Contamination of the aquatic environments by pollutants
has become a very serious problem in the recent years. In lakes
and rivers heavy metals are discharged from final effluents of
waste treatment plants, and are common pollutants of sewage,
particularly where there is input of industrial waste. Heavy
metals are toxic to most microorganisms at certain concentrations.
The characteristics and intensity of damage depend on
the nature and level of the metal, i.e., there are differences
between the effects of essential and non-essential metals
(Irato and Piccinni, 1996). Sediment and water properties, such
as pH and organic matter content will also affect the bioavailability
of the metals. It has been suggested that heavy metals
may block enzyme systems of microorganisms or interfere
with some essential cellular metabolites of bacteria and protozoa