The activity of microorganisms in natural samples can be assessed very sensitively using radioisotopes or microelectrodes. In most cases these measurements are of the net activity of a microbial guild or guilds rather than of a population of a single species. Section 16.8 Isotopic fractionation can yield information on the biological origin of various substances. Fractionation is a result of the activity of certain enzymes that can discriminate, usually against the heavier form of an element, when acting on their substrates. The soil is a complex habitat with numerous microenvironments and niches.
Microorganisms are present in the soil primarily attached to soil particles.
In aquatic ecosystems phototrophic microorganisms are usually the main primary producers. Most of the organic matter produced is consumed by bacteria, which can lead to depletion of oxygen in the environment. BOD is a measure of the oxygen-consuming properties of a water sample. The deep sea is a cold, dark habitat where high hydrostatic pressure, and low nutrient availability prevails. Barophiles grow best under pressure, and extreme barophiles, obtained from the greatest depth, require high pressures for growth.