A short survey of the characteristics of ecological niches in aquatic environments, settled by microbes (procaryotes), is given. Cell density and substrate concentration for heterotrophic species are strongly correlated in natural water masses (oceans, lakes). The low concentration in water of conventional substrates for organotrophic microbes (order of magnitude 10-9 Mol/l for amino acids, carbohydrates, etc.) affects the kinetics of the substrate uptake and the growth rate of water bacteria. It must be assumed that substrate receptor activities at cell surfaces of so-called oligocarbophilic bacteria are much higher than with saprophytes from more concentrated media (e.g. terrestrial strains).