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 fix carbon dioxide. Green sulfur bacteria have been found in depths of up to 145m in the Black Sea, with low light availability. Photosynthesis is achieved using a Type 1 reaction centre, which contains bacteriochlorophyll a, and is taken place in chlorosomes. Type 1 reaction centre is equivalent to photosystem I found in plants and cyanobacteria. Green sulfur bacteria use sulfide ions, hydrogen or ferrous iron as electron donors and the process is mediated by the Type I reaction centre and Fenna-Matthews-Olson complex. Reaction centre contains bacteriochlorophylls, P840, which donates electrons to cytochrome c-551 when it is excited by light. Cytochrome c-551 then passes the electrons down the electron chain. P840 is returned to its reduced state by the oxidation of sulfide. Sulfide donates two electrons to yield elemental sulfur. Elemental sulfur is deposited in globules on the extracellular side of the outer membrane. When sulfide is depleted, the sulfur globules are consumed and oxidized to sulfate. However, the pathway of sulfur oxidation is not well-understood. These autotrophs fix carbon dioxide using the reverse tricarboxylic acid (RTCA) cycle. Energy is consumed to incorporate carbon dioxide in order to assimilate pyruvate and acetate and generate macromolecules. Chlorobium tepidum, a member of green sulfur bacteria was found to be mixotroph due to its ability to use inorganic and organic carbon sources. They can assimilate acetate through the oxidative (forward) TCA (OTCA) cycle in addition to RTCA. In contrast to the RTCA cycle, energy is generated in the OTCA cycle, which may contribute to better growth. However, the capacity of the OTCA cycle is limited because gene that code for enzymes of the OTCA cycle are down-regulated when the bacteria is growing phototrophically. 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. The currently accepted phylogeny is based on 16S rRNA-based LTP release 123 by The All-Species Living Tree Project.