5. Ecology of Microorganisms in Freshwater

Freshwaters are defined as natural waters containing less than 1,000 mg per liter of dissolved solids, most often salt. Globally, freshwaters are scarce commodities and make up only 0.009% of the earth’s total water. Although they generate only about 3% of the earth’s total primary biological productivity, they contain about 40% of the world’s known fish species. Natural freshwaters are classifiable into atmospheric, surface, and underground waters each type having a unique microbial ecology. Atmospheric waters lose their microorganisms as they fall as rain or snow. Surface freshwaters are found in rivers and lakes, and contain large and diverse groups of microorganisms. Using molecular methods such as 16S rRNA analysis, which enables the study of unculturable microorganisms, new information regarding freshwater microbial ecology has emerged in recent times: There are more phylogenetic groups of bacteria than are observed by cultural methods; there is a unique and distinct bacterial group, which can be termed “typical freshwater bacteria”; contrary to previous knowledge when aquatic bacteria were thought to be mostly Gram-negative bacteria, Gram-positive bacteria are in fact abundant in freshwaters; finally, marine-freshwater transitional populations exist in coastal waters. Ground waters suffer contamination from chemicals and less from microorganisms; the deeper the groundwater, the less likely it is to contain microorganisms, which are filtered away by soil.