Kingdom Protista

The kingdom Protista is unique among the six classification kingdoms. If one were to look for a common bond linking all the organisms of this kingdom together it would be difficult to do so. Most of the organisms are unicellular, though many group together forming colonies. Some are autotrophic, while others are heterotrophic. The main reason these organisms are grouped into Protista is that they cannot be satisfactorily placed in any of the other kingdoms. Protists form two groups: heterotrophic protists and autotrophic protists.

Heterotrophic Protists:

Protozoa are generally motile, unicellular or syncytial, wall-less heterotrophic protists. They may be free living predators or scavengers, ingesting other organisms or bits of organic matter, or parasites or mutualistic symbionts. members of the first two phyla (plus Phylum Actinopoda) were formally placed into a single phylum Sarcodina, based on their common feature: pseudopods (false feet ).


Phylum Rhizopoda : Amoebas Phylum Rhizopoda contains about 200 species. The Amoeba proteus, commonly called the Ameba is one of the most thoroughly studied protists. This formless group of organisms use pseudopodia for movement and feeding. These organisms are found worldwide, in soil, in salt and fresh water, and in the bodies of animals. The species Entamoeba histolytica, can cause a serious disease in humans if it finds its way into the intestine. These organisms are usually naked, though some have a hard shell called a test. They do not contain cilia or flagella. They never undergo meiosis and do not produce mitotic spindles during cell division.

Phylum Granuloreticulosa: Foraminifera Foraminiferans have slender, granular pseudopods used to capture food than for movement. They have a shell (test) made of calcium carbonate or of sand grains cemented by mucus. The pseudopods poke out through holes in the test where they may touch and form a netlike structure to capture food. They contain numerous nuclei and are mostly marine. England's White Cliffs of Dover are made of deposits of these organisms dating from ancient times. Reproduction varies from none to a complex life cycle containing mitosis and meiosis.

Phylum Acrasiomycota: Cellular Slime Molds Members of this phylum combine characteristics of fungi and the amoeba. Since the fungus-like features are readily observable they were first studied by mycologists. During good times they move gathering food like an amoeba, but when the going gets rough they settle down and take on a fungus-like existence and produce fruiting bodies. These organisms live in the soil eating as they go. When their food supply becomes restricted they send out chemical signals attracting others of their species and for a large pseudoplasmodium. This may crawl around for awhile and then develop into fruiting bodies.

Phylum Myxomycota: Plasmodial Slime Molds The feeding stage is a plasmodium which moves around in soil, wood, dung, or decayed vegetation, engulfing bacteria or particles of food. When conditions become too dry, the plasmodium forms a fruiting body with cell walls. Spores are produced by meiosis. Germinating spores release haploid amoebas, which may develop flagella. Two compatible amoeba fuse and form a plasmodium with a diploid nucleus. The diploid nucleus divides but the cytoplasm does not.

Phylum Zoomastigina: Zooflagellates The organisms in this group contain whip-like flagella. The nutritional habits of these organisms range from free-living, freshwater or marine, and symbionts or parasitic. Termites cannot live without a certain zooflagellete in their intestine producing cellulose digesting enzymes. Trypanosomes, which live in the blood of vertebrates causing sleeping sickness and Chagas' disease. Leishmaniasis is transmitted by sand flies in Africa, causing ulcers on the skin and internal organs. If untreated, it is fatal within two years.

Phylum Apicomplexa All apicomplexans are parasites. In typical parasite fashion, most have complicated life cycles, often with two different hosts. The phylum name is named for the "apical complex" found on the end of the motile stage used to enter the host. Human malaria is caused by four species of Plasmodium.


Phylum Ciliophora: Ciliates All protists with cilia belong to one highly successful linage, placed in phylum Ciliophora. They have rows of cilia either all over the body or in specialized areas of the cell surface. Ciliates have a very complex organization. The cell covering, the pellicle, consists of two layers of membrane sandwiching a layer of vesicles between them. The outermost layer of cytoplasm, the cortex, contains a network of protein fibers connecting the basal bodies of the cilia. It may also contain many trichocysts, barbed or poisoned threadlike organelles that can be discharged to the outside. Most ciliates prey on bacteria, small animals, or other protists. A unique feature of all ciliates is the presence of two nuclei. The macronucleus controls the cells growth and contains hundreds of copies of DNA. The micronucleus is a small diploid nucleus used during the process of conjugation when genetic material between paramecia is swapped.




Phylum Oomycota: Water Molds The Oomycota resemble fungi in having bodies made up of threadlike filaments called hyphae and in reproducing and dispersing by means of spores. Oomycotes live as saprobes and parasites. They feed by growing hyphae into a food source, releasing digestive enzymes, and absorbing the resulting molecules. The vegetative hyphae are coenocytic, with diploid nuclei and cellulose walls. They form motile asexual spores, which swim through water by means of two unlike flagella, one hairy and forward pointing, the other smooth and trailing. The parasitic oomycote Phytophtora infestans caused the Irish potato famine in the 1800's where millions of people died and many emigrated to the United States.

Autotrophic Protists:

Algae The term algae embraces all photosynthetic protists. It refers to an aquatic, photosynthetic way of life, not an evolutionary kinship. Most algae live in water, but some are terrestrial. Most algae live near the surface of the water producing 30 to 50 percent of the earth's oxygen. Algae are classified on the basis of conservative characteristics such as the type of cell wall, flagella, photosynthetic pigments, and the form in which food is stored.

Phylum Dinoflagellata (Pyrrophyta) Dinoflagellates are unicellular or colonial organisms with two flagella: one attached centrally and the other at the rear of the organism. About half the species contain a cellulose "armor" just under the plasma membrane. Half the species are photosynthetic containing chlorophylls a and c and various carotenoids, and store their food in the form of oils and starch. Many dinoflagellates are colorless and live as heterotrophs and parasites. Some produce nerve poisons toxic to vertebrates. "Red Tide" is caused by a bloom of red pigmented dinoflagellates.

Phylum Euglenida Most members of this group live in fresh water, being especially abundant in polluted habitats. Many euglenoids contain two flagella and contain a hard pellicle made of protein just under the plasma membrane. Their chloroplasts contain chlorophyll a and b and carotenoids. Many contain a red eyespot, which is thought to be used as a photoreceptor. Euglenoids reproduce asexually by dividing lengthwise into two. They do not reproduce sexually.



Phylum Bacillariophyta: Diatoms Diatoms are probably the most abundant aquatic eukaryotes in number of individuals and species. They live singly or in simple filaments or colonies, they occur in either type of aquatic environment. They reproduce sexually and are basically non motile. Most unicellular algae are haploid but diatoms are diploid. They contain chlorophyll a and c and the accessory pigment fucoxanthin, a carotenoid that give them a yellow-brown color. They store food as oil and the polysaccharide chrysolaminarin. The most distinctive feature of diatoms is the intricately patterned cell wall. The two piece cell wall is impregnated with silica. Silica does not decay so large amounts of glasslike material is deposited on the ocean floor as the diatoms die.

Phylum Chrysophyta: Golden Algae Golden algae occur as single cells or as colonies of great diversity and complexity. Most are freshwater, a few marine. They contain two unlike flagella, chlorophyll a and c and fucoxanthin pigments. Some lake dwelling golden algae are both heterotrophic and autotrophic.

Phylum Phaeophyta: Brown Algae These algae are all multicellular. They contain chlorophyll a and c and fucoxanthin pigments. Since the pigments and stored foods are identical with those of the Chrysophyta, it is thought that the brown algae evolved from the less complex golden algae. Most brown algae live in the cool waters off the temperate and sub polar areas. Members of the genus Fucus are good examples. Their algal body is the thallus, a multicellular structure that looks like a plant but has no vascular tissue. It is attached to the surface of rocks by a structures called a holdfast.

Phylum Rhodophyta: Red Algae Red algae contains single cells as well as thalli that grow as filaments, branching structures, and broad flat plates or ruffles. The chloroplast of the red algae show strong evidence of descent from cyanobacteria. The arrangement of photosynthetic membranes are similar. Both have chlorophyll a as their only chlorophyll and the accessory phycobilin pigments: phycocyanin and phycoerytherin. Red algae stores their food as floridean starch. No red algae has flagella, even in sperm cells. Most red algae are marine, with a few freshwater and terrestrial forms.

Chlorophyta: Green Algae The ancestors of all plants were undoubtedly members of phylum Chlorophyta. Green algae show great diversity of form and live in a variety of habitats. Many are single celled; others form simple or branched filaments, or hollow balls of cells, or broad flat sheets. There is no tissue differentiation. The chloroplasts of algae contain chlorophylls a and b as well as beta carotene. They store their food as starch. The reproductive cycle demonstrates an alternation of generations.


Spirogyra a typical green algae.