Included among the diverse organisms called protistans (kingdom Protista) are algae, protozoans, slime molds, and the oomycota. Algae were long considered to be simple plants and were assigned to kingdom Plantae but lack themore highly differentiated tissues and organs characteristic of “higher plants” such as mosses, ferns, and seed plants.
Protozoa, all of which are unicellular, were assigned to the kingdom Animalia; considered more animal-like than plant-like, they are not considered here in detail.
The two remaining groups of protists, the slime molds and oomycota, were traditionally considered fungi, as indicated by their names (mycoto means “fungus”), and accordingly were assigned to the kingdom Fungi, largely due to their being heterotrophs (nonphotosynthetic). As information about the true nature of these life-forms has accumulated, however, they were grouped together with the protists.
Nevertheless, Protista is a heterogeneous kingdom, composed of both the heterotrophic slime molds and oomycetes and the autotrophic (photosynthetic) algae. Thus, the kingdom Protista is a “catch-all” group, containing various organisms that seemnot to fit into any of the other kingdoms.
Biology of the Protista
Most protistans are unicellular, but some, notably the large species of marine algae called kelps and seaweed, are multicellular. All are eukaryotic (with cells that have nuclei, making them members of the domain Eukarya, along with fungi, plants, and animals), in contrast to prokaryotic bacteria (which are divided between the domains Archaea and Bacteria).
In addition to having a distinct nucleus surrounded by a nuclearmembrane (envelope), the cytoplasm of a eukaryotic cell contains various types of organelles, each specialized for performing a particular task (or relatedones).
Examples aremitochondria (cellular respiration), chloroplasts (photosynthesis), Golgi bodies (packaging of molecules), and the endoplasmic reticulum (to lend rigidity). As a result of this specialization, eukaryotic cells are able to function more efficiently than do prokaryotic cells, which must perform the same functions but within the cell as a whole.
In this way, protistans are similar to plants, animals, and fungi. Furthermore, the evolution of eukaryotic cells from prokaryotic ones was a pivotal event in the evolution of life, leading not only to protistans but also from them to the more highly evolved kingdoms.
As previously stated, protists exist in great variety. Included are some that, due to the absence of a rigid cell wall, are able to change shape rapidly. Other protists have cellwalls surrounding their cell membranes, resulting in a more permanent shape.
Some possess fringe like cilia or whip like flagella which they use to swim; others move by other means; many are nonmotile. Some protists live solitary lives, while others aggregate to form colonies. Some are parasitic, whereas most are free-living (nonparasitic).
In size there is also great variation: Many are unicellular and microscopic, while some algae, such as the kelps and seaweeds, grow to many meters in length. Furthermore, many protists have complex life cycles, with the various stages possessing different combinations of these characteristics.
Biologists generally agree that fungi, plants, and animals are derived from ancient protists. Thus, the study of protists, which continue to inhabit the earth, sheds light on the origin of these groups of more highly evolved organisms.
Also, each of these modern protists plays an ecological role together with the other organisms that occupy its ecosystem. Some protists affect humans more directly as agents of disease; some serve as a source (or potential source) of medicines that can be used to combat various diseases.
The blue-green “algae,” historically considered to be algae, are now recognized as a separate group of organisms.As they are prokaryotic, they are now considered to be a special type of bacteria, the cyanobacteria. They differ from other bacteria primarily because they possess chlorophyll and therefore have the ability to photosynthesize.
Within the kingdom Protista, algae are currently divided by phycologists (scientists who study algae) among anywhere from four to thirteen phyla. Here, a systemis used in which nine phyla of the kingdom Protista include algae. The nine phyla include the following.
Euglenophyta (euglenoids). This small group of protists (about nine hundred species) is a good starting point for studying algae, as euglenoids combine traits characteristic of plants, fungi, and animals. All are unicellular, and nearly all are mobile by means of two flagella that emerge from a groove at the end of the cell.
By means of an eyespot near the base of the flagella, euglenoids can detect light and swim toward it. Most members of the phylum lack chlorophyll and therefore must absorb food from external sources.
Others, though, such as Euglena, have chloroplast swith chlorophylls a and b, along with carotenoids and accessory pigments. They reproduce by fission (cell division). Most are freshwater organisms, but a few occur in brackish or marine environments.
Cryptophyta (cryptomonads). These algae are single-celled flagellates that are commonly brownish, blue-green, or red. Their name (Greek kryptos means “hidden”) refers to their small size (3-50 micrometers), which makes the minconspicuous. Like euglenoids, they include both colorless (unpigmented) and pigmented photosynthetic members.
Pigments includes chlorophylls a and c and carotenoids. Evidence exists to indicate that cryptomonads arose fromthe fusion of two different kinds of eukaryotic cells. Of the two hundred known species, some are marine; others live in fresh water.
Rhodophyta (red algae). Red algae are primarily multicellularmarine organisms and are commonly referred to as seaweeds. However, about one hundred of the five hundred species are unicellular and live in fresh water.
The chloroplasts of red algae contain chlorophyll a, but the presence of phycobilins (red pigments) usually masks the chlorophyll, giving thema red or reddish appearance.
The red pigment aids in light absorption in deep water, where many red algae are found.As chloroplasts of red algae resemble those of cyanobacteria, there is reason to believe that they probably evolved from these prokaryotic organisms.
One group of red algae, the coralline algae, deposit calcium carbonate in their cell walls, resulting in stony formations in the ocean. They are often associatedwith coral reefs, which they help to stabilize.
Many red algae have complicated life histories. The simplest type is that in which a haploid gametophyte alternates with a diploid sporophyte. This pattern, known also throughout the plant kingdom, is known as alternation of generations.
In most red algae, there are three generations: a haploid gametophyte, a carposporophyte, and a tetrasporophyte (both diploid). It has now been recognized that some red algae previously considered to be different species are actually different stages of the same species.
Dinophyta (dinoflagellates). Most dinoflagellates are unicellularmarine algae, each with two flagella. Of the two, one is within an equatorial groove around the cell; the other passes down a longitudinal groove before extending outward. In addition to chlorophyll, they have accessory pigments that give them a golden-brown color.
Many dinoflagellates are associated symbiotically with various invertebrates; for example, many corals benefit from the food they derive from these algae. Other dinoflagellates are nonphotosynthetic and live as parasites within other marine organisms.
Many dinoflagellates are bioluminescent; they emit a faint light that can be seen in darkness from a passing ship. Others are responsible for fish kills when they become superabundant in warm stagnant water. The death of the fish within these “red tides” is often due to toxins produced by the dinoflagellates.
Haptophyta (haptophytes). This group, consisting of only about three hundred known species, includes a diversity of primarily marine species, but some freshwater and even terrestial species are known. Included are both unicellular and colonial flagellates, along with others that are nonmotile.
The distinctive feature of haptophytes is the haptonema, a threadlike structure that bends and coils as it apparently helps the cell to catch food particles. It differs from a flagellum by lacking the 9 + 2 arrangement of microtubules, which is characteristic of flagella of eukaryotic cells.
Most haptophytes are photosynthetic and possess chlorophylls a and c together with an accessory pigment such as fucoxanthin. Although the phenomenon is not as well documented as for dinoflagellates, haptophytes also cause marine fish kills by releasing toxins.
Chrysophyta (chrysophytes). Some one thousand species of chrysophytes exist, including both unicellular and colonial organisms that are often abundant in both fresh and marine habitats. Some lack chlorophylls a and c.
However, the golden color of fucoxanthin, which they possess, usually masks the green pigments, giving them their characteristic golden hue and accounting for their name (chrysos means “gold”). Some chrysophytes feed on bacteria. Some are responsible for “brown tides” that causes damage to shellfish and salmon fisheries.
Bacillariophyta (diatoms). The name “diatom” comes from the two overlapping shells that fit together like the two parts of a candy box. Composed of silica (silicon dioxide), the shells persist long after the living cell inside has died. Most species are photosynthetic, possessing chlorophylls a and c as well as fucoxanthin. The life cycles of diatoms include both asexual (cell division) and sexual phases.
It would be difficult to overestimate the importance of diatoms. There are more than 100,000 known species, and they are abundant in practically all aquatic and marine habitats. Due to the persistence of their shells, it is known that there are many extinct species also. Diatoms are responsible for as much as 25 percent of total world food production (photosynthesis).
Especially in polar waters, they are the primary food source for aquatic animals. Large accumulations of diatom shells, known as “diatomaceous earth,” are mined, cleaned, and used in filters, in gas masks, in toothpaste, and for a variety of other purposes.
Phaeophyta (brown algae). Found only in salt water, this group includes large, conspicuous, multicellular forms generally called kelps or seaweeds. Often seen on rocky shores, various of the fifteen hundred species inhabit the ocean, especially in temperate and cooler waters. Laminaria are called kelps and often form “kelp forests” along the gently sloping shores off the coast of California.
Although lacking true roots, stems, and leaves, kelps have the most highly differentiated bodies of any of the algae. Some of their cells resemble the phloem (food-conducting cells) of vascular plants. The brown pigment fucoxanthin is present in addition to chlorophylls a and c.
Chlorophyta (green algae). The seventeen thousand or more species of green algae are perhaps the most diverse group of the algae. Most live in fresh water, but some live in the ocean, and others live in soil or on tree trunks. Many form symbiotic associations with sponges, protozoa, and other invertebrates; others are associated with fungi within lichens.
As green algae resemble plants more than do any other group of algae, plants are believed to have evolved from green algae. They, like plants, possess chlorophyll a and b; food is stored in specialized cytoplasmic organelles called plastids.
The phylum is divided into three classes. In the class Chlorophyceae are included a diversity of forms, nearly all of which are freshwater species. Chlamydomonas is a motile unicellular species which, nevertheless, exhibits a complex life cycle.
Volvox and several other large spherical colonial forms are composed of cells, each of which strongly resembles Chlamydomonas. Other members of this class are filamentous.
The class Ulvophyceae includes primarily marine species. A common example is the Ulva species (sea lettuce), composed of flat sheets of cells; it is found in shallow seas around the world.
Included in the class Charophyceae is the familiar Spirogyra, a freshwater filamentous species with spiral chloroplasts.
Human Uses of Algae
References have already been made to ways that algae are involved in the overall “economy of nature.” Because algae are autotrophs (producers) and photosynthesize, they generate food that is made available to the heterotrophic animals (consumers).
At the same time, oxygen,which results as a by-product, is made available to these same animals. As algae perform functions in marine and freshwater environments that are similar to those performed by grasses (and other plants) on land, algae have been called “the grasses of many waters.”
Algae are often involved also in human affairs in more direct ways. In Asian countries especially, kelps and other multicellular algae have been used for food for centuries. Nori, a red alga of the genus Porphyra has been collected and eaten as a vegetable in Japan and China.
It is now cultivated on a large scale, thus increasing its availability and popularity. Unfortunately, most seaweeds are not high in food value, although they do provide some needed minerals and vitamins. Another problem is their taste, which is not acceptable to many Westerners.
Of more commercial value in Europe and North America are a number of products derived from kelps and various seaweeds: alginates, carrageenans, and agar.
Alginates are hydrophobic (water-attracting) compounds derived from various brown algae such as Laminaria. After they are harvested mechanically from the ocean, the algae are processed. The resulting products are salts of sodium and potassium alginate.
These alginates are used in the paper industry as a sizing and polishing agent and in the manufacture of paints, cosmetics, and a wide variety of foods. In each case, the role of the alginate is to improve the consistency of the product and to prevent the separation of its ingredients.
Carrageenans are obtained primarily from Irish moss (Chondrus crispus), a red alga found off the coast of New England. After processing, the resulting carrageenans are used for some of the same purposes as alginates. However, because of their higher melting point, they have been found superior for uses in many kinds of foods, especially desserts.
The pioneer German bacteriologist Robert Koch popularized the use of agar for the culture of bacteria. Agar is obtained from certain red algae. After processing and cleaning, agar is added in small quantities (1-2 percent) to water along with nutrients required by the bacteria. The result is a solid medium on which bacteria can be isolated from a mixed culture.
Although Asians have long used certain algae for folk medicinal purposes, their use in Western medicine has until recently been limited largely to serving as a binder in medicinal tablets or as a laxative.
Their potential as a source of therapeutic drugs is being pursued by an increasing number of researchers. Included are those from which antibiotics and anticancer drugs may be extracted.
Plasmodial slime molds (phylum Myxomycota) often exist as a conspicuous fan-shaped mass of protoplasm that creeps along a surface somewhat as do amoebas. From this stage, known as a plasmodium, are formed sporangia with spores inside. From the spores are formed the single-celled amoebalike stage; they converge to form the plasmodium.
Cellular slime molds (phylum Dictyosteliomycota) are amoeba-like organisms that combine at one stage to form “slugs,” or pseudoplasmodia.
Like that in plasmodial slime molds, reproduction in cellular slime molds is both sexual and asexual, but, unlike plasmodial slime molds, no flagellated cells are known. In both types of slime molds, particulate food such as bacteria can be ingested (fungi absorb only digested food).
Also previously considered to be fungi, oomycetes (phylum Oomycota) are probably more nearly related to certain algae than to fungi. Like algae, their cell walls are of cellulose. Some species are unicellular, whereas others are filamentous or highly branched.
Some of the filamentous forms are coenocytic (no cell walls separate adjacent cells). The name of the group reflects the large female gamete or egg; this type of sexual reproduction is called oogamy.
Some terrestial oomycetes are plant pathogens of considerable importance. Downy mildew of grapes, caused by Plasmopara viticola, has often threatened the wine industry of France. Species of the genus Phytophthora cause diseases of many fruit crops and other plants of economic importance.
Among these is P. infestans,which causes the potato late blight. One particular outbreak of this parasite caused the infamous Irish Potato Famine of the mid-1840’s, during which more than amillion people (some estimates say four million) were affected.
Saprolegnia is a prominent member of a group of aquatic oomycetes called “water molds.” Most are saprophytic on dead plants and animals, but a few are parasitic.