Further analysis of kingdom Protista has suggested the need for restructuring phylogenic classification. Genetic and morphological research has led to subdividing the Protista kingdom into numerous separate kingdoms, each with its own lineage of protist. In truth, the classification of the protists remains in flux. This chapter provides descriptions of representative members of the Protista group and their roles in our lives and the environment, using the still-accepted kingdom and phyla nomenclature.
Members of the kingdom Protista are a highly varied group of organisms, all of which are eukaryotic. In addition, protists are unicellular or, in some cases, colonial. Many species are autotrophs, creating their own food, while others are heterotrophs, feeding on organic matter. Many species are nonmotile, but the majority of protists are able to move by various means. Many protists have contractile vacuoles, which help them to remove excessive amounts of water from their cytoplasm. The kingdom Protista includes the protozoa, slime molds, and algae.
Protozoa can be divided into four phyla based on their locomotion: Mastigophora, Sarcodina, Ciliophora, and Sporozoa.
Members of the phylum Mastigophora move about by using one or more whiplike flagella. The genus Euglena contains flagellated species. Members are freshwater protists with typical eukaryotic properties, including two flagella, reproduction by mitosis, and flexible nutritional requirements. Euglena species also possess chlorophyll within chloroplasts. This pigment allows the organisms to synthesize organic compounds in the presence of sunlight. When no sunlight is available, the organism feeds on dead organic matter in the surrounding environment. Thus, the organism is autotrophic and heterotrophic. Some biologists consider Euglena to be the basic stock of evolution for both animals and plants.
Certain species of Mastigophora are zooflagellates, while some are phytoflagellates. The zooflagellates live within the bodies of animals and are typified by the wood-digesting flagellates in the intestines of termites. Among the pathogenic zooflagellates are those that cause sleeping sickness, trichomoniasis, and giardiasis. The phytoflagellates have photosynthetic abilities and are often discussed with algae in textbooks.
Some species of Mastigophora organize themselves into colonies. Members of the genus Volvox are typical colonial forms. The cell colonies are not differentiated into tissues or organs, but the colonies show how a preliminary step in evolutionary development might have occurred.
Members of the phylum Sarcodina are the amoebas and their relatives. Amoebas consist of a single cell without a definite shape. They feed on small organisms and particles of organic matter, and they engulf the particles by phagocytosis. Extensions of the cytoplasm called pseudopodia (the singular is pseudopodium) assist phagocytosis and motion in the organisms.
Amoebas are found in most lakes, ponds, and other bodies of freshwater. They move by a creeping form of locomotion called amoeboid motion. One amoeba called Entamoeba histolytica causes a type of dysentery in humans.
Two interesting amoebas are the foraminiferans and the radiolarians. Both are marine amoebas that secrete shells. Their shells have been identified as markers for oil deposits because both were present in the ocean communities that became the organic deposits that, under pressure, became oil fields.
Members of the phylum Ciliophora move by means of cilia. The organisms are all heterotrophic and have specialized organelles in their cytoplasm. For example, they have two nuclei: a large macronucleus and a number of smaller micronuclei. The micronuclei carry the genetic information of the cell.
The ciliate Paramecium typifies the phylum Ciliophora. This organism has a slipper-shaped body with a covering called a pellicle. Defensive organelles called trichocysts are present in the pellicle. The organism reproduces by mitosis and by an elaborate form of sexual behavior called conjugation, which occurs when two Paramecium join to one another in the oral region and exchange nuclear material. The cilia of Paramecium provide a precise form of motion not provided by flagella or pseudopodia. The cilia can propel the Paramecium either forward or backward and move it in a spiral manner.
Members of the phylum Sporozoa are exclusively parasites. They are so named because some members produce sporelike bodies. Often they have an amoeboid body form, but they are not related to the Sarcodina.
Sporozoans are parasitic organisms with complex life cycles involving several stages. One of the best-known members of the group is the Plasmodium species, which are the agents of malaria. The organisms spend portions of their life cycle within mosquitoes. After being injected into the human bloodstream by the mosquito, the parasites invade the red blood cells, undergo numerous changes, and emerge from the red blood cells, destroying them. The infected human experiences a malaria attack soon after.
Another important member of the Sporozoa group is Toxoplasma gondii, which causes toxoplasmosis, a disease of the white blood cells. Toxoplasmosis is normally not a dangerous disease, but pregnant women can pass it to their fetuses, where it can cause tissue damage. Also, people who are immunocompromised are susceptible to severe cases of toxoplasmosis.
Two other important pathogens of the Sporozoa group are Cryptosporidium coccidi and Pneumocystis carinii. Both of these organisms cause severe opportunistic disease in immunocompromised patients. The first causes a severe intestinal diarrhea that is accompanied by the loss of substantial volumes of water; the second causes a type of pneumonia.