Chlorobium

Chlorobium
Scientific classification
Kingdom:	Bacteria
Phylum:	Chlorobi
Class:	Chlorobia
Order:	Chlorobiales
Family:	Chlorobiaceae
Genus:	Chlorobium Nadson 1906
Some species
C. clathratiforme (Szafer 1911) Imhoff 2003 C. limicola Nadson 1906 (type species) C. luteolum (Schmidle 1901) Imhoff 2003 C. phaeobacteroides Pfennig 1968 C. phaeovibrioides Pfennig 1968 C. tepidum Wahlund et al. 1996 C. vibrioforme Pelsh 1936 C. aggregatum

Chlorobium (also known as Chlorochromatium) is a genus of green sulfur bacteria. They are photolithotrophic oxidizers of sulfur and most notably utilise a noncyclic electron transport chain to reduce NAD+. Hydrogen sulfide is used as an electron source and carbon dioxide its carbon source.^[1]

Chlorobium species exhibit a dark green color; in a Winogradsky column, the green layer often observed is composed of Chlorobium. This genus lives in strictly anaerobic conditions below the surface of a body of water, commonly the anaerobic zone of a eutrophic lake.^[1]

Chlorobium aggregatum is a species which exists in a symbiotic relationship with a colorless, nonphotosynthetic bacteria. This species looks like a bundle of green bacteria, attached to a central rod-like cell which can move around with a flagellum. The green, outer bacteria use light to oxidize sulfide into sulfate. The inner cell, which is not able to perform photosynthesis, reduces the sulfate into sulfide. These bacteria divide in unison, giving the structure a multicellular appearance which is highly unusual in bacteria.^[2]

Chlorobium species are thought to have played an important part in mass extinction events on Earth. If the oceans turn anoxic (due to the shutdown of ocean circulation) then Chlorobium would be able to out compete other photosynthetic life. They would produce huge quantities of methane and hydrogen sulfide which would cause global warming and acid rain. This would have huge consequences for other oceanic organisms and also for terrestrial organisms. Evidence for abundant Chlorobium populations is provided by chemical fossils found in sediments deposited at the cretaceous mass extinction.

The complete C. tepidum genome, which consists of 2.15 megabases (Mb), was sequenced and published in 2002.^[3] It synthesizes chlorophyll a and bacteriochlorophylls (BChls) a and c, of which the model organism has been used to elucidate the biosynthesis of BChl c.^[4] Several of its carotenoid metabolic pathways (including a novel lycopene cyclase) have similar counterparts in cyanobacteria.^[5][6]

References

1. ^ ^{a b} Prescott, Harley, Klein. (2005). Microbiology pp. 195, 493, 597, 618-619, 339.
2. ^ Postgate, John: "The Outer Reaches of Life", page 132-134. Cambridge University Press, 1994
3. ^ J.A. Eisen; Nelson, KE; Paulsen, IT; Heidelberg, JF; Wu, M; Dodson, RJ; Deboy, R; Gwinn, ML et al. (2002). "The complete genome sequence of Chlorobium tepidum TLS, a photosynthetic, anaerobic, green-sulfur bacterium". Proc. Natl Acad. Sci. USA 99 (14): 9509–9514. doi:10.1073/pnas.132181499. PMC 123171. PMID 12093901. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=123171. 
4. ^ N.-U Frigaard, et al. (2006). B. Grimm et al., eds. ed. Chlorophylls and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and Applications. 25. Springer. Vol. 25, pp. 201–221. 
5. ^ N.-U. Frigaard, et al. (2004). "Genetic manipulation of carotenoid biosynthesis in the green sulfur bacterium Chlorobium tepidum". Proc. Natl Acad. Sci. USA 186: 5210–5220. 
6. ^ J.A. Maresca, et al. (2005). A. van der Est & D. Bruce, eds. ed. Photosynthesis: Fundamental Aspects to Global Perspectives. Allen Press. pp. 884–886.