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Green bacteria

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Figure 10.21 Schematic of light absorption characteristics (absorption vs. wavelength in nm) of phototrophic bacteria. (Based on Prescott et al., 1999.)

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Figure 10.21 Schematic of light absorption characteristics (absorption vs. wavelength in nm) of phototrophic bacteria. (Based on Prescott et al., 1999.)

Chemolithotrophs The Proteobacteria include a number (but not all) of the organisms able to obtain energy from the oxidation of reduced inorganic compounds, including hydrogen and forms of nitrogen, sulfur, and iron (Table 10.6). Most of these organisms are also autotrophic (getting their carbon from carbon dioxide), and aerobic, although some are also able to use nitrate as an electron acceptor. They also tend to be slow growing, probably because the amount of energy available from oxidizing most of these compounds is relatively small.

The ability to utilize hydrogen is actually quite widely distributed. Examples of hydrogen-oxidizing Proteobacteria include some species of Pseudomonas and Alcaligenes. Most such bacteria are able to utilize organic substrates as well (unlike most other che-molithotrophs). Some (e.g., Xanthobacter) can fix nitrogen gas. Since hydrogen is typically produced under anaerobic conditions, and because the enzyme (hydrogenase) involved in its oxidation is oxygen sensitive, most hydrogen oxidizers prefer lower oxygen conditions (i.e., 5 to 10% O2 rather than the 21% of air). Some hydrogen oxidizers are also able to grow on carbon monoxide, and such carboxydotrophic bacteria probably play a major role in elimination of this toxic gas.

TABLE 10.6 Examples of Chemolithotrophic Proteobacteria
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