Green sulfur bacteria

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Green sulfur bacteria
Green d winogradsky.jpg
Green sulfur bacteria in a Winogradsky column
Scientific classification
Domain: Bacteria
Superphylum: FCB group
(unranked): Bacteroidetes-Chlorobi group
Phylum: Chlorobi
Iino et al. 2010
Genus

The green sulfur bacteria (Chlorobiaceae) are a family of obligately anaerobic photoautotrophic bacteria. Together with the non-photosynthetic Ignavibacteriaceae, they form the phylum Chlorobi.[1]

Green sulfur bacteria are nonmotile (except Chloroherpeton thalassium, which may glide) and capable of anoxygenic photosynthesis.[1][2] In contrast to plants, green sulfur bacteria mainly use sulfide ions as electron donors.[3] They are autotrophs that utilize the reverse tricarboxylic acid cycle to fix carbon dioxide.[4] Green sulfur bacteria have been found in depths of up to 145m in the Black Sea, with low light availability.[5]

Metabolism[edit]

Catabolism[edit]

Photosynthesis is achieved using a Type 1 reaction centre, which contains bacteriochlorophyll a, and is taken place in chlorosomes.[1][2] 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.[3]

Anabolism[edit]

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.[4]

Habitat[edit]

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.[5]

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.[6]

Phylogeny[edit]

The currently accepted phylogeny is based on 16S rRNA-based LTP release 123 by The All-Species Living Tree Project.[7]

Ignavibacteriaceae

Ignavibacterium Iino et al. 2010 emend. Podosokorskaya et al. 2013

Melioribacter Podosokorskaya et al. 2013

Chlorobiaceae

Chloroherpeton thalassium Gibson et al. 1985

Prosthecochloris

P. aestuarii Gorlenko 1970 emend. Imhoff 2003 (type sp.)

P. vibrioformis (Pelsh 1936) Imhoff 2003

Chlorobium chlorovibrioides[notes 2](Gorlenko et al. 1974) Imhoff 2003

Chlorobaculum

C. tepidum (Wahlund et al. 1996) Imhoff 2003 (type sp.)

C. thiosulfatiphilum Imhoff 2003

Chlorobium

C. luteolum (Schmidle 1901) emend. Imhoff 2003

C. phaeovibrioides Pfennig 1968 emend. Imhoff 2003

C. limicola Nadson 1906 emend. Imhoff 2003 (type sp.)

C. clathratiforme (Szafer 1911) emend. Imhoff 2003

C. phaeobacteroides Pfennig 1968 emend. Imhoff 2003

Taxonomy[edit]

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LSPN)[8][9]

Notes[edit]

  1. ^ a b c d e f g h i Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LSPN)
  2. ^ a b c Tang KH, Blankenship RE (November 2010). "Both forward and reverse TCA cycles operate in green sulfur bacteria". The Journal of Biological Chemistry. 285 (46): 35848–54. doi:10.1074/jbc.M110.157834. PMC 2975208. PMID 20650900.
  3. ^ a b Prokaryotes 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[edit]

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.[10]

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.[11]

Carbon Fixation of Green Sulfur Bacteria[edit]

The reactions of reversal of the oxidative tricarboxylic acid cycle are catalyzed by four enzymes:[4]

  1. pyruvate:ferredoxin (Fd) oxidoreductase:
    acetyl-CoA + CO2 + 2Fdred + 2H+ ⇌ pyruvate + CoA + 2Fdox
  2. ATP citrate lyase:
    ACL, acetyl-CoA + oxaloacetate + ADP + Pi ⇌ citrate + CoA + ATP
  3. α-keto-glutarate:ferredoxin oxidoreductase:
    succinyl-CoA + CO2 + 2Fdred + 2H+ ⇌ α-ketoglutarate + CoA + 2Fdox
  4. fumarare reductase
    succinate + acceptor ⇌ fumarate + reduced acceptor

See also[edit]

References[edit]

  1. ^ a b c Bryant DA, Frigaard NU (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends in Microbiology. 14 (11): 488–96. doi:10.1016/j.tim.2006.09.001. PMID 16997562.
  2. ^ a b Green BR (2003). Light-Harvesting Antennas in Photosynthesis. p. 8. ISBN 0792363353.
  3. ^ a b Sakurai H, Ogawa T, Shiga M, Inoue K (June 2010). "Inorganic sulfur oxidizing system in green sulfur bacteria". Photosynthesis Research. 104 (2–3): 163–76. doi:10.1007/s11120-010-9531-2. PMID 20143161.
  4. ^ a b c Tang KH, Blankenship RE (November 2010). "Both forward and reverse TCA cycles operate in green sulfur bacteria". The Journal of Biological Chemistry. 285 (46): 35848–54. doi:10.1074/jbc.M110.157834. PMC 2975208. PMID 20650900.
  5. ^ a b Marschall 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. PMID 20236170.
  6. ^ a b Beatty JT, Overmann J, Lince MT, Manske AK, Lang AS, Blankenship RE, Van Dover CL, Martinson TA, Plumley FG (June 2005). "An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent". Proceedings of the National Academy of Sciences of the United States of America. 102 (26): 9306–10. Bibcode:2005PNAS..102.9306B. doi:10.1073/pnas.0503674102. PMC 1166624. PMID 15967984.
  7. ^ See the All-Species Living Tree Project [1]. Data extracted from the "16S rRNA-based LTP release 123 (full tree)" (PDF). Silva Comprehensive Ribosomal RNA Database. Retrieved 2016-03-20.
  8. ^ See the List of Prokaryotic names with Standing in Nomenclature. Data extracted from J.P. Euzéby. "Chlorobi". Archived from the original on 2013-01-27. Retrieved 2016-03-20.
  9. ^ See the NCBI webpage on Chlorobi Data extracted from Sayers; et al. "NCBI Taxonomy Browser". National Center for Biotechnology Information. Retrieved 2016-03-20.
  10. ^ Hauska G, Schoedl T, Remigy H, Tsiotis G (October 2001). "The reaction center of green sulfur bacteria(1)". Biochimica et Biophysica Acta. 1507 (1–3): 260–77. doi:10.1016/S0005-2728(01)00200-6. PMID 11687219.
  11. ^ 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. ISBN 0-7923-6334-5.

External links[edit]