Protozoa are microscopic single cell animals. They utilise solid substances and bacteria as a food source. They can only function aerobically, and in a stream which contains little organic degradable matter they can become a predominant microbial type. They play an important part in sewage treatment where they remove free-swimming bacteria and help to produce a clear effluent.
In an aquatic environment, there are three main types of protozoa:
Those which have an amoeboid structure and move by means of an extruded pseudopod or false foot (e.g. Amoeba).
Those which move by utilising a flagellum or whip-like tail.
Those which move and gather food using hair-like projections (cilia); these can be free-swimming (e.g. Paramecium) or held stationary by means of a stalk (e.g. Vorticella campanula).
Minute multicellular animals which also feed on debris and bacteria are the rotifers (e.g. Keratella), which also play an important part in sewage treatment.