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Protista

Protista

The Protista, or Protoctista, are a kingdom of simple eukaryotic organisms, usually composed of a single cell or a colony of similar cells. Protists live in water, in moist terrestrial habitats, and as parasites and other symbionts in the bodies of multicellular eukaroytes.

Other eukaryotic kingdomsthe Plantae, Fungi, and Animaliaare each believed to be monophyletic . That is, all plants evolved from one ancestral plant, all animals from one ancestral animal, and all fungi from one ancestral fungus. The Protista, however, are not; they are almost certainly polyphyletic and did not arise from a single ancestral protist. Rather, the Protista are a category of miscellaneous eukaryotes, not closely related to each other and not sharing many characteristics, but not fitting any other kingdom of life. Some authorities divide the Protista into as many as twenty-seven phyla, and some feel the Protista should be discarded as a kingdom name, and these organisms divided into as many as twelve kingdoms.

Historically, the Protista were divided into three main categories: the plantlike algae, animal-like protozoans, and funguslike slime molds. This classification persists in many elementary textbooks; however, current molecular evidence indicates that these are not natural groups related by common descent, but groups with merely superficial , deceptive similarities. Classifying them together is probably no more scientific than it would be to classify bees, birds, and bats in one group simply because they all have wings and fly. The two flagellated protozoan groups called trypanosomes and dinoflagellates, for example, are probably less related to each other than a human is to a fish. Genetic evidence (base sequences in their mitochondrial deoxyribonucleic acid [mtDNA] and ribosomal ribonucleic acid [rRNA]) now indicates that the following are more natural (evolutionarily related) groups of Protista.

Basal Protista

These are the most primitive protists. Some lack mitochondria and suggest what the first eukaryotes may have been like, while others have primitive mitochondria that closely resemble bacteria. Some basal Protista without mitochondria are Trichomonas, a vaginal parasite of humans; Giardia, an intestinal parasite; and Entamoeba, the cause of amoebic dysentery. The lack of mitochondria is not necessarily the primitive (original) condition of all these protists, however. Although Giardia lacks mitochondria, it does have mitochondrial genes. Apparently it once had mitochondria, and these genes transferred to its nuclear DNA before the mitochondria were lost.

Basal Protista with mitochondria include Trypanosoma, a genus of blood parasites that cause African sleeping sickness and other diseases; Euglena, a green freshwater flagellated protozoan with chloroplasts; and Physarum, a common terrestrial slime mold.

Alveolates

Alveolates are named for flattened sacs called alveoli just beneath their plasma membranes. They have mitochondria with tubular cristae rather than the flattened cristae typical of most mitochondria. Alveolates include dinoflagellates, aquatic forms with two flagella and a cell wall made of armorlike cellulose plates; Paramecium and other familiar ciliates; and the Apicomplexa, a group of intracellular parasites that includes Plasmodium, the cause of malaria, and Toxoplasma, the cause of toxoplasmosis.

Stramenopiles

Stramenopiles include water molds, golden and brown algae, and diatoms. The funguslike water molds (oomycetes) live in fresh water and soil, feeding on living or decaying organisms. Despite their name, some of them are important pests of row crops, including potato blight, downy mildew, and white rust. The golden algae (Chrysophyta ) and brown algae (Phaeophyta ) include many familiar seaweeds easily found on rocky coasts. Kelp is a gigantic marine brown alga (Macrocystus ) that grows up to 30 meters (100 feet) long and forms dense "forests" in some coastal waters. Diatoms are microscopic unicellular algae encased in siliceous (glasslike) walls, often with delicate lacy designs like tiny jewel boxes or Christmas ornaments.

Red Algae

The red algae (Rhodophyta ) include most seaweeds and are most abundant in tropical seas. Coral reefs are made not only by corals but also by coralline red algae that deposit calcium carbonate in the reef. Some red algae produce viscous polysaccharides such as agar and carrageenan, used to thicken ice cream, desserts, salad dressings, toothpaste, cosmetics, paints, and bacterial culture media.

Green Algae

The green algae (Chlorophyta ) include the single-celled Chlamydomonas, the spherical colonies of Volvox, and large seaweeds such as Codium magnum. Some unicellular green algae, notably Chlorella, live within the cells of animals, imparting a green color to some sponges, hydras, and flatworms. The plant kingdom probably evolved from a green alga.

The Study of Protista

Biologists in several subdisciplines of biology specialize in the Protista or have interests that overlap with this kingdom. Microbiologists study bacteria and some unicellular protists. Phycologists specialize in algae. Protozoologists study protozoans. Mycologists specialize in fungi but also often study water molds and slime molds, formerly classified as fungi. Parasitologists study disease-producing protists.

see also Algae; Coral Reef; Fungi; Mitochondrion; Plant Pathogens and Pests; Protozoa; Protozoan Diseases; Slime Molds

Kenneth S. Saladin

Bibliography

Margulis, L., J. O. Corlis, M. Melkonian, and D. J. Chapman. Handbook of Protoctista, Boston: Jones & Bartlett, 1990.

Margulis, L., and K. Schwartz. The Five Kingdoms, 3rd ed. New York: W. H. Freeman and Company, 1998.

Patterson, D. J. "The Diversity of Eukaryotes." American Naturalist 154 (1999): S96S124.

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Protista

Protista (prōtĬs´tə) or Protoctista (prō´tŏktĬs´tə), in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms. Protists, which are eukaryotes, have cells that have a membrane-bound nucleus, DNA that is associated with histone proteins, and organelles (e.g., mitochondria and chloroplasts). A recently proposed system of classification designates the eukaryotes as one of three great groups of life (beside bacteria and archaea) and places the protists within it.

It has been hypothesized that the organelles in protists descend evolutionarily from specialized symbiotic bacteria living within the cells of other bacteria, contributing at least in part to the transition from prokaryotic (bacterial) cells (the earliest form of life on the planet, dating back at least 3.5 billion years) to early eukaryotic cells (the cells that define protists, dating back 1.5 billion years) and the more complex life forms of later plants and animals.

The protists comprise a very diverse group of organisms. They include some algae, the protozoans, and multicellular or multinucleate autotrophs, such as the water molds. Many have flagella that enable them to move about. Before the advent of modern biochemistry and the electron microscope, these organisms were fit into the plant and animal kingdoms. It is now thought that, although green plants probably evolved from the green algae and animals from some other early forms, most modern protists have followed independent evolutionary lines. There are approximately 60,000 living species of protists.

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Protista

Protista (domain Eukarya) A paraphyletic kingdom comprising single-celled and multicelled eukaryotes that are not classified as animals, green plants, or true fungi. There are about 60 types and about 200000 species of protists. These include the Oomycota, parasitic and free-living protozoa, and various single-celled and multicelled algae (see ALGA). The multicellular organisms arose from the protists by various independent routes, possibly as many as 17. They were already a diverse group 1000 Ma ago. Protoctista is sometimes used as an alternative name. In earlier classifications the Protista formed a kingdom (or superphylum or phylum) of relatively simple, undifferentiated organisms.

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Protista

Protista A kingdom (superkingdom Eukarya) comprising single-celled eukaryotes that are neither animals (developing from a blastula) nor plants (developing from an embryo). Animal-like members include naked and shelled amoebas, foraminiferans, zooflagellates, and ciliates; plant-like members include dinoflagellates, diatoms, and algae. In the widely used five-kingdom system of classification some multicellular organisms with protist affinities but previously classed as fungi or plants have been transferred into this kingdom and the name of the kingdom changed to Protoctista, which is now the name most often used.

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Protista

Protista(Protoctista) A kingdom in the domainEucaryota that comprises single-celled, eukaryotic organisms (see eukaryote) that may resemble animals or plants. Naked and shelled amoebas, foraminiferans, zooflagellates, ciliates, dinoflagellates, diatoms, and algae are protists. In an earlier five-kingdom classification, Protista was ranked as a kingdom; later some multicellular organisms with protist affinities but previously classed as fungi or plants were transferred into the Protista and the name was changed to Protoctista. Protoctista is now a synonym for Protista.

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Protista

Protista In some classifications, a kingdom containing unicellular eukaryotes that cannot be classified as animals, plants, or fungi. Originally proposed by Ernst Haeckel in 1866 to include the algae, bacteria, fungi, and protozoa, it was later restricted first to unicellular organisms, and then to protozoa, unicellular algae, and organisms then regarded as simple fungi. In most modern classifications it has been replaced by the Protoctista.

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Protista

Protista See PROTIST.

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Protista

Protista

Background

Classification

Protozoa

Algae

Slime molds and water molds

Disease-causing protists

Beneficial protists

Resources

The kingdom Protista is the most diverse of all of the eukaryotic kingdoms. It is certainly not monophyletic and its members really only share the fact that they have a simple structure, without any obvious tissues or organs. There are more than 200,000 known species of protists with many more yet to be discovered. Protists inhabit just about any area where water is found some or all of the time. They form the base of ecosystems as primary producers, as is the case with photosynthetic protists, or as low trophic level consumers that are in turn eaten by larger organisms. They range in size from microscopic, unicellular organisms to huge seaweeds that can grow up to 300 ft (100 m) long. The word protist comes from the Greek word for the very first, which indicates that protists were the first eukaryotes.

Background

The German zoologist Ernst Haeckel (18341919) first proposed the kingdom Protista in 1866. This early classification included any microorganism that was not a plant or an animal. Biologists did not readily accept this kingdom, and even after the American botanist Herbert F. Copeland again tried to establish its use 90 years later, there was not much support from the scientific community. Around 1960, R.Y. Stanier and C.B. Van Niel (18971985) proposed the division of all organisms into two groups, the prokaryotes and the eukaryotes. Eukaryotes are organisms that have membrane-bound organelles in which metabolic processes take place, while prokaryotes lack these structures. In 1969, Robert Whittaker proposed the five-kingdom system of classification. The kingdom Protista was one of the five proposed kingdoms. At this time, only unicellular eukaryotic organisms were considered protists. Since then, the kingdom has expanded to include multicellular organisms, although biologists still disagree about what exactly makes an organism a protist.

Classification

Protists are difficult to characterize because of the great diversity of the kingdom. These organisms vary in body form, nutrition, and reproduction. They may be unicellular, colonial, or multicellular. As eukaryotes, protists can have many different organelles, including a nucleus, mitochondria, contractile vacuoles, food vacuoles, eyespots, plastids, pellicles, and flagella. The nuclei of protists contain chromosomes, with DNA associated with proteins. Protists are also capable of sexual,

as well as asexual reproduction, meiosis, and mitosis. Protists can be free-living, or they may live symbiotically with another organism. This symbiosis can be mutualistic, where both partners benefit, or parasitic, where the protist uses its host as a source of food or shelter while providing no advantage to the other organism.

Many protists are economically important and beneficial to humans, while others cause fatal diseases. Protists make up the majority of the plankton in aquatic systems, where they serve as the base of the food chain. Many protists are motile, using structures such as cilia, flagella, or pseudopodia (false feet) to move, while others are sessile. They may be autotrophs, producing their own food from sunlight, or heterotrophs, requiring an outside source of nutrition. Researchers are currently comparing the RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) sequences of the protists with those of plants and animals, but the evidence is inconclusive. It is unknown whether protists were the precursors to plants, animals, or fungi. It is likely that several lines of protists evolved separately. Biologists consider the protists as a polyphyletic group, meaning they probably do not share a common ancestor.

Despite the great diversity evident in this kingdom, scientists classify the protests into three main categories: animal-like, plant-like, and fungus-like. Grouping into one of the three categories is based on an organisms mode of reproduction, method of nutrition, and motility. The animal-like protists are known as the protozoa, the plant-like protists are the algae, and the fungus-like protists are the slime molds and water molds.

Protozoa

The protozoa are all unicellular heterotrophs. They obtain their nutrition by ingesting other organisms or dead organic material. The word protozoa comes from the Latin word for first animals. Classification among protozoans is, to some extent, on modes of locomotion. Protozoans that use pseudopodia to move are known as amoebas, those that use flagella are called flagellates, those that use cilia are known as the ciliates, and those that do not move are called the sporozoans. Sporozoans are sessile and they are often parasitic, since they cannot actively capture food. They live in an area that has a constant food supply, such as the intestines or bloodstream of an animal host.

Amoebas belong to the phylum Rhizopoda. They have no wall outside of their cell membrane, giving the cell flexibility and allowing it to change shape. The word amoeba, in fact, comes from the Greek word for change. Amoebas use extensions of their cell membrane, called pseudopodia, to move as well as to engulf food. When the pseudopodium traps a bit of food, the cell membrane closes around the meal. This encasement forms a food vacuole. Digestive enzymes are secreted into the food vacuole, which break down the food. The cell then absorbs the nutrients. Because amoebas live in water, dissolved nutrients from the environment can diffuse directly through their cell membranes. Most amoebas live in marine environments, although some freshwater species exist. Freshwater amoebas live in a hypotonic environment, so water is constantly moving into the cell by osmosis. To remedy this problem, these amoebas use contractile vacuoles to pump excess water out of the cell. Most amoebas reproduce asexually by pinching off a part of the cell membrane to form a new organism. Amoebas may form cysts when environmental conditions become unfavorable. These cysts can survive conditions such as lack of water or nutrients. Two forms of amoebas have shells, the foraminiferans and the radiolarians.

The foraminiferans have a hard shell made of calcium carbonate. These shells are called tests. Foraminiferans live in marine environments and are very abundant. When they die, their shells sink and become a part of the muddy ocean floor. Geologists use the fossilized shells to determine the ages of rocks and sediments. The shells at the ocean floor are gradually converted into chalky deposits, which can be uplifted to become a land formation, such as the white cliffs of Dover in England. Radiolarians have shells made of silica instead of calcium carbonate. Both organisms have many tiny holes in their shells, through which they extend their pseudopodia. The pseudopodia act as a sticky net, trapping bits of food.

Flagellates, also called zooflagellates, may have one or more flagella and they belong to the phylum Zoomastigina. Flagellates whip their flagella from side to side in order to move through their aquatic surroundings. Flagellates are mostly unicellular with a spherical or oblong shape. A few are also amoeboid. Many ingest their food through a primitive mouth, called the oral groove. While most are motile, one class of flagellates, called the Choanoflagellates, is mostly made up of sessile species. These organisms attach to a rock or other substrate by a stalk.

The ciliates are members of the phylum Ciliophora. There are approximately 8,000 species of ciliates. These organisms move by the synchronized beating of the cilia covering their bodies. They can be found almost anywhere in freshwater or marine environments. Probably the best-known ciliate is the organism Paramecium. Paramecia have many well-developed organelles. Food enters the cell through the oral groove (lined with cilia, to sweep the food into the cell), where it moves to the gullet, which packages the meal into a food vacuole. Enzymes released into the food vacuole break down the food, and the nutrients are absorbed into the cell. Wastes are removed from the cell through an anal pore. Contractile vacuoles pump out excess water, since paramecia live in freshwater (hypotonic) surroundings.

Paramecia have two nuclei, a macronucleus and a micronucleus. The larger macronucleus controls most of the metabolic functions of the cell. The smaller micro-nucleus controls much of the pathways involved in sexual reproduction. Thousands of cilia appear through the pellicle, a tough, protective covering surrounding the cell membrane. These cilia beat in a synchronized fashion to move the Paramecium in any direction. Underneath the pellicle are trichocysts, which discharge tiny spikes that help trap prey. Paramecia usually reproduce asexually, when the cell divides into two new organisms after all of the organelles have been duplicated. When conditions are unfavorable, however, the organism can reproduce sexually. This form of sexual reproduction is called conjugation. During conjugation, two paramecia join at the oral groove, where they exchange genetic material. They then separate and divide asexually, although this division does not necessarily occur immediately following the exchange of genetic material.

The sporozoans belong to the phylum Sporozoa. These organisms are sessile, so they cannot capture prey. Therefore, the sporozoans are all parasites. As their name suggests, many of these organisms produce spores, reproductive cells that can give rise to a new organism. Sporozoans typically have complex life cycles, as they usually live in more than one host in their lifetimes.

Algae

The plant-like protists, or algae, are photosynthetic autotrophs. These organisms form the base of many food chains. Other creatures depend on these algae either directly for food or indirectly for the oxygen they produce. Algae are responsible for over half of the oxygen produced by photosynthesizing organisms. Many macroscopic forms of algae look like plants, but they differ in many ways. Algae do not have roots, stems, or leaves. They do not have the waxy cuticle plants have to prevent water loss. As a result, algae must live in areas where water is readily available. Algae also do not have multicellular game-tangia as the plants do. They contain chlorophyll along with other photosynthetic pigments. These pigments give the algae characteristic colors and are used to classify algae into various groups. Other characteristics used to classify algae are energy reserve storage and cell wall composition.

Members of the phylum Euglenophyta are known as euglenoids. These organisms are both autotrophic and heterotrophic. There are hundreds of species of euglenoids. Euglenoids are unicellular and share properties of both plants and animals. They are plant-like in that they contain chlorophyll and are capable of photosynthesis. They do not have a cell wall of cellulose, as do plants; instead, they have a pellicle made of protein. Euglenoids are like animals in that they are motile and responsive to outside stimuli. One particular species, Euglena, has a structure called an eyespot. This is an area of red pigments that is sensitive to light. An Euglena can respond to its environment by moving towards areas of bright light, where photosynthesis best occurs. In conditions where light is not available for photosynthesis, euglenoids can be heterotrophic and ingest their food. Euglenoids store their energy as paramylon, a type of polysaccharide.

Members of the phylum Bacillariophyta are called diatoms. Diatoms are unicellular organisms with silica shells. They are autotrophs and can live in marine or freshwater environments. They contain chlorophyll as well as pigments called carotenoids, which give them an orange-yellow color. Their shells resemble small boxes with lids. These shells are covered with grooves and pores, giving them a decorated appearance. Diatoms can be either radially or bilaterally symmetrical. Diatoms reproduce asexually in a very unique manner. The two halves of the shell separate, each producing a new shell that fits inside the original half. Each new generation, therefore, produces offspring that are smaller than the parent. As each generation gets smaller and smaller, a lower limit is reached, approximately one quarter the original size. At this point, the diatom produces gametes that fuse with gametes from other diatoms to produce zygotes. The zygotes develop into full sized diatoms that can begin asexual reproduction once more. When diatoms die, their shells fall to the bottom of the ocean and form deposits called diatomaceous earth. These deposits can be collected and used as abrasives, or used as an additive to give certain paints their sparkle. Diatoms store their energy as oils or carbohydrates.

The dinoflagellates are members of the phylum Dinoflagellata. These organisms are unicellular autotrophs. Their cell walls contain cellulose, creating thick, protective plates. These plates contain two grooves at right angles to each other, each groove containing one flagellum. When the two flagella beat together, they cause the organism to spin through the water. Most dinoflagellates are marine organisms, although some have been found in freshwater environments. Dinoflagellates contain chlorophyll as well as carotenoids and red pigments. They can be free-living, or live in symbiotic relationships with jellyfish or corals. Some of the free-living dinoflagellates are bioluminescent. Many dinoflagellates produce strong toxins. One species in particular, Gonyaulax catanella, produces a lethal nerve toxin. These organisms sometimes reproduce in huge amounts in the summertime, causing a red tide. There are so many of these organisms present during a red tide that the ocean actually appears red. When this occurs, the toxins released by the dinoflagellates ca reach such high concentrations in the ocean that many fish are killed. Dinoflagellates store their energy as oils or polysaccharides.

The phylum Rhodophyta includes the red algae. Nearly all of the 4,000 species in this phylum are multicellular and live in marine environments. Red algae are typically found in tropical waters and sometimes along the coasts in cooler areas. They live attached to rocks by a structure called a holdfast. Their cell walls contain thick polysaccharides. Some species incorporate calcium carbonate from the ocean into their cell walls as well. Red algae contain chlorophyll as well as phycobilins, red and blue pigments involved in photosynthesis. The red pigment is called phycoerythrin and the blue pigment is called phycocyanin. Phycobilins absorb the green, violet, and blue light waves that can penetrate deep water. These pigments allow the red algae to photosynthesize in deep water with little light available. Reproduction in these organisms is a complex alternation between sexual and asexual phases. Red algae store their energy as floridean starch.

The 1,500 species of brown algae are members of the phylum Phaeophyta. The majority of the brown algae live in marine environments, on rocks in cool waters. They contain chlorophyll as well as a yellow-brown carotenoid called fucoxanthin. The largest of the brown algae are the kelp. The kelp use holdfasts to attach to rocks. The body of a kelp is called a thallus, which can grow as long as 180 ft (60 m). The thallus is composed of three sections, the holdfast, the stipe, and the blade. Some species of brown algae have an air bladder to keep the thallus floating at the surface of the water, where more light is available for photosynthesis. Brown algae store their energy as laminarin, a carbohydrate.

The members of the phylum Chlorophyta are known as the green algae. This phylum is the most diverse of all the algae, with greater than 7,000 species. The green algae contain chlorophyll as their main pigment. Most live in fresh water, although some marine species exist. Their cell walls are composed of cellulose, which indicates the green algae may be the ancestors of modern plants. Green algae can be unicellular, colonial, or multicellular. An example of a unicellular green alga is Chlamydomonas. The algae Volvox is a colonial chlorophyte. A Volvox colony is a hollow sphere of thousands of individual cells. Each cell has a single flagellum that faces the exterior of the sphere. The individual cells beat their flagella in a coordinated fashion, allowing the colony to move. Daughter colonies form inside the sphere, growing until they reach a certain size and are released when the parent colony breaks open. Spirogyra and Ulva are both examples of multicellular green algae. Reproduction in the green algae can be both sexual and asexual. Green algae store their energy as starch.

Slime molds and water molds

The fungus-like protists resemble the fungi during some part of their life cycle. The slime molds and the water molds are members of this group. They all obtain energy by decomposing organic materials, and as a result, are important for recycling nutrients. They can be brightly colored and live in cool, moist, dark habitats. The slime molds are classified as either plasmodial or cellular by their modes of reproduction. The plasmodial slime molds belong to the phylum Myxomycota, and the cellular slime molds belong to the phylum Acrasiomycota.

The plasmodial slime molds form a structure called a plasmodium, a mass of cytoplasm that contains many nuclei but has no cell walls or membranes to separate individual cells. The plasmodium is the feeding stage of the slime mold. It moves much like an amoeba, slowly sneaking along decaying organic material. It moves at a rate of 1 in (2.5 cm) per hour, engulfing microorganisms. The reproductive structure of plasmodial slime molds occurs during unfavorable conditions when the

KEY TERMS

Bilateral symmetry Body plan in which the left and right halves of the animal are mirror images of each other.

Bioluminescent A flashing of light that emanates from an organism.

Cilia Short projections consisting of microtubules that cover the surface of some cells and provide for movement.

Colonial A member of a localized population of organisms.

Contractile vacuole In some protistans, a membranous chamber that takes up excess water in the cell body, then contracts, expelling the water outside the cell through a pore.

Flagellum Tail-like motile structure of many free-living eukaryotic cells.

Food vacuole A membranous chamber that engulfs food and secretes digestive enzymes to break down the food into nutrients.

Gamete Specialized cells capable of fusion in the sexual cycle; female gametes are termed egg cells; male gametes may be zoospores or sperm cells.

Hypotonic A solution with a lower salt concentration than inside a cell.

Meiosis Two-stage nuclear division process that is the basis of gamete formation and of spore formation.

Mitochondria An organelle that specializes in ATP formation, the powerhouse of the cell.

Mitosis Type of nuclear division that maintains the parental chromosome number for daughter cells, the basis of bodily growth, and asexual reproduction.

Motile Able to move.

Multicellular More than one cell.

Nucleus A membrane-bound organelle that isolates and organizes the DNA.

Organelle An internal, membrane-bound sac or compartment that has a specific, specialized metabolic function.

Osmosis The diffusion of water from an area of high concentration to low concentration through a membrane.

Plankton Any community of floating organisms, mostly microscopic, living in freshwater and marine environments.

Radial symmetry An arrangement of the floral parts characterized by their radiation from the center of the flower, like spokes on a bicycle wheel.

Unicellular Single celled.

Zygote The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

plasmodium forms a stalked structure. This structure produces spores that can be released and travel large distances. The spores land and produce a zygote that grows into a new plasmodium.

The cellular slime molds exist as individual cells during the feeding stage. These cells can move like an amoeba as well, engulfing food as they move. The feeding cells reproduce asexually through cell division. When conditions become unfavorable, the cells come together to form a large mass of cells resembling a plasmodium. This mass of cells can move as one organism and looks much like a garden slug. The mass eventually develops into a stalked structure capable of sexual reproduction.

The water molds and downy mildews belong to the phylum Oomycota. They grow on the surface of dead organisms or plants, decomposing the organic material and absorbing nutrients. Most live in water or in moist areas. Water molds grow as a mass of fuzzy white threads on dead material. The difference between these organisms and true fungi is the water molds form flagellated reproductive cells during their life cycles.

Disease-causing protists

Many protists can cause serious illness and disease. Malaria, for example, is caused by the protist Plasmodium. Plasmodia are sporozoans and are transferred from person to person through female Anopheles mosquitoes. People who suffer from malaria experience symptoms such as shivering, sweating, high fevers, and delirium. African sleeping sickness, also known as African trypanosomiasis, is caused by another sporozoan, Trypanosoma. Trypanosoma is transmitted through the African tsetse fly. This organism causes high fever and swollen lymph nodes. Eventually the protist makes its way into the victims brain, where it causes a feeling of uncontrollable fatigue. Giardiasis is another example of a disease caused by a protist. This illness is caused by Giardia, a sporozoan carried by muskrats and beavers. Giardiasis is characterized by fatigue, cramps, diarrhea, and weight loss. Amoebic dysentery occurs when a certain amoeba, Entamoeba histolytica, infects the large intestine of humans. It is spread through infected food and water. This organism causes bleeding, diarrhea, vomiting, and sometimes death.

Beneficial protists

Members of the kingdom Protista can also be very beneficial. Many species of red algae are edible and are popular foods in certain parts of the world. Red algae are rich in vitamins and minerals. Carageenan, a polysaccharide extracted from red algae, is used as a thickening agent in ice cream and other foods. Giant kelp forests are rich ecosystems, providing food and shelter for many organisms. Trichonymphs are flagellates that live in the intestines of termites. These protozoans break down cellulose in wood into carbohydrates the termites can digest.

Resources

BOOKS

Cambell, Neil A. and Jane B. Reece. Biology. San Francisco, CA: Benjamin Cummings, 2004.

Tortora, Gerard J., Berdell R. Funke and Christine L. Case. Microbiology: An Introduction. San Francisco, CA: Benjamin Cummings, 2006.

PERIODICALS

Adl, S. M. et al. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology. 52 (2005): 399-451.

OTHER

University of California Museum of Paleontology. Eukaryota: Systematics.<http://www.ucmp.berkeley.edu/alllife/eukaryotasy.html> (accessed October 27, 2006).

University of Montreal. Protist Image Data. April 12, 2004.<http://megasun.bch.umontreal.ca/protists/protists.html> (accessed October 27, 2006).

Jennifer McGrath

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Protista

Protista

The Kingdom Protista is the most diverse of all six kingdoms. There are more than 200,000 known species of protists with many more yet to be discovered. The protists can be found in countless colors, sizes, and shapes. They inhabit just about any area where water is found some or all of the time. They form the base of ecosystems by making food, as is the case with photosynthetic protists, or by themselves being eaten by larger organisms. They range in size from microscopic, unicellular organisms to huge seaweeds that can grow up to 300 ft (100 m) long.


Background

The German zoologist Ernst Haeckel (1834-1919) first proposed the kingdom Protista in 1866. This early classification included any microorganism that was not a plant or an animal . Biologists did not readily accept this kingdom, and even after the American botanist Herbert F. Copeland again tried to establish its use 90 years later, there was not much support from the scientific community. Around 1960, R.Y. Stanier and C.B. Van Niel (1897-1985) proposed the division of all organisms into two groups, the prokaryotes and the eukaryotes. Eukaryotes are organisms that have membrane-bound organelles in which metabolic processes take place, while prokaryotes lack these structures. In 1969, Robert Whittaker proposed the five-kingdom system of classification. The kingdom Protista was one of the five proposed kingdoms. At this time, only unicellular eukaryotic organisms were considered protists. Since then, the kingdom has expanded to include multicellular organisms, although biologists still disagree about what exactly makes an organism a protist.



Classification

Protists are difficult to characterize because of the great diversity of the kingdom. These organisms vary in body form, nutrition , and reproduction. They may be unicellular, colonial, or multicellular. As eukaryotes, protists can have many different organelles, including a nucleus, mitochondria, contractile vacuoles, food vacuoles, eyespots, plastids, pellicles, and flagella . The nuclei of protists contain chromosomes, with DNA associated with proteins . Protists are also capable of sexual, as well as asexual reproduction , meiosis , and mitosis . Protists can be free-living, or they may live symbiotically with another organism. This symbiosis can be mutualistic, where both partners benefit, or parasitic, where the protist uses its host as a source of food or shelter while providing no advantage to the other organism. Many protists are economically important and beneficial to mankind, while others cause fatal diseases. Protists make up the majority of the plankton in aquatic systems, where they serve as the base of the food chain. Many protists are motile, using structures such as cilia, flagella, or pseudopodia (false feet) to move, while others are sessile. They may be autotrophs, producing their own food from sunlight, or heterotrophs, requiring an outside source of nutrition. Researchers are currently comparing the RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) sequences of the protists with those of plants and animals, but the evidence is inconclusive. It is unknown whether protists were the precursors to plants, animals, or fungi . It is possible that several evolutionary lines of protists developed separately. Biologists consider the protists as a polyphyletic group, meaning they probably do not share a common ancestor. The word protist comes from the Greek word for the very first, which indicates that researchers believe protists may have been the first eukaryotes to evolve on Earth .

Despite the great diversity evident in this kingdom, scientists have been able to classify the protists into several groups. The protists can be classified into one of three main categories, animal-like, plant-like, and fungus-like. Grouping into one of the three categories is based on an organism's mode of reproduction, method of nutrition, and motility. The animal-like protists are known as the protozoa , the plant-like protists are the algae , and the fungus-like protists are the slime molds and water molds.

Protozoa

The protozoa are all unicellular heterotrophs. They obtain their nutrition by ingesting other organisms or dead organic material. The word protozoa comes from the Latin word for first animals. The protozoans are grouped into various phyla based on their modes of locomotion. They may use cilia, flagella, or pseudopodia. Some protozoans are sessile, meaning they do not move. These organisms are parasitic, since they cannot actively capture food. They must live in an area of the host organism that has a constant food supply, such as the intestines or bloodstream of an animal. The protozoans that use pseudopodia to move are known as amoebas, those that use flagella are called flagellates, those that use cilia are known as the ciliates, and those that do not move are called the sporozoans.

The amoebas belong to the phylum Rhizopoda. These protists have no wall outside of their cell membrane . This gives the cell flexibility and allows it to change shape. The word amoeba , in fact, comes from the Greek word for change. Amoebas use extensions of their cell membrane (called pseudopodia) to move, as well as, to engulf food. When the pseudopodium traps a bit of food, the cell membrane closes around the meal. This encasement forms a food vacuole. Digestive enzymes are secreted into the food vacuole, which break down the food. The cell then absorbs the nutrients . Because amoebas live in water, dissolved nutrients from the environment can diffuse directly through their cell membranes. Most amoebas live in marine environments, although some freshwater species exist. Freshwater amoebas live in a hypotonic environment, so water is constantly moving into the cell by osmosis . To remedy this problem, these amoebas use contractile vacuoles to pump excess water out of the cell. Most amoebas reproduce asexually by pinching off a part of the cell membrane to form a new organism. Amoebas may form cysts when environmental conditions become unfavorable. These cysts can survive conditions such as lack of water or nutrients. Two forms of amoebas have shells, the foraminiferans and the radiolarians.

The foraminiferans have a hard shell made of calcium carbonate . These shells are called tests. Foraminiferans live in marine environments and are very abundant. When they die, their shells fall to the ground where they become a part of the muddy ocean floor. Geologists use the fossilized shells to determine the ages of rocks and sediments. The shells at the ocean floor are gradually converted into chalky deposits, which can be uplifted to become a land formation, such as the white cliffs of Dover in England. Radiolarians have shells made of silica instead of calcium carbonate. Both organisms have many tiny holes in their shells, through which they extend their pseudopodia. The pseudopodia act as a sticky net, trapping bits of food.

The flagellates have one or more flagella and belong to the phylum Zoomastigina. These organisms whip their flagella from side to side in order to move through their aquatic surroundings. These organisms are also known as the zooflagellates. The flagellates are mostly unicellular with a spherical or oblong shape. A few are also amoeboid. Many ingest their food through a primitive mouth, called the oral groove. While most are motile, one class of flagellates, called the Choanoflagellates, is sessile. These organisms attach to a rock or other substrate by a stalk.

The ciliates are members of the phylum Ciliophora. There are approximately 8,000 species of ciliates. These organisms move by the synchronized beating of the cilia covering their bodies. They can be found almost anywhere, in freshwater or marine environments. Probably the best-known ciliate is the organism Paramecium. Paramecia have many well-developed organelles. Food enters the cell through the oral groove (lined with cilia, to "sweep" the food into the cell), where it moves to the gullet, which packages the meal into a food vacuole. Enzymes released into the food vacuole break down the food, and the nutrients are absorbed into the cell. Wastes are removed from the cell through an anal pore. Contractile vacuoles pump out excess water, since paramecia live in freshwater (hypotonic) surroundings. Paramecia have two nuclei, a macronucleus and a micronucleus. The larger macronucleus controls most of the metabolic functions of the cell. The smaller micronucleus controls much of the pathways involved in sexual reproduction . Thousands of cilia appear through the pellicle, a tough, protective covering surrounding the cell membrane. These cilia beat in a synchronized fashion to move the Paramecium in any direction. Underneath the pellicle are trichocysts, which discharge tiny spikes that help trap prey . Paramecia usually reproduce asexually, when the cell divides into two new organisms after all of the organelles have been duplicated. When conditions are unfavorable, however, the organism can reproduce sexually. This form of sexual reproduction is called conjugation. During conjugation, two paramecia join at the oral groove, where they exchange genetic material. They then separate and divide asexually, although this division does not necessarily occur immediately following the exchange of genetic material.

The sporozoans belong to the phylum Sporozoa. These organisms are sessile, so they cannot capture prey. Therefore, the sporozoans are all parasites . As their name suggests, many of these organisms produce spores, reproductive cells that can give rise to a new organism. Sporozoans typically have complex life cycles, as they usually live in more than one host in their lifetimes.


Algae

The plant-like protists, or algae, are all photosynthetic autotrophs. These organisms form the base of many food chains. Other creatures depend on these protists either directly for food or indirectly for the oxygen they produce. Algae are responsible for over half of the oxygen produced by photosynthesizing organisms. Many forms of algae look like plants, but they differ in many ways. Algae do not have roots, stems, or leaves. They do not have the waxy cuticle plants have to prevent water loss. As a result, algae must live in areas where water is readily available. Algae do not have multicellular gametangia as the plants do. They contain chlorophyll , but also contain other photosynthetic pigments. These pigments give the algae characteristic colors and are used to classify algae into various phyla. Other characteristics used to classify algae are energy reserve storage and cell wall composition.

Members of the phylum Euglenophyta are known as euglenoids. These organisms are both autotrophic as well as heterotrophic. There are hundreds of species of euglenoids. Euglenoids are unicellular and share properties of both plants and animals. They are plant-like in that they contain chlorophyll and are capable of photosynthesis . They do not have a cell wall of cellulose , as do plants; instead, they have a pellicle made of protein. Euglenoids are like animals in that they are motile and responsive to outside stimuli. One particular species, Euglena, has a structure called an eyespot. This is an area of red pigments that is sensitive to light . An Euglena can respond to its environment by moving towards areas of bright light, where photosynthesis best occurs. In conditions where light is not available for photosynthesis, euglenoids can be heterotrophic and ingest their food. Euglenoids store their energy as paramylon, a type of polysaccharide.

Members of the phylum Bacillariophyta are called diatoms . Diatoms are unicellular organisms with silica shells. They are autotrophs and can live in marine or freshwater environments. They contain chlorophyll as well as pigments called carotenoids, which give them an orange-yellow color . Their shells resemble small boxes with lids. These shells are covered with grooves and pores, giving them a decorated appearance. Diatoms can be either radially or bilaterally symmetrical. Diatoms reproduce asexually in a very unique manner. The two halves of the shell separate, each producing a new shell that fits inside the original half. Each new generation, therefore, produces offspring that are smaller than the parent. As each generation gets smaller and smaller, a lower limit is reached, approximately one quarter the original size. At this point, the diatom produces gametes that fuse with gametes from other diatoms to produce zygotes. The zygotes develop into full sized diatoms that can begin asexual reproduction once more. When diatoms die, their shells fall to the bottom of the ocean and form deposits called diatomaceous earth. These deposits can be collected and used as abrasives , or used as an additive to give certain paints their sparkle. Diatoms store their energy as oils or carbohydrates.

The dinoflagellates are members of the phylum Dinoflagellata. These organisms are unicellular autotrophs. Their cell walls contain cellulose, creating thick, protective plates. These plates contain two grooves at right angles to each other, each groove containing one flagellum. When the two flagella beat together, they cause the organism to spin through the water. Most dinoflagellates are marine organisms, although some have been found in freshwater environments. Dinoflagellates contain chlorophyll as well as carotenoids and red pigments. They can be free-living, or live in symbiotic relationships with jellyfish or corals. Some of the free-living dinoflagellates are bioluminescent. Many dinoflagellates produce strong toxins. One species in particular, Gonyaulax catanella, produces a lethal nerve toxin. These organisms sometimes reproduce in huge amounts in the summertime, causing a red tide . There are so many of these organisms present during a red tide that the ocean actually appears red. When this occurs, the toxins that are released reach such high concentrations in the ocean that many fish are killed. Dinoflagellates store their energy as oils or polysaccharides.

The phylum Rhodophyta consists of the red algae. All of the 4,000 species in this phylum are multicellular (with the exception of a few unicellular species) and live in marine environments. Red algae are typically found in tropical waters and sometimes along the coasts in cooler areas. They live attached to rocks by a structure called a holdfast. Their cell walls contain thick polysaccharides. Some species incorporate calcium carbonate from the ocean into their cell walls as well. Red algae contain chlorophyll as well as phycobilins, red and blue pigments involved in photosynthesis. The red pigment is called phycoerythrin and the blue pigment is called phycocyanin. Phycobilins absorb the green, violet, and blue light waves that can penetrate deep water. These pigments allow the red algae to photosynthesize in deep water with little light available. Reproduction in these organisms is a complex alternation between sexual and asexual phases. Red algae store their energy as floridean starch.

The 1,500 species of brown algae are the members of the phylum Phaeophyta. The majority of the brown algae live in marine environments, on rocks in cool waters. They contain chlorophyll as well as a yellow-brown carotenoid called fucoxanthin. The largest of the brown algae are the kelp. The kelp use holdfasts to attach to rocks. The body of a kelp is called a thallus, which can grow as long as 180 ft (60 m). The thallus is composed of three sections, the holdfast, the stipe, and the blade. Some species of brown algae have an air bladder to keep the thallus floating at the surface of the water, where more light is available for photosynthesis. Brown algae store their energy as laminarin, a carbohydrate .

The phylum Chlorophyta is known as the green algae. This phylum is the most diverse of all the algae, with greater than 7,000 species. The green algae contain chlorophyll as their main pigment. Most live in fresh water, although some marine species exist. Their cell walls are composed of cellulose, which indicates the green algae may be the ancestors of modern plants. Green algae can be unicellular, colonial, or multicellular. An example of a unicellular green alga is Chlamydomonas. An example of a colonial algae is Volvox. A Volvox colony is a hollow sphere of thousands of individual cells. Each cell has a single flagellum that faces the exterior of the sphere. The individual cells beat their flagella in a coordinated fashion, allowing the colony to move. Daughter colonies form inside the sphere, growing until they reach a certain size and are released when the parent colony breaks open. Spirogyra and Ulva are both examples of multicellular green algae. Reproduction in the green algae can be both sexual and asexual. Green algae store their energy as starch.


Slime molds and water molds

The fungus-like protists resemble the fungi during some part of their life cycle. These organisms exhibit properties of both fungi and protists. The slime molds and the water molds are members of this group. They all obtain energy by decomposing organic materials, and as a result, are important for recycling nutrients. They can be brightly colored and live in cool, moist, dark habitats. The slime molds are classified as either plasmodial or cellular by their modes of reproduction. The plasmodial slime molds belong to the phylum Myxomycota, and the cellular slime molds belong to the phylum Acrasiomycota.

The plasmodial slime molds form a structure called a plasmodium, a mass of cytoplasm that contains many nuclei but has no cell walls or membranes to separate individual cells. The plasmodium is the feeding stage of the slime mold . It moves much like an amoeba, slowly sneaking along decaying organic material. It moves at a rate of 1 in (2.5 cm) per hour, engulfing microorganisms . The reproductive structure of plasmodial slime molds occurs when the plasmodium forms a stalked structure during unfavorable conditions. This structure produces spores that can be released and travel large distances. The spores land and produce a zygote that grows into a new plasmodium.

The cellular slime molds exist as individual cells during the feeding stage. These cells can move like an amoeba as well, engulfing food along the way. The feeding cells reproduce asexually through cell division . When conditions become unfavorable, the cells come together to form a large mass of cells resembling a plasmodium. This mass of cells can move as one organism and looks much like a garden slug. The mass eventually develops into a stalked structure capable of sexual reproduction.

The water molds and downy mildews belong to the phylum Oomycota. They grow on the surface of dead organisms or plants, decomposing the organic material and absorbing nutrients. Most live in water or in moist areas. Water molds grow as a mass of fuzzy white threads on dead material. The difference between these organisms and true fungi is the water molds form flagellated reproductive cells during their life cycles.


Disease-causing protists

Many protists can cause serious illness and disease . Malaria , for example, is caused by the protist Plasmodium. Plasmodia are sporozoans and are transferred from person to person through female Anophelesmosquitoes . People who suffer from malaria experience symptoms such as shivering, sweating, high fevers, and delirium. African sleeping sickness , also known as African trypanosomiasis, is caused by another sporozoan, Trypanosoma. Trypanosoma is transmitted through the African tsetse fly. This organism causes high fever and swollen lymph nodes. Eventually the protist makes its way into the victim's brain , where it causes a feeling of uncontrollable fatigue. Giardiasis is another example of a disease caused by a protist. This illness is caused byGiardia, a sporozoan carried by muskrats and beavers . Giardiasis is characterized by fatigue, cramps, diarrhea, and weight loss. Amoebic dysentery occurs when a certain amoeba, Entamoeba histolytica, infects the large intestine of humans. It is spread through infected food and water. This organism causes bleeding, diarrhea, vomiting, and sometimes death.


Beneficial protists

Members of the kingdom Protista can also be very beneficial to life on Earth. Many species of red algae are edible and are popular foods in certain parts of the world. Red algae are rich in vitamins and minerals . Carageenan, a polysaccharide extracted from red algae, is used as a thickening agent in ice cream and other foods. Giant kelp forests are rich ecosystems, providing food and shelter for many organisms. Trichonymphs are flagellates that live in the intestines of termites . These protozoans break down cellulose in wood into carbohydrates the termites can digest.

The kingdom Protista is a diverse group of organisms. Some protists are harmful, but many more are beneficial. These organisms form the foundation for food chains, produce the oxygen we breathe, and play an important role in nutrient recycling. Many protists are economically useful as well. As many more of these unique organisms are discovered, humans will certainly enjoy the new uses and benefits protists provide.


Resources

books

Blaustein, Daniel. Biology: The Dynamics of Life. Westerville, OH: McGraw-Hill Companies, 1998.

Johnson, George B. Biology: Principles and Explorations. Orlando: Holt, Rinehart and Winston, 1998.

Solomon, Eldra Pearl. Biology. Orlando: Saunders College Publishing, 1999.

Starr, Cecie. Biology: Concepts and Applications. Belmont, CA: Wadsworth Publishing Company, 1997.

Tobin, Allan J. Asking About Life. Orlando: Saunders College Publishing, 1998.


Jennifer McGrath

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bilateral symmetry

—Body plan in which the left and right halves of the animal are mirror images of each other.

Bioluminescent

—A flashing of light that emanates from an organism.

Cilia

—Short projections consisting of microtubules that cover the surface of some cells and provide for movement.

Colonial

—A member of a localized population of organisms.

Contractile vacuole

—In some protistans, a membranous chamber that takes up excess water in the cell body, then contracts, expelling the water outside the cell through a pore.

Flagellum

—Tail-like motile structure of many free-living eukaryotic cells.

Food vacuole

—A membranous chamber that engulfs food and secretes digestive enzymes to break down the food into nutrients.

Gamete

—Specialized cells capable of fusion in the sexual cycle; female gametes are termed egg cells; male gametes may be zoospores or sperm cells.

Hypotonic

—A solution with a lower salt concentration than inside a cell.

Meiosis

—Two-stage nuclear division process that is the basis of gamete formation and of spore formation.

Mitochondria

—An organelle that specializes in ATP formation, the "powerhouse" of the cell.

Mitosis

—Type of nuclear division that maintains the parental chromosome number for daughter cells, the basis of bodily growth, and asexual reproduction.

Motile

—Able to move.

Multicellular

—More than one cell.

Nucleus

—A membrane-bound organelle that isolates and organizes the DNA.

Organelle

—An internal, membrane-bound sac or compartment that has a specific, specialized metabolic function.

Osmosis

—The diffusion of water from an area of high concentration to low concentration through a membrane.

Plankton

—Any community of floating organisms, mostly microscopic, living in freshwater and marine environments.

Plastid

—Of many bacteria, a small, circular molecule of extra DNA that carries only a few genes and replicates independently of the bacterial chromosome.

Radial symmetry

—An arrangement of the floral parts characterized by their radiation from the center of the flower, like spokes on a bicycle wheel.

Unicellular

—Single celled.

Zygote

—The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

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