Protists are eukaryotic organisms with cells containing nuclei, including animal-like protozoa, plant-like protophytes, and mushroom-like slime molds. DNA analysis has shown that many are not closely related to each other, and reclassification may be needed. Protists can be roughly grouped into animals, plants, and mushrooms. Animal-like forms are unicellular and capable of independent movement, using cilia, flagella, or pseudopodia. Plant-like forms are mostly non-motile and photosynthetic, including red and green algae, and diatoms. Mushroom-like forms are slime molds that move in colonies and reproduce by forming spores.
Protists is a general term used to describe all eukaryotic organisms, i.e. those that have cells with nuclei, as well as animals, plants and fungi. Examples include the unicellular animal-like protozoa, mushroom-like slime molds, and plant-like protophytes. These organisms were previously considered to belong to a kingdom of their own, such as plants and animals, but it is now known through DNA analysis that many of them are not closely related to each other and that some may be closer to organisms within other kingdoms than their fellow protists. The term derives from the Greek protiston, which means “first of all”. These organisms are often microscopic and are either unicellular or an undifferentiated multicellular mass.
Classification
Superficial similarities and differences between protists can be misleading. As more analyzes have been done at the molecular level, surprising relationships have emerged. For example, kelp which can grow up to 164 feet (50 meters) in length is found to be more closely related to microscopic diatoms than to some other algae, while green algae are more closely related to members of the plant kingdom than to other apparently similar protists like red algae. It is generally recognized that some sort of reclassification is needed among this category of life forms; this may involve splitting them into different kingdoms or grouping some types with other kingdoms. The classification of protists is still an ongoing area of research and may be subject to further revision as more information emerges, but, on a slightly less scientific basis, these organisms can be roughly grouped into animals, plants, and the less numerous types of mushrooms.
Animal forms
These types are all unicellular and are often capable of independent movement. Most actively hunt for food or are parasites that infect other organisms, although some generate their own food by photosynthesis. They include the life forms generally known as protozoa, as well as numerous other organisms.
Those that are capable of locomotion use one of three methods. Ciliates, like the famous paramecium, use tiny moving hairs known as cilia to travel through the water. Flagellates use a long, whip-like structure called a flagellum to propel themselves, while amoeba-like forms have a soft, flexible cell membrane and are able to move across surfaces by gliding along, producing pseudopodia – foot-like projections. Some types can transition from a flagellate to an amoeba-like form, while some parasitic forms, such as plasmodium, which causes malaria, and toxoplasma, which causes toxoplasmosis, are incapable of independent movement and often have life cycles complexes involving more than one guest.
Another interesting group are the dinoflagellates. These mobile freshwater and marine organisms propel themselves using flagella, but, like plants and algae, many create their own food through photosynthesis. Some types exhibit bioluminescence and, when present in large numbers, can produce a visible glow in seawater at night. Other types produce powerful toxins and are responsible for the “red tide” that can kill fish and other organisms: the red color comes from the photosynthetic pigment they use.
Plant-like shapes
This group consists mostly of non-motile photosynthetic organisms. Red and green algae are perhaps the best known types, although the category includes some very different looking life forms. They range from microscopic unicellular organisms to very large multicellular forms, such as algae.
Red algae get their name from a red pigment called phycoerythrin which performs the same photosynthetic function as chlorophyll in green algae and plants, but absorbs blue light. This allows them to live at greater depths than green algae, as light in the blue range of the spectrum penetrates the water more deeply. Red seaweed includes a number of types of seaweed, some of which are edible. Some types produce a calcium carbonate crust around themselves and are important in reef formation in some areas.
Green algae resemble members of the plant kingdom in that they use the same forms of chlorophyll. They are, in fact, considered the ancestors of green plants. They range from unicellular to multicellular types and can be found in a variety of aquatic or wet environments. Some species are marine and include many algae, while others are found in fresh water or in damp, shady locations. A number of types have formed a symbiotic relationship with some fungi, in the form of lichens.
The chromistas are an extremely diverse group of plant-like protists. They include the huge kelp algae that can form large forests on the ocean floor, as well as diatoms, which are single-celled microscopic organisms that are encased in silica coatings, often with very complex and beautiful structures. Diatoms are found in freshwater and the sea, where they are an important part of the phytoplankton that form the basis of the marine food chain.
Mushroom-like shapes
These consist of various types of slime molds; however, although they resemble mushrooms in their outward appearance and were once classified as such, they are actually completely unrelated. They consist of motile single-celled organisms that move in colonies, feeding on microorganisms, such as bacteria. Cells can fuse together at some stage, sometimes forming what is, in effect, a huge cell with many nuclei. They can reproduce by forming structures containing spores, which are released to form new slime molds when they land in a favorable environment. Slime molds can be found on the ground, tree bark, and decaying organic matter, such as decaying wood.
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