Types of Microbial Metabolism
All microbial metabolisms can be arranged according to three principles:
1. How the organism obtains carbon for synthesising cell mass:
- autotrophic – carbon is obtained from carbon dioxide (CO2)
- heterotrophic – carbon is obtained from organic compounds
- mixotrophic – carbon is obtained from both organic compounds and by fixing carbon dioxide
2. How the organism obtains reducing equivalents used either in energy conservation or in biosynthetic reactions:
- lithotrophic – reducing equivalents are obtained from inorganic compounds
- organotrophic – reducing equivalents are obtained from organic compounds
3. How the organism obtains energy for living and growing:
- chemotrophic – energy is obtained from external chemical compounds
- phototrophic – energy is obtained from light
In practice, these terms are almost freely combined. Typical examples are as follows:
- chemolithoautotrophs obtain energy from the oxidation of inorganic compounds and carbon from the fixation of carbon dioxide. Examples: Nitrifying bacteria, Sulfur-oxidizing bacteria, Iron-oxidizing bacteria, Knallgas-bacteria
- photolithoautotrophs obtain energy from light and carbon from the fixation of carbon dioxide, using reducing equivalents from inorganic compounds. Examples: Cyanobacteria (water (H2O) as reducing equivalent donor), Chlorobiaceae, Chromatiaceae (hydrogen sulfide (H2S) as reducing equivalent donor), Chloroflexus (hydrogen (H2) as reducing equivalent donor)
- chemolithoheterotrophs obtain energy from the oxidation of inorganic compounds, but cannot fix carbon dioxide (CO2). Examples: some Thiobacilus, some Beggiatoa, some Nitrobacter spp., Wolinella (with H2 as reducing equivalent donor), some Knallgas-bacteria, some sulfate-reducing bacteria
- chemoorganoheterotrophs obtain energy, carbon, and reducing equivalents for biosynthetic reactions from organic compounds. Examples: most bacteria, e. g. Escherichia coli, Bacillus spp., Actinobacteria
- photoorganoheterotrophs obtain energy from light, carbon and reducing equivalents for biosynthetic reactions from organic compounds. Some species are strictly heterotrophic, many others can also fix carbon dioxide and are mixotrophic. Examples: Rhodobacter, Rhodopseudomonas, Rhodospirillum, Rhodomicrobium, Rhodocyclus, Heliobacterium, Chloroflexus (alternatively to photolithoautotrophy with hydrogen)
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