ABSTRACT
During the past decade, an increasing number of ecological studies have considered the complexity of freshwater ecosystems. One major outcome of these studies has been an accelerated interest in the role of heterotrophic microorganisms (particularly bacteria) in the functioning of aquatic environments and the processes by which organic matter is made available to them (1-4). These heterotrophic microorganisms are the key trophic level at which the metabolism of the whole ecosystem is affected, i.e., organic matter decomposition, nutrient cycling, and structure of aquatic food webs. The demonstration of the importance of heterotrophic bacteria as a particulate carbon source for higher trophic levels and a major respiratory sink has created a renewed interest in the production and utilization of organic substrates by these microorganisms.
