Organisms can be divided into prokaryotes — primitive, single-celled life forms with no cell nucleus — and eukaryotes — which have a cell nucleus and include animals and plants. Protists belong to the eukaryotes, but form an extremely diverse group that includes everything except animals, plants and fungi. Some are single-celled, while others are multicellular, but are distinguished from non-protists by not having organs or different types of tissue. Although they seem to provide no proven direct benefits to humans, they play a crucial ecological role, and without them, more advanced life forms could not have evolved. They also have a number of important uses.
Most protists are microscopic, but some are just visible to the naked eye, and others can form large colonies and structures. They include protozoa — mobile single-celled organisms — and various types of algae, such as diatoms — tiny, photosynthetic life forms with silica shells — and seaweeds that can grow to 100 feet (30.5 meters) in length. Although some protozoa are responsible for serious human diseases, the majority are harmless, and protists in general are crucial to the world’s ecosystems.
Algae and diatoms use photosynthesis to manufacture food, absorbing carbon dioxide from the atmosphere and releasing oxygen. These protists are extremely numerous in the oceans and in fresh water, and so play an important role in maintaining the planet’s oxygen levels, and storing carbon. In fact, algae produce about half of the oxygen generated by photosynthesis on the planet. Along with other protists, they form a large part of the marine plankton that is the lowest level of the ocean food chain, and are therefore a crucial part of marine ecology. On land, soil protozoa help plant growth by consuming bacteria and releasing their nutrients in a form that can be easily absorbed by roots.
Sewage and other forms of wastewater can pose a serious threat to human health, and cause great harm to ecosystems, if they are simply allowed to run into rivers or dumped in the ocean. It is therefore sent to treatment plants to remove harmful bacteria, offensive odors and suspended organic matter. Protozoa play an important role in this process by preying on bacteria, and consuming large amounts of organic material, helping to clarify the water and render it safe for disposal.
Protists as a Food Source
Many types of seaweed are edible, and are an important food source in some coastal regions. Seaweed is farmed in some countries, such as Indonesia and the Philipines, both for food and because of the useful substances that can be extracted from it. Some of these are used as food additives, for example, as gelling agents, and to improve water-retention. Seaweed is also a good source of the essential element, iodine.
It may be that protozoa help control bacterial populations in the human intestine. There are a great variety of bacteria normally present in the human gut, and most of the time, they are beneficial, helping to break down food, or harmless. However, it is thought that protozoa that prey on these organisms may help to keep their numbers in check, and prevent imbalances between different types.
Scientific Research and Other Uses
Protists have been the subject of a great deal of research, not only to improve knowledge of the organisms themselves, but also to help understand more general biological processes. Slime molds, for example, are large colonies of amoeba-like organisms that creep in unison over soil, tree bark, and other damp surfaces, consuming various microorganisms as they go along. They are unusual in that instead of consisting of clearly defined cells, they simply have large numbers of nuclei floating within cellular fluid, rather like one huge cell with many nuclei. At a certain point, this mobile, animal-like, phase comes to an end and the slime mold ceases moving to form structures that release spores. Biologists study slime molds to learn more about the ways cells behave and differentiate.
The remains of diatoms are also useful to scientists. When these organisms die, their tiny silica shells fall to the ocean or lake floor, forming sediments known as diatomaceous earth. Different species, which can be identified by their shells, have different preferences regarding water temperature, and so, by studying ancient samples of diatomaceous earth, scientists can learn a lot about past climates.
There are a number of other practical uses for diatomaceous earth. It is porous, but the spaces between the particles of which it is made up are very small, making it useful for filtering water, and other liquids. It is used in many filtration systems, and has the advantage of being chemically very inert, and so can be used to filter liquids that would react with filter paper. Since diatom shell fragments are hard and often sharp-edged, diatomaceous earth can be used as an abrasive in cleaning powders and pastes. It is also added to animal feed to rid livestock of intestinal worms; the abrasive effect damages worm tissue, eventually killing them.