Protozoans: Useful and Harmful Protozoans
you may have seen some of these unicellular organisms in the drop of pond water you observed under the microscope. They are traditionally looked upon as animals, because most protozoans are animal-like. 'Protozoa', in fact, means 'the first animal'. Even protozoans like Euglena, which contain chlorophyll and can photosynthesise, lack a cell wall.
The others are either saprophytic or parasitic. Some live in freshwater, some in salt water, and some in the soil. Many live in the bodies of animals and plants. Protozoans are the link in the food chain between unicellular algae and small aquatic organisms. In other words, they eat algae and are eaten by small fish and other organisms. They reproduce by binary fission.
Perhaps you remember that Amoeba engulfs food with the help of pseudopodia. It is the simplest of protozoans, found in the soil and freshwater bodies. Paramoecium is another freshwater protozoan found in ponds and ditches. It has hair-like projections, called cilia, which help it swim. They also help to direct food and water into the oral groove of this organism.
1. Protozoans serve as food for many small aquatic organisms. Zooplankton are tiny protozoans which live in the sea. They form the principal diet of blue whales, who gulp them in with sea water.
2. They are the ultimate decomposers in nature, as they feed on bacteria and fungi, which decompose dead organic matter. They are, thus, useful in the treatment of sewage.
3. Some protozoans live in the body of other organisms and help them. Termites, for example, have protozoans living in their body. The protozoans digest the cellulose in the wood eaten by termites and convert it into carbohydrates that the termites can use.
Some protozoans cause diseases. The protozoan Entamoeba histolytica causes amoebiasis (amoebic dysentery) in human beings, Giardia causes giardiasis (dysentery), while Plasmodium causes malaria. trypanosome, a parasitic protozoan which lives in the bloodstream of human beings, cattle and other animals, causes a dangerous disease called sleeping sickness.