Predicting the rate of microbial respiration in geochemical environments

A new rate law of general form can be applied to predict rates of microbial respiration in various geochemical environments. Because it accounts for the energy available to a microbe, the rate law can be applied in over a spectrum of conditions, ranging from environments rich in chemical energy to those fully oligotrophic. Two sets of parameters, one related to thermodynamic constraints and the other set appearing in kinetic terms, need be determined to apply the new law. We show that the thermodynamic parameters can be estimated from knowledge of the details of a microbe's respiratory chain. The kinetic parameters are best determined by fitting the rate law to experimental observations. We find that for respiration characterized by a strong thermodynamic driving force, such as many types of aerobic respiration and denitrification, the kinetic controls on respiration rate almost invariably dominate. In such cases, the thermodynamic control can safely be ignored. For respiration utilizing less powerful thermodynamic driving forces, such as sulfate reduction and reductive methanogenesis, the thermodynamic control on respiration rate can be dominant. It is of critical importance in these cases to account for thermodynamic as well as kinetic controls when calculating respiration rates. Taking as examples methanogenesis and sulfate reduction in hydrothermal fluids, we show how the new rate law can be applied over a spectrum of conditions and energy availability.