Protozoa are the tiniest and simplest of all soil animals, and occur in all soils on Earth. Although they are only formed from one cell and are usually quite tiny, some of these cells can be quite “large” - up to 0.5 mm in diameter.

Soil Protozoa have a “stop/start” existence. They need a continuous film of water surrounding soil particles to live in otherwise they dehydrate. But Protozoa have a survival trick up their sleeves. These water-dependent animals can encyst and survive severe droughts in a state of suspended animation. They hatch out of encystment and start activity once it rains, then stop being active and encyst as the soil dries again.

They will also encyst if it gets too cold. In Antarctica, Protozoa encyst below 1.5 degrees Celsius. During the Antarctic day, there may be only 9 hours above this temperature but Protozoa can hatch out, be active and feed and then re-encyst come evening when the temperature drops.

Protozoa swim through water using whip-like tails (flagellae) at one end of the cell, or a covering of tiny pulsating hairs (cilia) which propel it through water, or they can ooze their way over water-covered surfaces by changing their shapes. The different groups of Protozoa can take their names from the way they move. Those that use flagellae are called flagellates and those that use cilia, are ciliates. Protozoa with a changeable or plastic form are called amoebae.

Video of single-celled citiates moving in soil water.

To feed, amoebae change their body shape so as to completely surround, engulf and then absorb the food into themselves in a bubble or food vacuole. In ciliates and flagellates, food is engulfed in a similar way but only in specialised bits of the cell. Most soil Protozoa eat bacteria. Amoebae can insert bits of themselves into fungal filaments and feed on the cell contents. They can feed on cell after cell within the same fungal filament so that whole mats of soil fungi may seem to be intact, but the cells are dead. Protozoa also eat particulate matter and can even eat other Protozoa.

Protozoa help provide nutrients to plants. After feasting on bacteria, they’ve imbibed nitrogen to excess and excrete it into the soil solution where roots can absorb it.

Aerial plankton: Some of the tiny soil animals, including Protozoa are so light, they can get wafted up into the air and carried by the wind to become part of the “aerial plankton”. It is pretty dry in the atmosphere so it is the encysted Protozoa that transport themselves from place to place in the wind. There is an average of 2 Protozoan cysts in each cubic meter of air.