Protozoa are an extremely diverse range of single celled organisms which have representatives living in nearly every habitat. They obtain their energy from preformed organic sources and so are heterotrophs This can be in the form of soluble organic molecules, bacteria or detritus (Patterson, 2009). However, some may possess chloroplasts and so obtain some of their energy from the sun.
Estimates have put the number of protozoa species at around 30,000 with more than 250 species of protozoa isolated from soils, with sometimes as many as 40 or 50 of such groups occur in a single sample of soil. They generally thrive best in moist well-drained soils and are most numerous in surface horizons, the biomass ranging from 20 to 200 kg/ha. The main groups of protozoa can be distinguished by their appearance (or morphology) and how they feed: ciliates, flagellates and amoebae. Most ciliates are about 5-1000 µm in length and can be either be free living and are good swimmers or be attached via a stalk to particles such as soil or plant debris. They may even form small colonies of about a dozen individuals. They have rows of cilia (very short, fine hairs) which allow them to move and to feed. The cilia beat creating currents which draw their prey (bacteria or organic matter debris) to their mouths. The flagellates have between 1-8 flagella which are elongated hairs which whip back and forth gathering food and providing motility. The flagella may appear singly or in pairs usually at one end of the cell. The amoebae are probably the most familiar of the protozoa. They feed and move by temporary extensions to their cell body (pseudopodia). Some of them, the testate amoeba, excrete or use material from the environment to form a hard shell which protects them against predation and the environment.
Although they prefer moist conditions, under drought some can form cysts which are protective structures enabling them to survive adverse conditions.
Protozoa are usually restricted to the top 15-20 cm of soil and being larger than bacteria or fungi they tend to inhabit the spaces around soil pores rather than in the small pores inhabited by bacteria. Many protozoa are found in freshwater where they are free to move. In soil, their movement is much more restricted and tend to be smaller in size than their fresh water counterparts. They feed largely on bacteria and fungal spores and so are an important part in recycling much of the nutrient associated with bacteria. In soil, numbers of protozoa tend to increase around nutrient hotspots such as the rhizosphere or where nutrients have been made readily available (Krivtsov et al., 2007). Some flagellated species posses chloroplasts providing some nutrients to the cell through photosynthesis.
Protozoa interact with other soil microflora, such as soil nematodes. In one study, the amoeba Acanthamoeba castellanii has been found too produce excretions which can repel the nematode Caenorhabditis elegans (Neidig et al., 2010). Conversely, the nematode was found to produce enzymes which could inhibit the amoeba.
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