Protozoa are unicellular. Protozoa are motil; they produce flagella (flagellates) or cilia (ciliates) or cause their cytoplasm to stream (amoebae). All protozoa consume bacteria. Some amoebae can use enzymes to drill through fungal cell walls and consume fungal cytoplasm. most flagellates and amoebae cannot tolerate reduced-oxygen conditions and either die (if change in conditions occurs rapidly) or become dormant, if they have time to go through the physiological change before conditions become too bad. Ciliates can tolerate low-oxygen conditions but also die or go dormant when oxygen becomes extremely limited. When conditions become difficult, protozoa will become dormant by the process known as encysting. If time occurs between the protozoan detecting that things are becoming bad and the limit of survival, then the protozoan will encyst and, or for the most part, survive through bad times. Otherwise, rapid changes will kill the protist before it can become dormant. Temperature, moisture, salt concentration, oxygen, types of bacteria present, and so on, are all conditions that can cause a protest to become dormant (i.e., product cysts). For additional information on Amoebae, visit Microworld, World of Amoeboid Organisms.
The smallest (usually, 3 to 5 micrometers diameter) of the three types of major groups of protozoa. One or two flagella per individual is the norm. The arrangement of the flagella on the body results in flagellates having a rolling motion as they swim through water. Flagellate cysts are small and generally circular, about the size of the flagellate when active. There is a single wall around a flagellate cyst. There are amoeboid flagellates, which bend and look amoeba-like but have flagella. These are flagellates. Flagellates are strict aerobes. All flagellates eat bacteria.
Amoebae extend a pseudopod (an oozing lobe, or "foot) and draw the rest of the body along. In this way, they slowly roll along surfaces, searching for their food resource, bacteria. Amoebae can be about the size of a flagellate (5 to 10 micrometers diameter), up to quite large about 100 micrometers across the body. Most soil amoebae are around 5 to 20 micrometers in diameter, however. They typically have to fit through small pore spaces to find their prey. Certain species of amoebae have shells, called tests, which they make to protect themselves. Some amoebae have long, slender pseudopods that make them look like a spiky star. Some amoebae put out a pseudopod (false foot) suddenly, whereas some smoothly ooze. Some amoebae look like thin, filigreed threads. Amoeba cycts are double walled and circular. The outer wall of the double-walled amoeba cyst is not always completely symmetrical. The inner wall of the double-walled amoeba cyst is always round. Amoebae are strict aerobes and eat bacteria, although they have been observed consuming small flagellates.
Ciliates are the largest of the protozoa (10 to 200 micrometers) and the fastest moving. Ciliates often come zipping out of organic matter, moving at fast speeds and pushing soil particles, bacteria, and other things out of the way. Cilia are short, slender "hairs" that cover all or part of the surface of the ciliate and are used to row the ciliate through water or the soil solution. Many of the most common soil or compost ciliates are kidney-bean shaped (Colpoda). Hairbrush, comb and vase shapes occur but are less common. Stalked ciliates are like vases on stalks, with the cilia coming out where the flowers would be seen in the flowerpot or vase. The cilia set up a vortex, pulling the soil solution into the vase. The ciliate filters bacteria it wants out of the solution, and sends the water stream back out into the soil along with the bacteria and other debris the ciliate does not want. Stalked ciliates can contract the stalk and spring off that surface, looking for a better place to stop and set up the bacteria-capturing situation again. Ciliate cysts are large, clear, and single walled. They can be mistaken for yeast, but yeasts are typically more oblong and have bud scare (round slits or markings) on their surfaces.