What They Are and A Few Interesting Facts
Protozoa are tiny single-celled animals that mainly feed on bacteria (think of them as little grazers), although some eat other protozoa and organic matter. While protozoa are many times larger than bacteria, you still need a microscope to see them. A pinch of soil can contain thousands.
Protozoa are classified into three groups based on structural peculiarities. Ciliates are the biggest and use many hair-like cilia to scoot through soil and water. Amoebae are small (relatively), amorphous, and use temporary feet to move around. Flagellates are the smallest and swim using a few whip-like flagella. All protozoa need water to move through soil, however, they only need a thin film surrounding the soil particles to get around. Believe it or not, they can be quite active even in very parched desert soils.
Protozoa are found in soils everywhere: even in very dry desert. However, they are most abundant near plant roots, because that's where both bacteria and organic matter (i.e., food) are concentrated in the soil.
Why They Are Important
Protozoa play an important role in the structure of the soil food web. Protozoa eat bacteria and release nitrogen and other nutrients in their waste. Since protozoa are concentrated near plant roots, the plant can benefit from this supply of nutrients. Protozoa can stimulate the rate of decomposition by maximizing bacterial activity.
Protozoa are in turn consumed by nematodes and microarthropods. Remember what we said about missing parts? Think of what would happen if there were no protozoa.
Okay, so they aren't all good guys. Some protozoa attack roots and cause disease in rangeland plants. However, other protozoa feed on root pathogens, thus reducing plant disease.
Clarholm, Marianne. 1985. Interactions of bacteria, protozoa and plants leading to mineralization of soil nitrogen. Soil Biol. Biochem. 17(2)181-187.
Ingham, Elaine R. 1998. The soil biology primer, soil protozoa. USDA, Natural Resources Conservation Service, Soil Quality Institute.
Whitford, Walter G. 1996. The importance of the biodiversity of soil biota in arid ecosystems. Biodiversity and Conservation 5: 185-195.
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