Protozoos

FACT SHEET
Agriculture and Natural Resources

SAG-15-11

The Role of Soil Protozoa and Nematodes
James J. Hoorman
Cover Crops and Water Quality
Assistant Professor and Extension Educator
Ohio State University Extension

Protozoa Introduction
Protozoa are single-celled animals that feed primarily
on bacteria, but also eat other protozoa, soluble organic
matter, and sometimes fungi.They are several times larger
than bacteria, ranging from 1/5000 to 1/50 of an inch
(5 to 500 µm) in diameter. Both protozoa and nematodes
are aquatic and live and move in soil water films and
water-filled pores of soil aggregates.
Protozoa are classified into three groups based on
their shape:
1. Species of Mastigophora or f lagellates are the smallest
(5 to 20 µm) and use one to fourwhip-like flagella to
move. Flagellates feed primarily on bacteria and are
the most numerous of soil protozoa.
2. Ciliophora or ciliates are the largest (10 to 80 µm)
protozoa and the least numerous and move by means
of hair-like cilia. The cilia use vibrating hairs to move.
Ciliates use the fine cilia along their bodies like oars
to move rapidly through the soil. They eat the other
two typesof protozoa, as well as bacteria. Cilates may
consume as many as ten thousand bacteria per day.
3. Sarcodina or amoebae also can be quite large and move
by means of a temporary foot or “pseudopod.” Amoebas
reside in the rhizosphere and at the root surface where
they graze on bacteria populations. There are two
types of amoebas: testate and naked. Testate amoebas
are encased in a rigidchitin shell or testa, while naked
amoebas lack a rigid shell. Naked amoebas can change
shape and explore tiny pore spaces making them
valuable for soil nutrient recycling.

Protozoa are found in greatest abundance near the
surface of the soil, particularly in the upper 15 cm (six
inches). The life cycle of many protozoa consists of an active
or trophozoite phase where the animal feeds andmultiplies
and a resting or cyst stage where the cell produces a thick
coating. In the cyst stage, many species can withstand
harsh environmental conditions and persist for many
years until environmental conditions improve.

Photos by Elaine L. Ingram, Oregon State University, and USDA
Natural Resource Conservation Service Soil Biology Primer.

Role of Protozoa
Protozoa play an importantrole in mineralizing
nutrients, making them available for use by plants and
other soil organisms. Protozoa (and nematodes) have a
lower concentration of carbon (C) and nitrogen (N) in

Copyright © 2011, The Ohio State University

The Role of Soil Protozoa and Nematodes—page 2

their cells than the bacteria they eat. (The ratio of C:N
for protozoa is 10:1 or much more than 3:1 to 10:1 forbacteria.) Bacteria eaten by protozoa contain too much
N for the amount of C protozoa need. They release the
excess N in the form of ammonium (NH4+). This usually
occurs near the root system of a plant. Bacteria and other
organisms rapidly take up most of the ammonium, but
some is used by the plant. See Understanding Soil Microbes
and Nutrient Recycling.
Another role that protozoa play isin regulating bacteria
and algae populations. Single protozoa (paramecium)
can consume as many as 5 million bacteria in one day.
The protozoa help maintain an ecological balance in the
soil. When they graze on bacteria, protozoa stimulate
growth of the bacterial population and decomposition
rates and soil aggregation. Protozoa grazing can be
thought of like pruning a tree—a small amountenhances
growth, too much reduces growth or will modify the mix
of species in the bacterial community. Protozoa are also
an important food source for other soil organisms and
help to suppress disease by competing with or feeding
on pathogens. Protozoa feed selectively so they influence
the bacteria composition and bacteria population in the
rhizosphere.
Living roots release many
types of...