At the microscopic level, soil organisms are actually aquatic. They spend their lives in the thin film of moisture between soil particles. The two large protozoa pictured here are ciliates of the genus Euplotes. Ciliates
are typically larger protozoa characterized by the presence of many
hair-like structures called cilia. They derive their name from the Latin
word for ‘eyelash’. This is because of the tiny hairs that cover some or
all of the ciliate bodies. When you are less than a millimeter in body size,
water acts like a thick syrup, making swimming like a fish very inefficient.
Instead, these protists use the hair like structures almost like paddles to
maneuver. These hair like structures also act to create currents in the
surrounding liquid and consequently help to sweep bacteria, detritus and
algae into their mouth pore into their gullet. That said, some ciliates are
mouthless and feed by absorption.
Individual ciliate species vary greatly in their tolerance of pollution, as well as oxygen levels. A predominance of ciliates seen under a microscope
can be an indicator of anaerobic conditions. If ciliates numbers are too high, then this may indicate that the soil is either compacted or water-logged, and lacking oxygen. Ciliates are aerobic organisms, but prefer to consume anaerobic bacteria. They tolerate reduced oxygen conditions better than the other protozoa, so high numbers of ciliates indicate problems with the movement of oxygen into the soil, which then needs to be fixed.
There are also some flagellates present in the background of this image and some strands of fungi weaving through the soil, trading nutrients with plants and helping create good soil structure along the way. Protozoa of all kinds are important members of the soil ecosystem. They are key to unlocking nutrients immobilized by bacteria and freeing them for use by plants.
Protozoa have also been found to promote plant health and disease resistance, and to improve plant growth independently of nutrients. The
presence of diverse protozoa is a good sign of a healthy, properly
functioning soil ecosystem.