Chlorobium ferrooxidans
Classification
Domain: Bacteria
Phylum: Chlorobi
Class: Chlorobia
Order: Chlorobiales
Family: Chlorobiaceae
Genus: Chlorobium
Species: ferrooxidans (2)
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Species
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NCBI: Taxonomy |
Chlorobium ferrooxidans
Description and Significance
This green phototrophic bacterium is short, rod-shaped, approximately 0.5x1.0-1.5 μm in size, with rounded ends. The organism is nonmotile, gram negative, and nonsporeforming. Chlorobium ferrooxidans is strictly anaerobic. Originally isolated from shallow freshwater ditches, the phottrophic bacterium (strain KoFox) has only been isolated as a coculture with a strain identified as a member of the ε-subclass of the proteobacteria closely related to Geospirillum arsenophilum (KoFum). When grown in coculture, Chlorobium ferrooxidans oxidizes ferrous iron to ferric iron with stoichiometric formation of cell mass from carbon dioxide. This bacterium is important due to the fact that it is a novel green phototroph, related to other species of Chlorobium yet unique in regards to the oxidation of ferrous iron to ferric iron. This process by bacteria is a relatively novel phenomenon that has only been observed with phototrophic purple sulfur or non-sulfur bacteria (Wkddel et al., 1993; Ehrenreich and Widdel, 1994; Heising and Scchink, 1998). This observation in green phototrophic bacteria may indicate phototrophic ferrous iron oxidation was a widespread metabolic capacity in an early phase of evolution (1).
Genome Structure
The Chlorobium-like partner (KoFox) in the coculture is genetically related to Chlorobium, Prosthecochloris, and Pelodictyon, however no relationship was found to any strain for which rRNA sequence data currently are available (1). Overall 16S rRNA sequence similarity values of 91.4 - 96.7% indicate that the strain represents a separate line of descent within a Pelodictyon/Prosthechloris cluster (see figure 2). According to the NCBI Genome project website (2), the genome of Chlorobium ferrooxidans is 2.53896 Mbp in length, contains 2158 proteins and 47 RNAs.
Cell Structure, Metabolism and Life Cycle
The bacterium (KoFox) while living in a coculture with a chemoheterotrophic partner, is presumed to obtain trace nutrients from the Geospirillium-like species. Bacteriochlorophyll c present in the organism makes this bacterium strictly phototrophic with an affinity for dim light excluding light of 740 nm in wavelength. This strian (KoFox) differs from all Chlorobium strains in its lack of sulfide oxidation. Instead KoFox oxidizes ferrous iron to ferric iron hydroxides and use a combination of iron and sunlight to reduce carbon dioxide for making organic compounds. Hydrogen and acetate have been shown to increase the cell yield of the strain KoFox in the presence of ferrous iron. Hydrogen has also been shown to be used as a sole electron source. Oxidation of ferrous iron by KoFox in the presence of KoFum is coupled to biomass formation from Carbon dioxide according to the equation: 17 FeCO3 + 29 H2O --> 17 FE(OH)3 + <C4H7O3> + 13 CO2 (1).
Ecology and Pathogenesis
The bacterium discussed here lives in a co-culture which seems to suggest a symbiosis. The description according to Heising et. al explained that the iron oxidizing strain (KoFox) was unable to be seperated its Geospirillum-like chemoheterotrophic partner strian (KoFum). Compared to other green phototrophs, strain KoFox behaves atypically in not utilizing sulfide as a sole electron source. Oxidation of ferrous iron by phototrophic bacteria is a relatively novel phenomenon that has typically only been observed with phototrophic purple sulfur or non-sulfur bacteria (3,4,5). Oxidation of ferrous iron by Phototrophic bacteria has had implications in our understanding of early earth evolution. Banded iron formations (BIF's) (FIGURE 4) are Precambrian sedimentary deposits that generally consist of alternating layers of iron minerals and silica. The question of when oxygen evolved on the planet and whether it was present in sufficient concentrations to be responsible for the deposition of these BIF's is still under debate. The finding of Chlorobium ferrooxidans and the fact that ferrous iron can be oxidized by a green anoxygenic phototrophic bacteria may add to further speculations on the question of how banded iron formations originated; since the green phototrophs represent a seperate group within the Bacteria domain, this may be an indication that phototrophic ferrous iron oxidation was a widespread metabolic capacity in an early phase of evolution (1,6).
References
(1) Heising, S., Richter, L., Ludwig, W., and Schink, B. 1999. Chlorobium ferrooxidans sp. nov., a phototrophic green sulfur bacterium that oxidizes ferrous iron in coculture with a “Geospirillum” sp. strain. Arch Microbiol. 172:116-124.]
(2) NCBI Genome Project [1]
(3) Widdel, F. Schnell, S., Heising, S., Ehrenreich, A., Assmus, B., Schink, B. 1993. Ferrous iron oxidation by anoxygenic phototrophic bacteria. Nature. 362:834-836.
(4) Ehrenreich, A. and Widdel, F. 1994. Anaerobic oxidation of ferrous iron by purple bacteria, a new type of phototrophic metabolism. Appl. Environ. Microbiol. 60:4517-4526.
(5) Heising S. and Schink, B. 1998. Phototrophic oxidation of ferrous iron by a Rhodmicrobium vannielii strain. Microbiology. 144:2260-2269.
(6) Kappler, A., Pasquero, C., Konhauser, K. O., and Newman, D. K. 2005. Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria. Geology. 33:865-868.
(7) Beatty, J.T.; Overmann, J.; Lince, M.T.; Mansket, A.K.; Lang, A.S.; Blankenship, R.E.; Van Dover, C.L.; Martinson, T.A.; Plumley, F.G. “ An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent”. PNAS June 28, 2005 vol. 102 no. 26 9306-9310
(8) www.britannica.com
Author
Page authored by Paul Giordano and Apram Ghuman, students of Prof. Jay Lennon at Michigan State University.
