The Ecological Context of Microbial Processes

Consider any habitat that interests you: terrestrial or aquatic; forest, grassland, lake sediments, or deep oceanic muds. Are these sites geochemically uniform and homogeneous, or are they spatially and temporally heterogeneous? Consider the physiological impact of gradients and spatial heterogeneity on microbially mediated physiological processes.

Spatial heterogeneity is widely accepted to occur in many habitats. Figure 1 shows the aerobic/anaerobic interface for a soil aggregate. Atmospheric oxygen is

Aerobic Microaerophilic

Anaerobic Microaerophilic Aerobic

Aerobic Microaerophilic

Anaerobic Microaerophilic Aerobic

Figure 1 Cross-sectional map of O2 concentrations in a soil particle. The numbers on the map are O2 concentrations, and the color of each zone represents a different microenvironment. Note: Atmospheric oxygen concentration is 21%.

available to microorganisms for respiratory processes (see below) at the surface of the soil aggregate. But, the rate of oxygen diffusion often fails to match its rate of utilization by microorganisms. Therefore, zones of oxygen depletion develop. In the absence of oxygen, microorganisms endowed with anaerobic physiologies can flourish.

Of the many millions of microorganisms found in each cubic centimeter of soil, sediment, and water, there are thousands of species, each with complex genomes conferring the potential to carry out a variety of biogeochemical processes. Many are linked to oxygen. Many are linked to other geochemical resources (especially, alternative electron acceptors described below). These processes are reflected in the stratification of physicochemical parameters around the aerobic and anaerobic interfaces that occur in ecosystems (e.g., soil aggregates; Figure 1). These spatially and temporally dynamic interfaces both govern and respond to the microbial respiration processes that occur. Moreover, waste products of microbial processes in one set of physiological conditions are valuable physiological assets (i.e., energy sources or final electron acceptors) in another. Thus, there is a constant geochemi-cal and physiological dialog between the microbial populations that inhabit adjacent habitats at anaerobic/ aerobic boundaries.

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