Green sulfur bacteria

The green sulfur bacteria (Chlorobiaceae) are a family of obligately anaerobic photoautotrophic bacteria. Together with the non-photosynthetic Ignavibacteriaceae, they form the phylum Chlorobi.
Green sulfur bacteria are nonmotile (except "Chloroherpeton thalassium", which may glide) and capable of anoxygenic photosynthesis. 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. Green sulfur bacteria have been found in depths of up to 145m in the Black Sea, with low light availability. 
Characteristics of green-Sulfur bacteria:
Major photosynthetic pigment: Bacteriochlorophylls a plus c, d or e
Location of photosynthetic pigment: Chlorosomes and plasma membranes
Photosynthetic electron donor: H2, H2S, S
Sulfur deposition: Outside of the cell
Metabolic type: Photolithoautotrophs
Habitat.
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.
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.
Phylogeny.
The currently accepted phylogeny is based on 16S rRNA-based LTP release 123 by The All-Species Living Tree Project.
Taxonomy.
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature.(LSPN)
Metabolism.
Photosynthesis in the green sulfur bacteria.
The green sulfur bacteria use a Type I reaction center for photosynthesis. Type I reaction centers are the bacterial homologue of photosystem I (PSI) in plants and cyanobacteria. The GSB reaction centers contain bacteriochlorophyll a and are known as P840 reaction centers due to the excitation wavelength of 840 nm that powers the flow of electrons. In green sulfur bacteria the reaction center is associated with a large antena complex called the chlorosome that captures and funnels light energy to the reaction center. The chlorosomes have a peak absorption in the far red region of the spectrum between 720-750 nm because they contain bacteriocholorophyll c, d and e. 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 fix and reduce NADPH. 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 ( = +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.
Instead of passing the electrons onto Fd, the Fe-S clusters in the P840 reaction center can transfer the electrons to menaquinone (MQ:) which returns the electrons to the P840+ via an electron transport chain (ETC). On the way back to the RC the electrons from MQH2 pass through a cytochrome bc1 complex (similar to the complex III of mitochondria) that pumps ions across the membrane. The electrochemical potential of the protons across the membrane is used to synthesize ATP by the FoF1 ATP synthase. This cyclic electron transport is responsible for converting light energy into cellular energy in the form of ATP.
Carbon fixation of green sulfur bacteria.
Green sulfur bacteria are photoautotrophs: they not only get energy from light, they can grow using carbon dioxide as their sole source of carbon. They fix carbon dioxide using the reverse tricarboxylic acid cycle (rTCA) cycle where energy is consumed to reduce carbon dioxide in order to synthesize pyruvate and acetate. These molecules are used as the raw materials to synthesize all the building blocks a cell needs to generate macromolecules. The rTCA cycle is highly energy efficient enabling the bacteria to grow under low light conditions. However it has several oxygen sensitive enzymes that limits it's efficiency in aerobic conditions.
The reactions of reversal of the oxidative tricarboxylic acid cycle are catalyzed by four enzymes:
Mixotrophy in green sulfur bacteria.
Green sulfur bacteria are obligate photoautotrophs: they cannot grow in the absence of light even if they are provided with organic matter. However they exhibit a form of mixotrophy where they can consume simple organic compounds in the presence of light and CO2.
Nitrogen fixation.
The majority of green sulfur bacteria are diazotrophs: they can reduce nitrogen to ammonia which is then used to synthesize amino acids.