Protoctists

In the genetic-, evolutionary-, and morphology-based five-kingdoms-of-life taxonomy, protoctists are defined as nonbacterial organisms that are neither plant, animal, nor fungal; ranging from unicellular to colonial, and from microscopic to macroscopic, protoctists are in evolutionary history the first organisms with cells that have nuclei and chromosomes. To this group belong all the beings once called protozoans—a term that must be scrapped because, literally, it means "first animals" or "pre-animals." But the protoctists are as likely to be "pre-plants"—that is, to have photo-synthetic inclusions in their cells yet not display the advanced tissue organization or development from an embryo characteristic of true plants. A typical example of a protoctist that illustrates the necessity of organizing this miscellany of eukaryotic organisms into their own kingdom—and the inadequacy of the obsolescent term protozoa—is the microbe Euglena.

Like an amoeba or sperm cell, Euglena is capable of locomoting itself through the aqueous medium, and thus it has "animal-like" characteristics. Yet, like a plant or algal cell, Euglena has green plastid—chloroplasts—in its cell. Neither plant, animal, nor "pre-animal," it is a protist. The term protoctist, from Greek words meaning, roughly, "first established being," was coined by John Hogg in 1860.

Unicellular members of the protoctist kingdom are known as protists. Although the name is ungainly and obscure, protoctists are of fundamental importance to biology, ecology, medicine, and, perhaps most intriguingly, an evolutionary understanding of who we are. In terms of biodiversity, they include many important globally distributed groups, such as amoebae; green, red, and brown algae; ciliates; diatoms; dinomastigotes (more often, if less correctly, known as dinoflagellates); flagellates (which should more logically be called mastig-otes); foraminifera; kelp; radiolaria; and slime molds. Evolutionarily, the protoctists were central players in two crucial transitions: the symbiotic genesis, some 2 billion years ago, of gene-trading bacteria into cells with nuclei; and the aggregation of these new cells with nuclei and chromosomes into the colonial ancestors to plants, animals, and fungi. (Although some still consider the green algae to be primitive plants, they are probably best considered protoctists, since they do not, like all other plants, develop from an embryo that itself comes from a fertilized egg.) Their study holds the keys not only to these major evolutionary transitions, but also to that major theme of our lives as individual animals—sex and reproduction. For it was in the protoctists, these free-living cells some of which look like bodiless eggs and sperm, that the origins of our kind of sex, meiotic sex, first evolved. Thus, although less important to medicine and genetic research than the bacteria, and less important to behavioral research and food production than animals, plants, and fungi, this previously overlooked and highly diverse kingdom is of great importance to understanding the evolution of eukaryotic cells, the origins of meiotic sex, present and past ecology, and the evolution from unicellular beings of multicellular plants, animals, and fungi.

History of Taxonomic Status of Protoctists

The term protoctista, coined by British biologist John Hogg in 1860, was used by him to refer to "organisms that are clearly neither animals nor plants." The great German champion of Charles Darwin's evolutionary ideas, Ernst Haeckel (1834-1914), had already proposed a new kingdom—Monera ("first beings")—to accommodate the taxonomi-cally equivocate microbes whose existence, revealed by the microscope, had now, with the increasing acceptance of Darwin's theories after 1859, to be grappled with in evolutionary terms. If microbes were our ancestors, and those of plants and animals, they could no longer be considered a drawing room curiosity, or only as pathogens.

Thus in 1956 biologist H. F. Copeland of the Sacramento City College in California proposed a four-kingdom refinement of Haeckel's Monera, clearly distinguishing (as Haeckel had not) between the nonnucleated prokary-otes (bacteria), and cells with nuclei (eukary-otes). Little attention was paid to Copeland's system, and the entrenched two-kingdom, plant-animal system held on. But with advances in microscopy and molecular biology in the 1960s and 1970s, the plant-animal dichotomy became increasingly untenable. Cornell University biologist R. H. Whittaker, whose studies showed how distinct bacteria and fungi are from plants, proposed a five-kingdom system, using the term protists. Copeland had used protoctists, but only to refer to single-celled organisms (leading to continuing the confusion surrounding the term protozoan), and this term was resuscitated by biologists Lynn Margulis and Karlene Schwartz in their further refinement of Whittaker's system.

Diversity of Protoctists

Today an estimated 200,000 species of protoc-tists exist, grouped into from seventeen to forty-five phyla, depending on who is doing the classifying. They are still actively being discovered. The smaller members of the great group are the protists. Protoctists range in size from tiny micromonads and chlorella algae, cells about a micrometer in diameter, to giant brown kelps meters across, at the shores of the ocean; most, however, range from 5 to 100 ^m in size. Some, such as the giant amoeba Pelomyxa palustris, which survives on reduced oxygen levels in muddy, freshwater ponds, have nuclei but no mitochondria in its cells: it thus is a potential "missing link" between early cells with nuclei and modern ones, almost all of which have mitochondria outside the nucleus involved in oxygen metabolism and energy acquisition in an oxidizing atmosphere. Despite the existence of Pelomyxa and other unusual protoctists, the metabolic diversity in this group is almost identical to that of plants, animals, and fungi— most protoctists are oxygen-breathing forms with mitochondria and, sometimes, plastids (called chloroplasts if green) in their cells. Their diversity thus tends to be morphological and behavioral, rather than biochemical or metabolic, as in the bacteria.

One area of diversity in this group that is of note is their genetic diversity. Unlike the plants and animals that arise from sexual merging of eggs and sperm, parts of protoctists can be grafted and then breed true. That is the case, for example, with protoctists with many waving appendages known as undulipodia (some times called, incorrectly, flagellates; the waving structures are fundamentally different from the flagella of bacteria). The protoctists known as ciliates have distinct patterns of cilia (undulipodia) which, if removed and replaced in a different location of the cell, will continue to be found at that place when the cell reproduces. Such genetics is the equivalent to a finger's being surgically grafted onto the leg of a man, whose children then appear with fingers on their legs. This genetic lability of cil-iates was one of the first indications that pro-toctists have genetic apparatus outside the nucleus. That discovery in turn led to the finding that all protoctists, the smaller ones of which were the first protists (equivalent to the first eukaryotes), are multiple-genome beings. Protists began from merged genetic systems of from two to four or more types of bacteria. Exploration of the genetic, morphological, and behavioral diversity of protoctists continues to play an important role in investigations of the evolution of mitosis, meiotic sex, and the establishment of mitochondria.

Amoebae

We have already mentioned the giant amoeba. It looks like a wine flask and is the largest member of the Class Arcamoebae in Phylum Archaeprotist (which includes all primitive mitochondrialess protists). Other amoebae, members of the Phylum Rhizopoda, are food-engulfing protists with retractable "limbs" called pseudopods ("false feet") that extend in a wobbly mass as the cell changes shape. There are many species of amoebae; most have chromosomes and undergo mitosis. These protoc-tists, whose nuclei can be removed and replaced, are used in experimental studies of the relationship between cell nuclei and cytoplasm, the area around the cell. Some amoebae are enveloped in tiny calcified shells, called tests. Some tests are made from sand and the shells of other organisms, held together by organic glue. Amoebae not only display the basic cell type of animals but also contain proteins, such as actin (used in animals as muscle tissue), found in "higher" organisms. With fossils from a billion years ago, this multifarious group is also known to form resistant structures, called cysts, able to withstand drying, digestion by animals, and other environmental insults.

Golden, Green, Red, Yellow-Green, Brown, and Eyespot Algae

Some words that may seem taxonomic—for example, plankton—are in fact ecological words; plankton refers to free-floating microscopic organisms, usually algae, whose transport is subject to wave movements. Technically, algae are defined as photoautotrophic protoctists—protoctists that make their own food photosynthetically. They all produce oxygen from photosynthesis, but the appellation does not include plants, which form from embryos. Nor does the group contain oxygen-producing bacteria such as cyanobacteria and chloroxybacteria. These latter beings are, nonetheless, still often called algae (especially cyanobacteria, which have historically been called "blue green algae"), in a looser way designating oxygen-producing photosynthesizers. There are more than 300 species of golden algae, whose photosynthetic cell parts, or plas-tids, are golden yellow in color and called chrysoplasts. Some scavenge silicon from the water and use it to make elaborate skeletons. Green algae, put by Copeland in the plant kingdom, are really protoctists, photosynthetic cells with nuclei that are neither plants nor cyanobacteria.

There are thousands of species of green algae. Like plants, they contain chlorophylls a and b. There are two major groups: the chlorophytes, which form (like some plants)

swimming, spermlike cells with undulipodia tails, and the gamophytes, which lack undulipodia. Green seaweed is a kind of green algae, and some closely allied species, such as those of Volvox and its relatives, show many variations on multicellularity—giving us a window into how it may have evolved. Brown algae contain the brown pigment fucoxan-thin as well as chlorophyll a. About 4,000 species of red algae or rhodophytes are known. They contain the red pigment phycoerythrin, which allows them to grow 180 meters beneath the ocean surface, where they capture longer wavelengths of the sun's radiation. They are used in the production of agar and some foods. The yellow-green algae have a characteristic plastid called a xanthoplast, which links them to the Eustigmatophyta, eyespot algae whose plastids appear to play a role in photolocation, "seeing."

Ciliates

Although not always visible on adult cells, ciliates are named for the hairlike undulipodia that protrude from their cells, called cilia, arranged into characteristic structures called kinetids. Most are unicellular, with complex cell structures, although some species, such as Sorogena, are multicellular and grow stalks that release "spores" that germinate into swimming cells. More than 8,000 species are known. The complex cell structure revealed by the electron microscope allows scientists to divide these organisms into three basic groups. The postciliodesmata have cell tail structures known as postciliary fibers. The rhapdophorans lack these fibers. The third group, the cyr-tophorans, reconstruct the pattern of their ciliated cell surface prior to reproduction. Some ciliates "hunt" by extending cords with poisons at the end; the body part resembles a fishing line with lure, and it is even reeled in. Sex in ciliates, which do not require sex to reproduce, involves the making of a subsidiary nucleus and exchanging it with a neighboring cell. These activities evince the protoctistan penchant for variations on the theme of sex in nucleated cells that has become more or less fixed in sexually reproducing plant and animal lineages. In the relatively familiar organism Paramecium, a ciliate, the extra nucleus will fuse with itself in a process known as autogamy if no partner is available. Most ciliates are incapable of photosynthesis, although Para-mecium bursaria, which contains within it the green alga (see above) Chlorella, thrives in sunlight. (If the Paramecium bursaria is restricted by darkness, however, it digests its inner gardens before it dies.)

Diatoms

Ubiquitous in the world's oceans, and traditionally classified as plants within the algal division Bacillariophyta, diatoms are better classified as photosynthetic protoctists. Single-celled or colonial, they are often of spectacular symmetry and beauty. Cell walls ("frus-tules" or "valves") are hardened by silica. Diatoms are important players in global ecology, and also were in the past; indeed, mineral beds up to 1,000 feet in depth, called diatoma-ceous earth, are composed of their fossil remains. Diatomaceous earth is used in paints, varnishes, and toothpastes, and as an insulator superior to asbestos. The varied exoskele-tons of diatoms range in shape from flowerlike to crown-shaped. The centric ones, disk-shaped, look like flowers or pillboxes, whereas the pennate ones like Navicula resemble boats. Made from silica, which they take from solution in the water, diatoms are so good at constructing their miniature hard parts that they can grow even in water where human instruments fail to detect measurable amounts of silica. The frustules are composed of two valves. Pennate diatoms have a split, or raphe,

Microscopic view of a diatom. (Jim Zuckerman/Corbis)

between their valves, along which they move. Diatom valves split after reproduction; each one goes to an offspring cell. In many, each offspring then grows a smaller valve to fit the original one. After shrinking a third in successive cell divisions, the diatoms then recapture their original size by flinging off their valves to nakedly conjugate in a mating act that reestablishes their larger size. Usually tan or brown, diatoms used to be classified with the golden algae.

Mastigotes (Flagellates)

The microbial world is still very new to science. In this world are no true plants or animals but only bacteria and thousands of species of protoctists still being classified and investigated. Protoctists is a new and relatively unusual word. But the older words are more reflective of familiarity than conciseness of meaning. Nowhere is this more true than in the eclectic group of organisms that are neither pho-tosynthetic nor ciliates, but instead have undulipodia. Undulipodia are whiplike structures (cilia and human sperm tails are examples) composed on the inside of tiny tubes, called microtubules, in specific formations, usually nine pairs arranged in a circle and often surrounding a central pair. The mastig-otes, often called flagellates, possess these undulipodia. Early scientists conflated them with the completely different tail-like structure of bacteria, flagella. The mastigotes are rapid swimmers, and they would be considered algae if they were photosynthetic. Instead, they are often called zooflagellates. But true flagella are rotating rods composed of flagellin proteins and found only in bacteria. The small undulipodiated swimming cells are not "zoo"— that is, not animals. In this group of mastigotes, then, can be found amoebomastigotes, diplomonads, retortomonads, kinetoplastids, bicoecids, opalinids, choanomastigotes, pyr-sonymphids, and parabasalids. Fine examination with the electron microscope is necessary to distinguish among these groups. Cho-anomastigote (choanoflagellate) cells look like individual cells from a sponge, one of the simplest sorts of animal. Kinetoplastids, another sort of mastigote, are named for their kinetoplast, a special large mitochondrion. These beings have been well studied, because they are involved in frightening tropical diseases such as leishmaniasis, sleeping sickness, and Chagas's disease. Other sorts of mastigotes are harbored within the complex microbial communities that break down wood into sugars and other food in the hindguts of wood-eating termites.

Dinomastigotes (Dinoflagellates)

Dinomastigotes have two cell tails, undulipo-dia: the first is inserted into a characteristic groove of their shell, which is made of cellulose (sometimes hardened with silica); the other circles the equator of the cell. Their name comes from their habit of slowly turning: dino means "whirl" in Greek. Fond of warm, marine waters, they provide food for whales and many other organisms. Some are bioluminescent, lighting up according to internal biological clocks, and also when disturbed. Some have evolved light-sensitive membranes, some even with miniature lenses that rove about the surface of the cell—tiny eyes.

Foraminifera

These giant marine unicells, familiarly called forams, may grow to several centimeters in diameter; they feed on algae and ciliates, even nematodes and the larvae of crustaceans. They are usually sand-dwelling, and many contain dinomastigotes, red algae, or other photosynthesizers, so that they behave as "marine plants," which they definitely are not. Forams are a main part of the diet of many invertebrate animals. Their complex shells, building up on the ocean floor, are used to date other fossils and as markers for geologists searching for petroleum reserves. Regular fossil forms, 10 cm in width, show up as nummulites, or "coin stones," in many places, including the limestones used to build the Egyptian pyramids.

Radiolaria

Their spiny silicate shells radiating like solid stars, radiolarians may possess up to several hundred spikes, technically known as axopods, shooting out from their bodies in all directions. The axopods, the identifying characteristic of this group, are used variously to row, to catch small protists, and to locomote like tum-bleweeds. Axopods attached to prey are used like high-tech straws to suck the nutritional contents of trapped protists and even small animals into the cytoplasm. The axopod spines allow some radiolarian species to float on the ocean's surface to look for food. They also increase the ability to accumulate nitrogen and phosphorus in the nutrient-poor open ocean, by increasing surface area. Radiolarian spines or spicules may be made from strontium sulfate or silica. Two main types of radiolari-ans exist, the polycystines and the phaeodar-ians. Polycystines have multiple sets of chromosomes; most plants and animals have two in their cells. Their skeletons are composed of hydrated amorphous silica, the substance of opal. The opaline skeletons begin as minute silica deposits linked with the internal cell membranes. Skeletons of phaeodarians, not directly evolutionarily related, are less well known but contain silica as well as other materials. Freshwater radiolaria are known as heliozoans and can be found stuck to rocks at the bottom of streams and ponds.

Slime Nets and Molds

Slime nets, usually marine microbes, move on a special net made from the secretion of their own slime, the so-called net plasmodium, which itself moves. Slime net microbes that grow to abundance on marine grasses, which support clam and oyster beds, can be associated with shellfish destruction. Cellular slime molds are interesting because they combine individual life with colonial, organismlike structures each generation. They never grow undulipodia. Amoebae break loose from a spore case suspended on a slender stalk. Although the amoebae look like neighboring amoebae, they will aggregate, under the influence of a small organic compound called cyclic AMP, when food is unavailable. The aggregating amoebae mount each other, forming a slug that continues to grow and may move in concert as a single being. Dic-tyostelids, for example, can be found moving across rotting logs or damp soils. The moldlike slime, which looks like tiny yellow or gray stalks, upon higher magnification is often a slime mold, not a fungus, despite its name. These protoctists consist of tightly aggregated amoebae cells behaving in social concert. The noncellular slime molds don't form slugs. They grow to be more extensive than the cellular slime molds, as they form slimy masses that move in which the cells lack their original structure and form a single entity—really a huge multinucleate cell called a plasmodium. If food is nearby, the cells lose their undulipo-dia and grow by nuclear division rather than whole cell division. The swimmers and amoebae can also engage in sex, but only with their own kind (swimmers or tail-less forms). The plasmodial mass can give rise to swimmers (mastigote stages) at any time. These organ isms feed on bacteria and plant material from decaying logs. They possess a protein, myx-omyosin, similar to the actinomyosin that contracts when we use our muscles. Still another group of slime molds are the parasitic slime molds, called plasmodiophorans or plasmodiophorids. These molds do not move but hide out inside plant tissue (they especially like members of the mustard family, such as cabbages and radishes). They feed by taking in plant juices in a growing plasmodial stage. Parts break off into individual cells to swim through the soil to find new plant victims.

Protoctist Diversity

Despite the abbreviated introductions to the above groups, which seem to us the most important, other protoctist groups, such as ellobiopsids, chytrids, and oomycotes, exist. Much of earth's limestone and chalk cliffs is calcium carbonate formed not by animals but by chalk-making protoctists (haptophytes and coccolithophorids). These ocean-going organisms are involved (before dying and being added to fossil mineral deposits) in global cycles of sulfur and carbon, and they may, through their production of gases, play a role in global weather and climate. Many other pro-toctists, not always recognized as such, do likewise. The suffix phyte means "plant," but haptophytes are not at all plants. So, too, mycos means "fungi," but many organisms previously classified with the fungi swim—and so belong better with the protoctists. For example, the oomycotes, or chytrids, are protoctists previously considered to be fungi; as we learn more, as instrumentation and microbiological techniques become more powerful, and more organisms are discovered, and as taxonomic categories become more reflective of evolutionary history, more microbes are sure to be added to this overlooked jungle of evolutionary and ecological diversity, the protoctists.

Some 213,000 species have been tallied, and every paleontologist agrees that the vast majority are extinct. Probably millions of species of these amazingly diverse beings, from which the original animals evolved, have come and gone.

—Dorion Sagan and Lynn Margulis

See also: Bacteria; Carbon Cycle; Classification, Biological; Climatology; Five Kingdoms of Nature; Fungi; Global Climate Change; Lakes; Microbiology; Mol-lusca; Paleontology; Soil

Bibliography

Grell, Karl G. 1973. Protozoology. New York: SpringerVerlag; Lee, John J., Seymour H. Hunter, and Eugene C. Bovee, eds. 1998. An Illustrated Guide to the Protozoa. Lawrence, KS: Allen; Margulis, Lynn, and Kar-lene Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3d ed. New York: W. H. Freeman; Margulis, Lynn, Karlene V. Schwartz, and Michael Dolan. 1999. Diversity of Life: The Illustrated Guide to the Five Kingdoms. Sudbury, MA: Jones and Bartlett; Margulis, Lynn, et al., eds. 1990. Handbook of Protoctista. Boston: Jones and Bartlett; Margulis, Lynn, et al., eds. 1993. Illustrated Glossary of Protoctista. Boston: Jones and Bartlett; Sagan, Dorion, and Lynn Margulis. 1993. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Dubuque, IA: Kendall/Hunt; Sleigh, Michael A. 1989. Protozoa and Other Protists. London: Edward Arnold.

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