Photosynthesis is achieved using a Type 1 reaction centre, which contains bacteriochlorophylla, 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.
^ abProkaryotes where no pure (axenic) cultures are isolated or available, i. e. not cultivated or can not be sustained in culture for more than a few serial passages
Photosynthesis in the green sulfur bacteria
The green sulfur bacteria use PS I for photosynthesis. Thousands of bacteriochlorophyll(BCHl) c, d and e of the cells absorb light at 720-750 nm, and the light energy is transferred to BChl a-795 and a-808 before being transferred to Fenna-Matthews-Olson (FMO)-proteins which are connected to reaction centers (RC). The FMO complex then transfers the excitation energy to the RC with its special pair which absorbs at 840 nm in the plasma membrane.
After the reaction centers receive the energy, electrons are ejected and transferred through electron transport chains (ETCs). Some electrons form Fe-S proteins in electron transport chains are accepted by ferredoxins (Fd) which can be involved in NAD(P) reduction and other metabolic reactions.
Carbon fixation of green sulfur bacteria
The reactions of reversal of the oxidative tricarboxylic acid cycle are catalyzed by four enzymes:
pyruvate:ferredoxin (Fd) oxidoreductase:
acetyl-CoA + CO2 + 2Fdred + 2H+ ⇌ pyruvate + CoA + 2Fdox
ATP citrate lyase:
ACL, acetyl-CoA + oxaloacetate + ADP + Pi ⇌ citrate + CoA + ATP
succinyl-CoA + CO2 + 2Fdred + 2H+ ⇌ α-ketoglutarate + CoA + 2Fdox
^ abMarschall E, Jogler M, Hessge U, Overmann J (May 2010). "Large-scale distribution and activity patterns of an extremely low-light-adapted population of green sulfur bacteria in the Black Sea". Environmental Microbiology. 12 (5): 1348–62. doi:10.1111/j.1462-2920.2010.02178.x. PMID20236170.
^Pranav kumar, Usha mina (2014). Life science fundamental and practice part I.
^Ke B (2003). "The Green Bacteria. II. The Reaction Center Photochemistry and Electron Transport". Photosynthesis. Advances in Photosynthesis and Respiration. 10. pp. 159–78. doi:10.1007/0-306-48136-7_9. ISBN0-7923-6334-5.