ABSTRACT
Natural environments are inhabited by a great diversity of microorganisms which encompass a complete spectrum of physiological and nutritional types, ranging from obligate aerobes to exacting anaerobes, and from autotrophs to heterotrophs. The majority of natural microbial ecosystems are heterogeneous, and microorganisms generally develop in spatially organized physico-chemical gradients. Chemostats can be used at low dilution rates and low nutrient concentrations to enrich microorganisms which never become dominant in nutrient-rich batch-culture experiments. The bidirectional system can be used to study microbial interactions such as ammensalism, commensalism, mutualism, and competition. A primary requirement of laboratory systems is to incorporate features which bear a close resemblance to the properties of the natural habitat. The microbial populations under investigation are retained within semipermeable membranes, for example, dialysis tubing, and are open to the dissolved nutrients of the surrounding environment. Equally most natural environments are spatially and temporally heterogeneous, and the emphasis has been on homogenous laboratory methods of culture.
