Algae:

A group of structurally simple plant-like organisms that carry out oxygen-evolving photosynthesis. Although most algae are unicellar and microscopic, some as small as 1 to 2 micrometres in diameter (a micrometre is 0.000001 metre, or 0.00004 in), many are conspicuous in the form of pond scum, seaweeds, red tide, blue-green discolouration of aquarium walls, the green coating on trees, and red snow. Representatives of many genera of algae live together in symbiosis with fungi to form lichens. In some forms of algae, the ability to photosynthesize has been lost through evolution. Algae differ from bryophytes (mosses, liverworts, and hornworts), which also lack complex tissues, in that their reproductive cells are produced in unicellular structures rather than multicellular structures. The study of algae is called phycology (Grk., phykos, seaweed) or algology (Lat., alga, sea wrack).

Fucus Vesiculosis (Seaweed)

Macroscopic forms of algae are usually attached to a firm surface and grow abundantly as seaweeds in intertidal and subtidal zones as deep as 268 m (879 ft), the depth depending on light penetration. Macroscopic algae also grow on rocks in flowing and standing fresh water, frequently becoming detached and floating as pond scum. Microscopic forms of algae, which are mostly unicellular and planktonic (free-floating or motile-that is, capable of movement), are an essential part of the food chain in all aquatic habitats.

Scientists have classified algae in several ways. Traditionally, nonmotile forms have been studied by botanists, while motile forms, even if they perform photosynthesis, have been studied by zoologists as well as botanists. However, simply dividing algae between plants and animals is inaccurate. Many biologists and this encyclopedia use a classification system that distributes algae among multiple kingdoms. Current research suggests that at least 16 phyletic lines (groups of organisms with a common ancestry) exist. A phyletic line is usually given the rank of a phylum in zoology, or of a division in botany.

Phyletic lines of algae are defined by certain characteristics. These include photosynthetic pigmentation, storage products, cell wall composition, flagellation of motile cells, and structure of the nucleus, chloroplast, pyrenoid (area of the chloroplast involved in starch formation), and eyespot (an organelle made up of closely packed lipids). Algae that are prokaryotic-that is, lacking a nuclear membrane-are classified in the kingdom Monera. Eukaryotic algae (those that have their cellular organelles bounded by a nuclear membrane) with no multicellular forms are classified in the kingdom Protista, whereas eukaryotic phyletic lines with multicellular forms are classified in the kingdom Plantae. One hypothesis suggests that the organelles of algal cells have arisen by evolution of endosymbionts (organisms that live symbiotically inside the tissues or cells of a host organism). Proponents of this theory place eukaryotic algae in the kingdom Protoctista with traditional protozoa.

Blue-Green Algae (Cyanophyta)

Blue-green algae, or cyanophytes, are sometimes called cyanobacteria or blue-green bacteria because, like bacteria, they lack a nuclear membrane. However, the fact that cyanophytes carry out oxygen-evolving photosynthesis in a similar way to higher plants supports their classification as algae, not bacteria. Their colour varies from blue-green to red or purple, and is determined by the proportions of two special photosynthetic pigments, c-phycocyanin (blue) and c-phycoerythrin (red), which tend to mask the chlorophyll. Whereas higher plants have two kinds of chlorophyll called a and b, cyanophytes contain only chlorophyll a. They reproduce by binary fission, production and germination of spores, or breakage of multicellular filaments. Like bacteria, cyanophytes are nearly everywhere, occurring in typical aquatic and terrestrial habitats as well as in such extreme sites as hot springs (at temperatures up to 73° C, or 160° F) and crevices of desert rocks. Fossils believed to represent cyanophytes are found in rocks 2.5 to 2.8 billion years old. Cyanophytes were probably responsible for the accumulation of oxygen in the primeval atmosphere and the formation of concentrically laminated reef-like structures called stromatolites, which continue to be formed (as at Shark Bay, Western Australia) by the cementing of sediments with calcium carbonate.

Some freshwater planktonic forms of blue-green algae, which occur in massive quantities called blooms under favourable conditions, produce toxins that can kill livestock. Some forms are nitrogen-fixing and thus enrich the habitats where they grow, such as rice paddies. Spirulina, one form of blue-green algae and a traditional food in parts of Mexico and central Africa, is grown commercially and marketed as a high-protein dietary supplement.

Red Algae (Rhodophyta)

Red algae are eukaryotic-that is, they have a nuclear membrane-and are mostly confined to marine habitats. Many are spectacularly beautiful. Like cyanophytes, they lack chlorophyll b and have special blue and red pigments. The presence of pit connections between cells, which result from incomplete cell division, is characteristic of most red algae. The reproductive cells of red algae lack flagella (whip-like appendages used for locomotion). The sexual life history of most red algae is extremely complicated, involving an alternation of two free-living phases that can be morphologically similar or different, and a post-fertilization phase that develops on the female plant. The cell walls of coralline algae are impregnated with a form of calcium carbonate called calcite. Coralline algae are important in the formation of coral reefs, producing new material and cementing together other organisms.

The cell walls of certain red algae are the sole source of two polysaccharide carbohydrates of major economic importance, agar and carrageenan. The two polysaccharides are closely related chemically, and both have suspending, emulsifying, stabilizing, and gelling properties. Agar is best known for its use in preparing media for culturing micro-organisms. Carrageenan is best known for its use in dairy products, but it is also used in the cosmetic, pharmaceutical, printing, and textile industries. Several red algae, of which nori is the best known, are an important component of the diet of various peoples, especially the Japanese.

Brown Algae (Phaeophyta)

Brown algae, like red algae, are found mostly in marine habitats. They also lack chlorophyll b, but have another type called chlorophyll c, as well as special photosynthetic, yellow to deep-red pigments. Reproductive cells often have flagella. Brown algae are best known for their rapid growth, immense size, and relatively complex tissues. Some kelps reach a length of 30 m (98 ft) and have a primitive food-conducting tissue. One genus, although common in all warm waters, is famous for floating in masses in the Sargasso Sea of the North Atlantic Ocean.

Alginate, a polysaccharide extracted from kelps, is used commercially in the same manner as agar and carrageenan. Large brown algae are also sources of vitamins, minerals, and fertilizers. Several species, including wakame, kombu, and hijiki, are important sources of food, especially in Japan.

Diatoms (Bacillariophyta)

Diatoms are unicellular organisms with pigmentation similar but not identical to that of brown algae. They are found in all types of water and in moist soil. Diatoms may float as plankton or attach to rocks or other surfaces. Diatoms contribute significantly to the food chain in aquatic habitats. Their cell walls are impregnated with silica and are fossilized as diatomaceous earth, which is used in filtration and as an abrasive.

Green Algae (Chlorophyta)

Green algae are similar to higher plants in having both chlorophyll a and b, and in storing food as starch. Most are unicellular or colonial and form an important part of the plankton of freshwater habitats. Many unicellular chlorophytes combine into filaments and are visible as river "moss" or pond scum. The most highly developed freshwater chlorophytes are the stoneworts, and the most highly developed marine chlorophytes are made up of multinucleate siphons, and reach a length of 10 m (33 ft). One genus has cell walls impregnated with a form of calcium carbonate called aragonite and makes an important contribution to coral reef formation. Studies indicate that stoneworts and certain other chlorophytes have probably evolved into bryophytes and higher plants.

Other Algal Phyletic Lines

At least 11 other algal phyletic lines have been defined, most members of which are unicellular or colonial flagellates. Dinoflagellates (Dinophyta or Pyrrophyta) are mostly marine. Although their role as primary producers in the food chain is of chief importance, they are best known for causing red tide, a massive growth of certain species that introduce toxins into the food chain. Coccolithophorids are members of the Prymnesiophyta (or Haptophyta) that have intricate, calcified scales called coccoliths attached to their cell bodies. Fossilized coccoliths, which form the white cliffs of Dover, are important in the geological study of strata (layers of sedimentary rock). The other algal phyletic lines with photosynthetic members are the Chrysophyta, Xanthophyta (Tribophyta), Eustigmatophyta, Raphidophyta, Cryptophyta, Euglenophyta, and Prasinophyta.