The kinetics of microbially catalyzed reactions are of special interest because of the control the reactions exert of the redox state of laboratory experiments and the natural environment. A general description of microbial kinetics must address the requirement of thermodynamic consistency, so the kinetic laws apply equally well far from chemical equilibrium, and close to it. This chapter shows how to formulate thermodynamically consistent rate laws for microbial respiration and fermentation, the process of incorporating such laws into multicomponent chemical reaction models, and a fully worked example demonstrating how such models behave.

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 this chapter we use multicomponent chemical reaction modeling combined with thermodynamically consistent kinetic expressions to explore how microbially catalyzed reactions proceed in the laboratory and in nature.

Any consideration of reaction processes in multicomponent chemical systems begins with a conceptual model of the setting of interest. This chapter describes how to develop a conceptual basis for constructing a geochemical reaction model and discusses the uncertainties inherent in evaluating such a model.

Using a historical sketch, this chapter traces the development and application of numerical methods for predicting speciation in multicomponent chemical systems. Discussion begins with attempts to predict the thrust provided by rocket fuels and continues through the application of advanced algorithms today to solve problems of scientific, practical, and societal importance in geochemistry and related fields.