Definition: Protists are organisms in the kingdom Protista. These organisms are eukaryotes, meaning they are made up of single or multiple cells which all contain a nucleus enclosed by a membrane. The protists are a diverse group of eukaryotes that cannot be classified as animals, plants, or fungi. Organisms in the Protista kingdom include amoebae, red algae, dinoflagellates, diatoms, euglena and slime molds. Also Known As: Protista
One type of protist in the marine environment is Irish moss, which is a species of red algae. Diversity of Protists
Protists are Eukaryotes that are not fungi, plants, or animals. This polyphyletic group includes a wide variety of organisms. Most groups of protists are unicellular but some are multicellular. Molecular evidence suggests that protists include several different lineages and therefore is not a kingdom. Some lineages are more closely related to either Fungi, plants, or animals than they are to other protist groups. Plants,fungi,and animals evolved from protist ancestors.
Protists are a very diverse group and include organisms that range in size from single cells to complex structures more than 100 meters long. They show a variety of reproductive and nutritional strategies. Some protists are photoautotrophs, others ingest food (heterotrophs) or they release digestive enzymes into the environment and absorb organic molecules (saprotrophs). Some protists are both autotrophs and heterotrophs (mixotrophs). Most protists are aquatic but they are also found in moist terrestrial environments. They are important components of plankton in many aquatic food chains. Some groups of photoautotrophic protists are referred to as algae (green algae, red algae, brown algae, golden algae). The word algae is not used as a taxonomic category. Plasmodial (Acellular) Slime Molds
Acellular slime molds are diploid, multinucleate masses that creep along the substrate and phagocytize dead organic material and microorganisms. The mass is one large cell referred to as a plasmodium. Note- Do not confuse the use of the word “plasmodium” here with the genus Plasmodium discussed under Apicomplexans above. Slime molds play an ecological role similar to that of fungi. They are decomposers, feeding on dead organic material. They differ from fungi in that slime molds ingest their food. Below: Physarum polycephalum. Click to view an enlargement.
When environmental conditions are unfavorable such as when sufficient food or moisture are unavailable, sporangia form, and spores are produced by meiosis. Spores are resistant to environmental extremes and germinate when environmental conditions become favorable. They germinate to produce haploid cells that are either biflagellate (two flagella) or amoeboid. These cells can act as gametes, fusing to produce a diploid zygote that matures into the plasmodium.
Cellular Slime Molds
Cellular slime molds exist as individual amoeboid cells that phagocytize bacteria and yeast. When food becomes scarce, the cells aggregate to produce a mass that resembles the plasmodium of a plasmodial slime mold. This mass of cells may continue to move about but eventually will settle down and cells within the mass will produce fruiting bodies (reproductive structures). The cells at the tips of the fruiting bodies become spores. The spores germinate when conditions become favorable. The amoeboid cells are haploid. In the sexual phase of the life cycle, two amoeboid cells fuse to form a zygote. New amoeboid cells are produced by meiosis.
Gymnamoebas move by cytoplasmic extensions called pseudopodia. They feed by phagocytizing (engulfing) their prey.
Click on the image below to view movement in Amoeba.
Gymnamoebas are found in soil, marine, and freshwater environments. Amoeba proteus (below) is found in freshwater. Nutrition in some different types of protists is variable. In flagellates, for example, filter feeding may sometimes occur where the flagella find the prey. Other protists can engulf bacteria and digest them internally, by extending their cell membrane around the food material to form a food vacuole. This is then taken into the cell via endocytosis (usually phagocytosis; sometimes pinocytosis).
Some protists reproduce sexually (gametes), while others reproduce asexually (binary fission). Some species, for example Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually. However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protists may be clonal lines that rarely exchange genes with other members of their species.
Role as pathogens
Some protists are significant pathogens of both animals and plants; for example Plasmodium falciparum, which causes malaria in humans, and Phytophthora infestans, which causes late blight in potatoes. A more thorough understanding of protist biology may allow these diseases to be treated more efficiently. Researchers from the Agricultural Research Service are taking advantage of protists as pathogens in an effort to control red imported fire ant (Solenopsis invicta) populations in Argentina. With the help of spore-producing protists such as Kneallhazia solenopsae the red fire ant populations can be reduced by 53-100%. Researchers have also found a way to infect phorid flies with the protist without harming the flies. This is important because the flies act as a vector to infect the red fire ant population with the pathogenic protist.