Microorganisms, as they interact with each other and with other organisms in biogeochemical cycling, also are influenced by their immediate physical environment, whether this might be soil, water, the deep marine environment, or a plant or animal host. It is important to consider the specific environments where microorganisms interact with each other, other organisms, and the physical environment.
The specific physical location of a microorganism is its microenvironment. In this physical microenvironment, the flux of required oxidants, reductants, and nutrients to the actual location of the microorganism can be limited. At the same time, waste products may not be able to diffuse away from the microorganism at rates sufficient to avoid growth inhibition by high waste product concentrations. These fluxes and gradients create a unique niche, which includes the microorganism, its physical habitat, the time of resource use, and the resources available for microbial growth and function (figure 28.26).
This physically structured environment also can limit the predatory activities of protozoa. If the microenvironment has pores with diameters of 3 to 6 ^m, it will protect bacteria in the pores from predation, while allowing diffusion of nutrients and waste products. If the pores are larger, perhaps greater than 6 ^m in diameter, protozoa may be able to feed on the bacteria. It is important to emphasize that microorganisms can create their own microenvironments and niches.
620 Chapter 28 Microorganism Interactions and Microbial Ecology
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