Free living protozoa are found in variety of habitats. The factor which influence the distribution and number of free living protozoa in a habitat are moisture, temperature, light, available nutrients, and other physical and chemical conditions.
The vegetative, or trophic stages of free living protozoa occur in every type of salt water, fresh water, sand, soil and decaying organic matter.
The association between these protozoa and any other organisms can differ in different ways. The term symbioticdescribes any type of coexistence between different organisms.
In commensalism the host is neither injured not benefited, but the commensal is benefited.
Ectocommensalism is often represented by protozoa which attach themselves to a host’s body.
Endocommensalism is the association when the protozoan is inside the host’s body. E.g. the protozoa that live in the lumen of the alimentary tract.
Mutalism occurs between some protozoa and their hosts. For example, certain flagellates are present in the gut of termites and digest the whole material eaten by the termite to a glycogenous substance which can be used by the host cells. If deprived of these flagellates, the termite dies; if the flagellates are removed from the termite gut, they too perish.
The Importance Of Protozoa
Protozoa serves as an important link in aquatic ecosystems. For example, in marine water, zooplankton are protozoa those feed on photosynthetic phytoplankton. They in turn become food for large marine organisms.
Role of protozoa in the treatment of sewage is more completely understood and appreciated. Biological sewage treatment involves both anaerobic digestion and/or aeration. Metopus, Saprodinim etc. Species of anaerobic protozoa play an important role in anaerobic sewage treatment, while those treatment steps requiring aeration and flocculation include the aerobic protozoa such as Bodo, Paramecium, Aspidisca etc.
In the treatment of industrial waste, where there is accumulation of nitrogen and phosphate waste, the settling tanks are illuminated to promote the growth of algae and protozoa. These protists remove the inorganic materials from the water for their own synthesis. Water quality is improved, and the autotrophs are skimmed from the water surface, dried, and may used as fertilizer.
Some protozoa causes disease in animals, including humans. Similar to the bacteria, some protozoa multiply within the host. Some live only as obligate parasites and produce chronic or acute diseases in humans. Some well known protozoan disease in humans are intestinal amoebiasis, African sleeping sickness, and malaria.
Reference: Microbiology Pelczar