biology]]