Green sulfur bacteria
Green sulfur bacteria in a Winogradsky column
Scientific classification
Domain:	Bacteria
(unranked):	Bacteroidota–Chlorobiota group
Phylum:	Chlorobiota Iino et al. 2021[3]
Class:	"Chlorobia" Garrity and Holt 2001[2]
Order:	Chlorobiales Gibbons and Murray 1978 (Approved Lists 1980)[1]
Families and Genera
Chlorobiaceae "Thermochlorobacteriaceae"
Synonyms
Chlorobiota: Chlorobi Iino et al. 2010 "Chlorobi" Garrity and Holt 2001 "Chlorobaeota" Oren et al. 2015 "Chlorobiota" Whitman et al. 2018 Chlorobiota: "Chlorobia" Whitman et al. 2018 Chlorobea Cavalier-Smith 2002 "Chlorobiia" Cavalier-Smith 2020 Chlorobiales: "Chlorobiales" Garrity and Holt 2001

The green sulfur bacteria are a phylum, Chlorobiota,^[4] of obligately anaerobic photoautotrophic bacteria that metabolize sulfur.^[5]

Green sulfur bacteria are nonmotile (except Chloroherpeton thalassium, which may glide) and capable of anoxygenic photosynthesis.^[5][6] They live in anaerobic aquatic environments.^[7] In contrast to plants, green sulfur bacteria mainly use sulfide ions as electron donors.^[8] They are autotrophs that utilize the reverse tricarboxylic acid cycle to perform carbon fixation.^[9] They are also mixotrophs and reduce nitrogen.^[10][11]

Characteristics

Green sulfur bacteria are gram-negative rod or spherical shaped bacteria. Some types of green sulfur bacteria have gas vacuoles that allow for movement. They are photolithoautotrophs, and use light energy and reduced sulfur compounds as the electron source.^[12] Electron donors include H₂, H₂S, S. The major photosynthetic pigment in these bacteria is Bacteriochlorophylls c or d in green species and e in brown species, and is located in the chlorosomes and plasma membranes.^[7] Chlorosomes are a unique feature that allow them to capture light in low-light conditions.^[13]

Habitat

The majority of green sulfur bacteria are mesophilic, preferring moderate temperatures, and all live in aquatic environments. They require anaerobic conditions and reduced sulfur; they are usually found in the top millimeters of sediment. They are capable of photosynthesis in low light conditions.^[7]

The Black Sea, an extremely anoxic environment, was found to house a large population of green sulfur bacteria at about 100 m depth. Due to the lack of light available in this region of the sea, most bacteria were photosynthetically inactive. The photosynthetic activity detected in the sulfide chemocline suggests that the bacteria need very little energy for cellular maintenance.^[14]

A species of green sulfur bacteria has been found living near a black smoker off the coast of Mexico at a depth of 2,500 m in the Pacific Ocean. At this depth, the bacterium, designated GSB1, lives off the dim glow of the thermal vent since no sunlight can penetrate to that depth.^[15]

Green sulfur bacteria has also been found living on coral reef colonies in Taiwan, they make up the majority of a "green layer" on these colonies. They likely play a role in the coral system, and there could be a symbiotic relationship between the bacteria and the coral host.^[16] The coral could provide an anaerobic environment and  a source of carbon for the bacteria. The bacteria can provide nutrients and detoxify the coral by oxidizing sulfide.^[17]

One type of green sulfur bacteria, Chlorobaculum tepidum, has been found in sulfur springs. These organisms are thermophilic, unlike most other green sulfur bacteria.^[7]

Phylogeny