Carbon And Energy Sources

The phylogenetic classification of prokaryotes described earlier in this chapter provides an indication of evolutionary relationships, which will have arisen, in part, through differences in physiological characteristics. As a consequence, molecular phylogeny often agrees with traditional classification based on combinations of

Photolithotrophic or photoautotrophic

Photoorganotrophic or photoheterotrophic

Chemolithotrophic or chemoautotrophic

Chemoorganotrophic or chemoheterotrophic

FIGURE 5.7 Physiological classification of soil bacteria in terms of their C and energy source (lithotrophs and organotrophs).

physiologies. Physiological classification is also valuable when considering the role of prokaryotes in soil processes. However, many physiological processes have evolved at different times in different organisms, and particular physiological characteristics may therefore be found in many genera and species. This is illustrated in Tables 5.1 and 5.2 and will be exemplified in greater detail in examples below.

Physiological classification of soil microorganisms is of great value to those interested in ecosystem functions as it provides one of the underpinning bases for considering ecological roles (Fig. 5.7). The first order of such a classification relates to the energy source of the microorganisms. Phototrophs use light and chemotrophs use chemical energy as their energy source. The second order usually relates to the C source. Autotrophs or lithotrophs use CO2, while organotrophs or heterotrophs use organic compounds. Cyanobacteria and green sulfur bacteria are examples of photoautotrophs, while the purple non-sulfur bacteria are photoheterotrophs. S- and Fe-oxidizing thiobacilli and nitrifiers (oxidizing reduced forms of N) are examples of chemoautotrophs, while pseudomonads and Rhizobium are examples of chemoheterotrophs.

Although the standard physiological classification based on energy/C source described above enables links to be made to potential ecosystem functions, there are exceptions to the above rules. For example, heterotrophic bacterial nitrifiers have a range of complex physiologies that cannot always be readily classified according to Fig. 5.7, not least because some appear to change their physiological strategy with time and with the availability of energy sources (Killham, 1986). This concept of physiological flexibility will be developed further below.

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