Since it is a major constituent of cell materials, all organisms need a source of carbon. Heterotrophs (including fungi, protozoans, and most bacteria) require organic carbon, whereas autotrophs (algae and some bacteria) consume inorganic carbon (carbon dioxide or bicarbonate). These terms can be combined with those above to refer, for example, to chemolithoautotrophs (Table 10.2). The term heterotrophy is also used more loosely to refer to chemoorganotrophy, since the two categories usually coincide (microbes using organic carbon as an energy source also use it for a carbon source). However, some lithoautotrophs are able to utilize amino acids, hence getting some of their carbon from organic sources, and a few lithotrophs, such as most species of the sulfur-oxidizer
TABLE 10.2 Categorization of Organisms by Energy and Carbon Source, with Examples
Energy Source Used
Organotroph Phototroph (Organic Carbon)
Lithotroph (Inorganic Compounds)
Chemolithoautotroph or lithoautotroph: nitrifying bacteria, Thiobacillus some cyanobacteria animals, protozoans, fungi, Pseudomonas, Escherichia
Chemolithoautotroph or lithoautotroph: nitrifying bacteria, Thiobacillus
Chemolithoheterotroph or mixotroph:a most Beggiatoa some cyanobacteria animals, protozoans, fungi, Pseudomonas, Escherichia aSome primarily chemolithoautotrophic microbes can also utilize amino acids.
Beggiatoa, get their carbon from small organic molecules. Such organisms are often referred to as mixotrophs.
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