2.4 Metabolic Groups

Classification of prokaryotes in terms of metabolic capacity and physiology is widely used as a framework to describe specific roles of microbes in ecosystem function (Amend and Teske, 2005; Anderson et al., 2006; Shirokova and Ferris, 2013). Groups defined in this way may include microorganisms that are unrelated by phylogenetic criteria used in taxonomy. For example, anaerobic respiration by sulfate reduction is carried out by prokaryotic microorganisms from several different phylogenetic lines, including various phyla and genera in the domain of Bacteria as well as some species of Archaea. Despite their major phylogenetic differences, sulfate-reducing prokaryotes represent a distinct metabolic (phenotypic) group that plays an important ecological role in the biogeochemical cycling of sulfur and carbon. Apart from ecophysiological categorization based on terminal electron acceptors, two additional criteria are used to define metabolic groups of prokaryotes: the source of energy used to fuel cell metabolism and the process used to acquire carbon to support cell growth.

Prokaryotic microorganisms capture energy from either light-driven reactions (phototrophs) or the oxidation of reduced chemical compounds (chemotrophs), as shown in Figure 10. While the availability of light energy from the sun restricts phototrophs to surface environments, chemotrophs thrive even in the complete darkness found in deep ocean and underground habitats. Among chemotrophs, those that oxidize reduced organic substances are known as organotrophs, whereas lithotrophs oxidize inorganics such as ammonium (NH₄⁺), sulfide (S^2-), or ferrous iron (Fe²⁺). When it comes to the acquisition of carbon, autotrophs rely on the reduction of carbon dioxide to produce cellular organic material. In contrast, heterotrophs (often taken to be synonymous with organotrophs) utilize pre-existing organic molecules to foster cell growth and division. Photoheterotrophs rely on light to generate energy and organic matter as a source of carbon. These various terms give practical ecophysiological definitions for the different metabolic groups of prokaryotic microorganisms found in natural systems.

Figure 10 – Functional groups of microorganisms based on energy and carbon sources.