The Kingdom-level grouping known as Protista or Protoctistae consists mainly of microscopic organisms. It is defined by exclusion: its members are neither animals (which develop from a blastula), plants (which develop from an embryo), fungi (which lack
undulipodia and develop from spores), nor prokaryotes. It includes the green algae (Gamophyta and Chlorophyta), red algae (Rhodophyta), brown algae (Phaeophyta), diatoms (Bacillariophyta), and other groups.
Different authorities have differing views on the taxonomy of these organisms.
Phylum of cellular slime molds.
Members of the kingdom Apicomplexa are parasitic protists responsible for many animal and human diseases. Many of them have complex life cycles involving more than one host and include Babesia, Coccidia, Eimeria, Toxoplasma and Plasmodium. They are able to reproduce by producing a burst of spores.
Members of the kingdom Bacillariophyta are the diatoms. They are unicellular organisms that have cell walls, or tests, that are made of silicon dioxide and composed of two valves, which fit together with the help of a set of girdle bands. Most are photosynthesizers and they are important members of the marine plankton. There are about 10,000 known species.
Members of the Chlorophyta are the green algae or green seaweeds. Although traditionally classified as protists they are now generally considered to be part of the plant kingdom. There are two main divisions; the Ulvophycaea, which are mostly marine and multicellular and form filaments or flat blades as in the sea lettuce, and the Chlorophyceae, which mostly live in fresh water and most of which are single-celled. Like other land plants, they contain chlorophyll a and b, with carotenoids as accessory pigments for photosynthesis. It seems likely that the ancestor of all the land plants was a green alga.
Members of the kingdom Chrysophyta are single-celled protists known as golden algae. They vary greatly, are very widespread and are mostly found in fresh, cool lakes and ponds, sometimes forming colonies which may be elaborately shaped. The marine types form intricate skeletons of silica.
Members of the kingdom Ciliophora are single-celled organisms commonly known as the ciliates. They all have cilia, sometimes a great many, over their surface, and many have more than one nucleus. There are several thousand species living in the sea and in freshwater, feeding mainly on bacteria.
Phylum of unicellular flagellates which botanists treat as a separate division, Cryptophyta, but which zoologists treat as the flagellate order Cryptomonadida. Some photosynthetic species form commensal relationships with marine corals to their mutual benefit.
Members of the kingdom Dinoflagellata are single-celled, have a long whip-like flagellum, and are encased in a shell or test. They are mostly marine, forming an important part of the plankton. Some photosynthesize and many of these form symbiotic relationships with corals, anemones and clams. Under certain conditions, dinoflagellates can multiply prodigiously and cause "blooms". Some produce powerful toxins that can be poisonous to marine organisms, affecting humans when, for example, shellfish are consumed.
The euglenoids are a group of single-celled protozoans shaped like cigars with a gullet at the front and a single, forward-pointing flagellum. Most live in fresh water, and these contain chloroplasts for photosynthesis. Others live as parasites, often in the guts of animals. The euglenoids and the kinetoplastids, such as the blood parasite Trypanosoma, are now considered to constitute the kingdom Euglenozoa.
Members of the phylum Foraminifera are single-celled amoeboid protists, typically with a shell or test. Known affectionately as forams, they are abundant all over the ocean, most living on the sea bottom but some are part of the marine plankton. There are about 4,000 known species.
Phylum of single-celled green algae found in the marine plankton and abundant as microfossils. They contain chlorophylls a and c but never b.
Members of the kingdom Myxomycota are the plasmodial or true slime moulds. The individuals group together to form patches of wet slime on fallen logs, with many nuclei in a continuous sheet of cytoplasm. The patches do not move bodily but may grow in one direction. When conditions become drier, they may form a mound from which stalked sporangia grow, and from which single-celled offspring emerge
Myxozoa used to be considered a phylum of protists. Modern thought is that they should be classified as animals in the phylum Cnidaria which includes such radially symmetric creatures as the jellyfish. They are mostly parasites of fish.
The kingdom Oomycota includes a number of fungus-like groups such as the downy mildews and white rusts. They produce similar threads called hyphae and secrete enzymes to digest their food before absorbing the released nutrients. They differ from fungi in that their cell walls are made of cellulose rather than chitin.
Phytopthera is responsible for blight in potatoes and Pythium causes the damping-off of seedlings.
Members of the kingdom Phaeophyta are the brown algae or seaweeds, a group of photosynthetic protists. They contain chlorophylls a and c, not a and b as do plants. The brown colour comes from the pigment xanthophyll. Many brown seaweeds have a blade or thallus and a holdfast for anchorage. They form great forests in some shallow oceans and there are about 1500 known species.
Radiolaria are protozoa distinguished by the segregation of their soft anatomy into a central capsule, and their siliceous tests which often have many spines extending outward. Radiolarians have existed since the beginning of the Paleozoic era, producing an astonishing diversity of intricate shapes during their 600 million year history. There are two superorders, Polycystina and Phaeodoria.
Members of the kingdom Rhodophyta are the red algae or seaweeds. Almost all are marine and they vary greatly, some being filaments, some cushions, some feathery and others flattened discs. Many have a coating of calcium carbonate and form crusts or calcified trees that resemble corals. They contain phycobilins and chlorophyll a, but not chlorophyll b or c, and there are about 4,000 species known.
Phylum of protozoa, including amoebas and flagellates (such as trypanosomes and giardia).
Members of this group tend to form colonies and are photosynthetic organisms which live primarily in freshwater. Their yellow-green colour comes from various xanthins as well as chlorophylls a, c, and e. About 100 different species are known.