Soil Microorganism – Protozoa
These are unicellular, eukaryotic, colourless, and animal like organisms (Animal kingdom). They are larger than bacteria and size varying from few microns to a few centimeters. Their population in arable soil ranges from l0,000 to 1,00,000 per gram of soil and are abundant in surface soil. They can withstand adverse soil conditions as they are characterized by "cyst stage" in their life cycle. Except few genera which reproduce sexually by fusion of cells, rest of them reproduces asexually by fission / binary fission. Most of the soil protozoa are motile by flagella or cilia or pseudopodia as locomotors organs. Depending upon the type of appendages provided for locomotion, protozoa are
Sporophora (not common Inhabitants of soil)
Class-Rhizopoda: Consists protozoa without appendages usually have naked protoplasm without cell-wall, pseudopodia as temporary locomotory organs are present some times. Important genera are Amoeba, Biomyxa, Euglypha, etc.
Class Mastigophora: Belongs flagellated protozoa, which are predominant in soil. Important genera are: Allention, Bodo, Cercobodo, Cercomonas, Entosiphon Spiromonas, Spongomions and Testramitus. Many members are saprophytic and some posses chlorophyll and are autotrophic in nature. In this respect, they resemble unicellular algae and hence are known as "Phytoflagellates".
The soil protozoa belonging to the class ciliate / ciliophora are characterized by the presence of cilia (short hair-like appendages) around their body, which helps in locomotion. The important soil inhabitants of this class are Colpidium, Colpoda, Balantiophorus, Gastrostyla, Halteria, Uroleptus, Vortiicella, Pleurotricha etc.
Protozoa are abundant in the upper layer (15 cm) of soil. Organic manures protozoa. Soil moisture, aeration, temperature and PH are the important factors affecting soil protozoa.
Function / Role of Protozoa
1. Most of protozoans derive their nutrition by feeding or ingesting soil bacteria belonging to the genera Enterobacter, Agrobacterium, Bacillus, Escherichia, Micrococcus, and Pseudomonas and thus, they play important role in maintaining microbial / bacterial equilibrium in the soil.
2. Some protozoa have been recently used as biological control agents against phytopathogens.
3. Species of the bacterial genera viz. Enterobacter and Aerobacter are commonly used as the food base for isolation and enumeration of soil protozoans.
4. Several soil protozoa cause diseases in human beings which are carried through water and other vectors, eg. Amoebic dysentery caused by Entomobea histolytica.