Green-sulphur bacteria
| Green sulfur bacteria | |
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| Green sulfur bacteria in a Winogradsky column | |
| Scientific classification | |
| Domain: | Bacteria |
| Phylum: |
Chlorobi Iino et al. 2010 |
| Genus | |
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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 are often found in the deep sea, which has 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 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.
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