Geochemists increasingly find a need to better understand the distribution of microbial life within the geosphere, and the interaction of the communities of microbes there with the fluids and minerals they contact. How do geochemical conditions determine where microbial communities develop, and what groups of microbes they contain? And how do those communities affect the geochemistry of their environments?

In many cases, microbial life in nature develops into zones within which communities are dominated by one or a few functional groups, such as aerobes, sulfate reducers, or methanogens. Distinct zoning is characteristic, for example, of microbial mats (Konhauser, 2007), hot springs (Fouke et al., 2003), marine sediments and freshwater muds (Berner, 1980), contaminated aquifers (Bekins et al., 1999), and pristine groundwater flows (Chapelle and Lovley, 1992). Communities develop as well in laboratory experiments, when microbes are cultivated in pure or mixed culture.

In this chapter, we consider how to construct quantitative models of the dynamics of microbial communities, building on our discussion of microbial kinetics in Chapter 18. In our modeling, we take care to account for how the ambient geochemistry controls microbial growth, and the effect of the growth on geochemical conditions.

Arsenate reduction by Bacillus arsenicoselenatis

Blum et al. (1998) isolated a bacterial strain Bacillus arsenicoselenatis from muds of Mono Lake, a hypersaline alkaline lake in northern California (see Section 24.2). Under anaerobic conditions in saline water, over an optimum pH range of 8.5–10, the strain can respire using As(V), or arsenate, as the electron acceptor, reducing it to As(III), arsenite.