Redox reactions in the geochemical environment, as discussed in previous chapters (Chapters 7 and 17), are commonly in disequilibrium at low temperature, their progress described by kinetic rate laws. The reactions may proceed in solution homogeneously or be catalyzed on the surface of minerals or organic matter. In a great many cases, however, they are promoted by the enzymes of the ambient microbial community.

In this chapter, we consider how the microbial community catalyzes redox reactions, perhaps changing in size and composition as it does. The kinetics of such reactions are of special interest, because of the close relationship between geochemical conditions and microbial ecology. The microbes promote reactions that change geochemical conditions, many times significantly, and the geochemistry controls the nature of the microbial community that can exist in a given environment.

From the geochemist's perspective, a microbe can be thought of as a selfreplicating bundle of enzymes. Microbes use their collections of enzymes to catalyze redox reactions, harvesting some of the energy released for their own purposes. Since microbial growth increases the ability of the community to catalyze redox reactions, and catalyzing the reactions provides microbes the energy they need to grow, a microbially mediated reaction is by nature autocatalytic.

Microbial respiration and fermentation

Chemosynthetic microorganisms derive the energy they need to live and grow from chemical species in their environment, reaping the benefits of the redox disequilibrium characteristic of geochemical environments (see Chapter 7).