In Chapter 6, we saw that organisms have two sources of energy: the sun (phototrophs) and chemical compounds (chemotrophs). Organisms also have two sources of the carbon needed for growth and other functions: inorganic molecules like CO2 (autotrophs) and organic molecules like glucose (heterotrophs). This means that there are four ways that organisms acquire the energy and carbon they need (see Fig. 6.1).
Up to this point, we have considered only two of these ways. Plants, algae, and cyanobacteria are photoautotrophs, gaining energy from the sunlight and carbon from CO2, whereas animals, fungi, and many prokaryotes are chemoheterotrophs, gaining both energy and carbon from organic molecules taken up from the environment. Among the Bacteria and Archaea, additional metabolisms are possible. Some use the energy from sunlight to make ATP, just as plants do, but rather than reducing CO2 to make their own organic molecules, they rely on organic molecules obtained from the environment as the source of carbon for growth and other vital functions. These organisms are photoheterotrophs.
Photoheterotrophy can be advantageous in environments rich in dissolved organic compounds. It allows organisms to use all their absorbed light energy to make ATP while directing all absorbed organic molecules toward growth and reproduction. Photoheterotrophic bacteria have a single photosystem and so live where oxygen is limited or absent. The purple layer of a microbial mat very likely contains heliobacteria or green nonsulfur bacteria that take advantage of the large amounts of organic matter generated by photoautotrophs in the mat.
Quick Check 3 How do photoautotrophs such as cyanobacteria differ from photoheterotrophs such as heliobacteria?
Cyanoabcteria use much of the ATP they generate to drive the reduction of CO2 into organic molecules used for growth and reproduction. In photoheterotrophs, the organic molecules needed for growth are taken in from the environment, leaving ATP available for other uses.