Because the need for carbon, energy, and electrons is so important, biologists use specific terms to define how these requirements are fulfilled. We have already seen that microorganisms can be classified as either heterotrophs or autotrophs with respect to their preferred source of carbon (table).
There are only two sources of energy available to organisms:
- light energy,
- the energy derived from oxidizing organic or inorganic molecules.
Sources of Carbon, Energy, and Electrons
|Autotrophs||CO2 sole or principal biosynthetic carbon source|
|Heterotrophs||Reduced, preformed, organic molecules from other organisms|
|Chemotrophs||Oxidation of organic or inorganic compounds|
|Lithotrophs||Reduced inorganic molecules|
Phototrophs use light as their energy source; chemotrophs obtain energy from the oxidation of chemical compounds (either organic or inorganic).
Microorganisms also have only two sources for electrons. Lithotrophs (i.e., “rock-eaters”) use reduced inorganic substances as their electron source, whereas organotrophs extract electrons from reduced organic compounds.
Despite the great metabolic diversity seen in microorganisms, most may be placed in one of five nutritional classes based on their primary sources of carbon, energy, and electrons (table). The majority of microorganisms thus far studied are either photolithographic autotrophs or chemoorganotrophic heterotrophs.
Photolithotrophic autotrophs (often called photoautotrophs or photolithoautotrophs) use light energy and have CO2 as their carbon source. Photosynthetic protists and cyanobacteria employ water as the electron donor and release oxygen (figure).
Other photolithoautotrophs, such as the purple and green sulfur bacteria (figure ), cannot oxidize water but extract electrons from inorganic donors like hydrogen, hydrogen sulfide, and elemental sulfur.
Chemoorganotrophic heterotrophs(often called chemoheterotrophs, chemoorganoheterotrophs, or just heterotrophs) use organic compounds as sources of energy, hydrogen, electrons, and carbon. Frequently the same organic nutrient will satisfy all these requirements.
Essentially all pathogenic microorganisms are chemoheterotrophs. The other nutritional classes have fewer known microorganisms but often are very important ecologically. Some photosynthetic bacteria (purple and green bacteria) use organic matter as their electron donor and carbon source.
These photoorganotrophic heterotrophs (photoorganoheterotrophs) are common inhabitants of polluted lakes and streams. Some of these bacteria also can grow as photoautotrophs with molecular hydrogen as an electron donor.
Chemolithotrophic autotrophs (chemolithoautotrophs), oxidize reduced inorganic compounds such as iron, nitrogen, or sulfur molecules to derive both energy and electrons for biosynthesis (figure).
Carbon dioxide is the carbon source. Chemolithoheterotrophs, also known as mixotrophs (figure), use reduced inorganic molecules as their energy and electron source, but derive their carbon from organic sources.
Chemolithotrophs contribute greatly to the chemical transformations of elements (e.g., the conversion of ammonia to nitrate or sulfur to sulfate) that continually occur in ecosystems.
Although a particular species usually belongs in only one of the nutritional classes, some show great metabolic flexibility and alter their metabolic patterns in response to environmental changes. For example, many purple non-sulfur bacteria act as photoorganotrophic heterotrophs in the absence of oxygen but oxidize organic molecules and function chemoorganotrophically at normal oxygen levels.
When oxygen is low, photosynthesis and chemoorganotrophic metabolism may function simultaneously.
This sort of flexibility seems complex and confusing, yet it gives these microbes a definite advantage if environmental conditions frequently change.