Green sulfur bacteria

The green sulfur bacteria are a phylum, Chlorobiota, of obligately anaerobic photoautotrophic bacteria that metabolize sulfur.

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

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.

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.

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. 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.

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

! colspan=1 | 120 marker proteins based GTDB 10-RS226

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Taxonomy

Family Chlorobiaceae Copeland 1956 ["Chlorobacteriaceae" Geitler & Pascher 1925]
?Ancalochloris Gorlenko and Lebedeva 1971
Chlorobaculum Imhoff 2003
Chlorobium Nadson 1906
?"Chloroplana" Dubinina and Gorlenko 1975
?"Clathrochloris" Geitler 1925
Prosthecochloris Gorlenko 1970
Family "Chloroherpetaceae" corrig. Bello et al. 2022
Chloroherpeton Gibson et al. 1985
Family "Thermochlorobacteriaceae" corrig. Liu et al. 2012
"Ca. Thermochlorobacter" Liu et al. 2012

Specific characteristics of genera

Green sulfur bacteria are family Chlorobiaceae. There are four genera; Chloroherpeton, Prosthecochloris, Chlorobium and Chlorobaculum. Characteristics used to distinguish between these genera include some metabolic properties, pigments, cell morphology and absorption spectra. However, it is difficult to distinguish these properties and therefore the taxonomic division is sometimes unclear.

Generally, Chlorobium are rod or vibroid shaped and some species contain gas vesicles. They can develop as single or aggregate cells. They can be green or dark brown. The green strains use photosynthetic pigments Bchl c or d with chlorobactene carotenoids and the brown strains use photosynthetic pigment Bchl e with isorenieratene carotenoids. Low amounts of salt are required for growth. A protein complex called the Fenna-Matthews-Olson complex (FMO) is physically located between the chlorosomes and the P840 RC. The FMO complex helps efficiently transfer the energy absorbed by the antena to the reaction center.

PSI and Type I reaction centers are able to reduce ferredoxin (Fd), a strong reductant that can be used to reduce NAD+ and fix . Once the reaction center (RC) has given an electron to Fd, it becomes an oxidizing agent (P840+) with a reduction potential of around +300 mV. While this is not positive enough to strip electrons from water to synthesize (E0 = +820 mV), it can accept electrons from other sources like , thiosulphate or ions. This transport of electrons from donors like to the acceptor Fd is called linear electron flow, or linear electron transport. The oxidation of sulfide ions leads to the production of sulfur as a waste product that accumulates as globules on the extracellular side of the membrane. These globules of sulfur give green sulfur bacteria their name. When sulfide is depleted, the sulfur globules are consumed and further oxidized to sulfate. However, the pathway of sulfur oxidation is not well-understood. The intermediate is usually sulfur, which is deposited outside of the cell, and the end product is sulfate. The sulfur, which is deposited extracellularly, is in the form of sulfur globules, which can be later oxidized completely. The oxidation of thiosulfate to sulfate could be catalyzed by the enzymes in the system. However it has several oxygen sensitive enzymes that limits its efficiency in aerobic conditions. Mixotrophy occurs during amino acid biosynthesis/carbon utilization and energy metabolism. The bacterium uses electrons, generated from the oxidation of sulfur, and the energy it captures from light to run the rTCA. C. tepidum also exhibits use of both pyruvate and acetate as an organic carbon source. Nitrogen fixation among green sulfur bacteria is generally typical of an anoxygenic phototroph, and requires the presence of light. Green sulfur bacteria exhibit activity from a Type-1 secretion system and a ferredoxin-NADP+ oxidoreductase to generate reduced iron, a trait that evolved to support nitrogen fixation. Like purple sulfur bacteria, they can regulate the activity of nitrogenase post-translationally in response to ammonia concentrations. Their possession of nif genes, even though evolutionarily distinct, may suggest their nitrogen fixation abilities arose in two different events or through a shared very distant ancestor.