ABSTRACT
This chapter describes the available methods for assessing rates of metabolic activity in the subsurface and understanding of the in situ activities of important functional groups of subsurface microbes. To understand the role of microorganisms as mediators of geochemical change in subsurface environments, one should assess the in situ metabolic activity for each of the major microbial functional groups in that environment. Estimates of subsurface microbial activities based on retrieved materials typically involve laboratory experiments with recovered ground waters or sediments. In most natural environments, the majority of microorganisms in subsurface environments are nonculturable. Once the flow unit and path are identified, analysis of known energy sources, electron donors, electron acceptors, end products, and their sinks can be used to identify predominant metabolic pathways used by resident microorganisms. Microorganisms in the unsaturated zones of arid and semiarid regions exist in extremely low nutrient flux environments.
