Organisms can be identified according to the source of carbon they use for metabolism as well as their energy source. The prefixes auto- (“self”) and hetero- (“other”) refer to the origins of the carbon sources various organisms can use. Organisms that convert inorganic carbon dioxide (CO2) into organic carbon compounds are autotrophs. Plants and cyanobacteria are well-known examples of autotrophs. Conversely, heterotrophs rely on more complex organic carbon compounds as nutrients; these are provided to them initially by autotrophs. Many organisms, ranging from humans to many prokaryotes, including the well-studied Escherichia coli, are heterotrophic. All pathogens are heterotrophic because their carbon source is their host.
Organisms can also be identified by the energy source they use. All energy is derived from the transfer of electrons, but the source of electrons differs between various types of organisms. The prefixes photo- (“light”) and chemo- (“chemical”) refer to the energy sources that various organisms use. Those that get their energy for electron transfer from light are phototrophs, whereas chemotrophs obtain energy for electron transfer by breaking chemical bonds. There are two types of chemotrophs: organotrophs and lithotrophs. Organotrophs, including humans, fungi, and many prokaryotes, are chemotrophs that obtain energy from organic compounds. Lithotrophs (“litho” means “rock”) are chemotrophs that get energy from inorganic compounds, including hydrogen sulfide (H2S) and reduced iron. Lithotrophy is unique to the microbial world.
The strategies used to obtain both carbon and energy can be combined for the classification of organisms according to nutritional type. Most organisms are chemoheterotrophs because they use organic molecules as both their electron and carbon sources. Table 22.214.171.124.1 summarizes this and the other classifications.
|Classifications||Energy Source||Carbon Source||Examples|
|Chemotrophs||Chemoautotrophs||Chemical||Inorganic||Hydrogen-, sulfur-, iron-, nitrogen-, and carbon monoxide-oxidizing bacteria|
|Chemoheterotrophs||Chemical||Organic compounds||All animals, most fungi, protozoa, and bacteria|
|Phototrophs||Photoautotrophs||Light||Inorganic||All plants, algae, cyanobacteria, and green and purple sulfur bacteria|
|Photoheterotrophs||Light||Organic compounds||Green and purple nonsulfur bacteria, heliobacteria|