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The Ecology of Ciliated Protozoa
What are Ciliated Protozoa?
Ciliates are microscopic, single celled organisms which are named for the abundance of tiny hairs (cilia) on the cell surface. They have been around for over 500 million years, and play a crucial role in maintaining water quality, as they feed on bacteria in the water. They are incredibly diverse organisms, and are found all over the world in habitats such as soils, freshwater, marine environments and even snow.
Most ciliates are described as phagotrophic, in that they feed on other organisms and cannot photosynthesize. There are some exceptions to this however, with some species such as Paramecium bursaria (pictured below) having endosymbiotic algae which live inside the cells. These organisms can be described as mixotrophic, because they can photosynthesize using the algae inside the cell, but can also feed on other organisms.
Ciliates feed on a wide variety of microbes, including bacteria, algae, flagellates (small single celled organisms with a tail-like flagellum) and even other ciliates. Indeed some larger have been species have been observed feeding on small animals such as rotifers! Ciliates are in turn also fed upon by zooplankton such as water fleas and copepods. Small fish feed on these zooplankton and essentially ciliates link microbial food webs with macroscopic food networks (involving invertebrates, fish, etc.)
Ciliates are also involved in nutrient cycling within the microbial loop. As ciliates feed on bacteria and algae, the organic matter accumulated by these organisms is passed up the food web, to larger organisms such as fish. However, when ciliate cells die, they release the organic matter accumulated through feeding and this is taken up by bacteria. A schematic representation of this is shown below:
Applications of Ciliates in Sewage Treatment Plants
Stalked ciliates such as Epistylis sp. are commonly used in wastewater treatment plants in order to clean sewage water. These ciliates feed on the bacteria and as such they help to clean up the water. In this sense, ciliates can also be used as indicators of water quality in natural bodies of water, such as rivers and lakes.
Overall, ciliates, despite their tiny size, play a significant role in the healthy functioning of aquatic ecosystems, by keeping blooms of bacteria and algae under control, and as such maintaining water quality. They also are fundamental in nutrient cycling in freshwater ecosystems, and as a key component of the microbial loop.