The Princeton Guide to Ecology

Laboratory studies of microbial populations have informed many early population models, and there is now an enormous literature describing the dynamics of microbial populations under controlled conditions. This chapter outlines the major developments in the study of microbial populations, from the simplest-case scenario of a single population feeding on a single substrate to situations where there are interactions among multiple populations. Currently, the greatest difficulty is in extrapolating the results of the laboratory studies to understand natural microbial communities.

batch/continuous culture. In batch culture, strains are grown for a fixed period (e.g., a few days) before being transferred to fresh medium. In continuous culture, there is a continuous input of nutrients and output of spent medium, resulting in constant environmental conditions. The rate at which nutrients are input (and output) into the microcosm is called the dilution rate. Continuous culture experiments are conducted in a chemostat.

cometabolism. Simultaneous metabolism of two substrates such that the metabolism of one substrate occurs only in the presence of a second substrate.

culturability. The ability to grow strains in the laboratory in pure culture. For example, it is estimated that as few as 1% of bacteria species are culturable. Surveys of microbial communities therefore often rely on culture-independent techniques. For example, it is possible to construct a clone library of amplified DNA sequences to characterize a particular microbial community.

diauxie. Literally “double growth”; diauxie describes the way in which bacterial populations feed on mixtures of substrates (usually sugars). Diauxic growth is characterized by an initial growth phase, followed by a lag where the strain switches from the first to the second substrate, which is in turn followed by a second growth phase as the second substrate is utilized.

microbe. Here defined as an organism that is small (<1 mm) and unicellular. The current discussion is also restricted to free-living microbes (i.e., excluding parasites).

Monod equation. Named after the microbiologist Jacques Monod, the equation describes the relationship between substrate concentration and the growth rate of a microbial population. The form of the equation is equivalent to the Michaelis-Menten equation of enzyme kinetics.

syntrophy. A mutualistic interaction where two strains can utilize a substrate that neither could utilize when the other is absent.

yield. The number of microbial cells produced per unit of substrate.

There is a popular conception of the microbial world as an unseen host of germs hiding in unwashed corners, intent on infecting people and crops, contaminating water and food. However, microbial populations are intrinsic to the ecology of animal and plant communities and play a vital role in the flow of nutrients and energy in ecosystems. In aquatic ecosystems, phytoplankton are often the principal source of primary production, thereby controlling the quantity of organic material available to higher trophic levels. Bacteria and fungi control the rate of decomposition in most ecosystems and, therefore, the amount of inorganic matter

-239-