Phycology Introductory article Ralph A Lewin, University of California, La Jolla, California, USA Article Contents Michael A Borowitzka, Murdoch University, Perth, Australia . General Features . Uses The study of algae is generally called ‘phycology’, from the Greek word phykos meaning . Noxious Algae ‘seaweed’. Just what algae are is difficult to define, because they belong to many different . Classification and unrelated classes including both prokaryotic and eukaryotic representatives. Broadly . Evolution speaking, the algae comprise all, mainly aquatic, plants that can use light energy to fix carbon from atmospheric CO2 and evolve oxygen, but which are not specialized land doi: 10.1038/npg.els.0004234 plants like mosses, ferns, coniferous trees and flowering plants. This is a negative definition, but it serves its purpose. General Features Algae range in size from microscopic unicells less than 1 mm several species are also of economic importance. Some in diameter to kelps as long as 60 m. They can be found in kinds are consumed as food by humans. These include almost all aqueous or moist habitats; in marine and fresh- the red alga Porphyra (also known as nori or laver), an water environments they are the main photosynthetic or- important ingredient of Japanese foods such as sushi. ganisms. They are also common in soils, salt lakes and hot Other algae commonly eaten in the Orient are the brown springs, and some can grow in snow and on rocks and the algae Laminaria and Undaria and the green algae Caulerpa bark of trees. Most algae normally require light, but some and Monostroma. The new science of molecular biology species can also grow in the dark if a suitable organic carbon has depended largely on the use of algal polysaccharides, source is available for nutrition. Algal photosynthesis may which are essential for the culture of bacteria and fungi and well account for half of the total global primary production in the purification and separation of proteins and nucleic and constitutes the basis of almost all aquatic food webs. acids. Brown algae or kelps such as Macrocystis, Lamin- Furthermore, the nitrogen-fixing cyanobacteria (blue- aria, Durvillea and Sargassum are sources of alginates, green algae) are an important source of fixed nitrogen, which can serve as thickening agents and colloidal stabi- which is necessary for the nutrition of other plants and lizers in the food, cosmetic, textile, pharmaceutical and animals. The name ‘algae’ is not a taxonomic term, but paper industries. The total world production of alginates is rather describes a very broad collection of organisms, both about 21 500 tonnes, equivalent to an algal harvest of prokaryotic and eukaryotic. These ‘algae’ are generally about 575 000 tonnes fresh weight. The principal genera photosynthetic, but do not have roots or leafy shoots and harvested are Macrocystis and Nereocystis (Pacific coast of lack true vascular tissue. See also: Algal ecology North America), Laminaria and Ascophyllum (Europe), Several species form important symbioses with animals, Durvillea (Australasia) and Laminaria and Undaria plants or fungi. For example, lichens are actually symbiotic (China). See also: Algal metabolism; Brown algae associations of specific fungi and algae. A few cyanobac- Many red algae are abundant seaweeds of great eco- teria form symbiotic associations with several kinds of nomic importance, especially as sources of gelling agents plants, enhancing their growth by providing fixed nitrogen. such as agar and carrageenan, which are used in the food Others form symbioses with invertebrates such as sponges. and cosmetic industries and in microbiological labora- The aquatic fern Azolla contains filaments of the nitrogen- tories. Agar is obtained from genera such as Gelidium, fixing cyanobacterium Anabaena azollae in special cham- Gracilaria and Pterocladia, with a total worldwide pro- bers in the leaves. An important alga/animal symbiosis is duction of about 7000 tonnes. Carrageenan is mainly pro- that between certain reef-forming corals and the dinoflag- duced from Chondrus crispus (Irish moss) and Mastocarpus ellates Symbiodinium spp. There are many other examples stellatus in Europe, and from Eucheuma and Kappaphycus of algal/invertebrate symbioses among the Protozoa, species in the Philippines, with an annual production ex- Porifera, Coelenterata and Mollusca. See also: Algal ceeding 13 000 tonnes. Many of these algae are farmed by symbioses; Lichens; Porifera sponges mariculture when supplies of ‘wild’ material are inade- quate to meet the high demand. Notably, the annual har- vest of cultivated Porphyra or ‘nori’ is now worth about Uses a billion dollars. The unicellular green alga Dunaliella salina, which be- Algae are not only important as primary producers in na- comes red in hypersaline media, is grown commercially in ture, providing oxygen and food for other organisms, but Australia, China and the USA as a source of the carotenoid ENCYCLOPEDIA OF LIFE SCIENCES & 2005, John Wiley & Sons, Ltd. www.els.net 1 Phycology b-carotene, which has been recommended as a nutritional toxic algae is monitored, and, when they are abundant, supplement and is widely used as a natural colourant for inshore fisheries may be closed to reduce the threat to foods. The cyanobacterium Spirulina is extensively cul- human health. When the algal bloom dies out, the shellfish tured in Thailand, the USA, China, India, Japan, Chile and eventually clear themselves of the toxic algae and then be- Australia and is sold as a food supplement with sales ap- come once more fit for human consumption. See also: proaching 1 billion dollars annually. The chlorophyte Harmful algal blooms; Protozoa Chlorella is also cultured for human consumption and has Some seaweeds, introduced to foreign shores, have be- a large world market. Other species of microalgae are being come troublesome weeds (e.g. certain species of Sargassum developed as sources of polyunsaturated fatty acids to and Caulerpa). See also: Biogeography of marine algae supplement human and animal diets. See also: Green algae The silica walls of diatoms fossilize well, and in many parts of the world there are extensive deposits of their shells Classification from the Tertiary era known as ‘diatomaceous earth’ or ‘Kieselguhr’. The fine holes and small size of these silica Although earlier classifications of algae were based mainly walls make them an excellent filter material for swimming on their pigments, more recent ultrastructural and molec- pools and for the food and chemical industries. See also: ular biological studies have led to considerable changes and Algal calcification and silicification; Diatoms reassessments of the phylogenetic relationships of the var- Nitrogen-fixing cyanobacteria (e.g. Anabaena, Nostoc) ious algal divisions to one another and to other organisms. are used as natural fertilizers in India and other parts of For example, since the blue-green algae (which used to be Asia, where they reduce the need for expensive synthetic called Cyanophyta) are prokaryotic, they have been re- fertilizers in rice farming. Some algae are also proving to be named Cyanobacteria and alternatively can be classified potential sources of new pharmaceutical products, includ- among the bacteria. Table 1 presents a resume´of the main ing antibiotics and agents to combat cancer and viral dis- distinguishing features of the presently recognized divisions eases. A number of algal compounds are currently under of algae, and some details of the structure, life histories development by drug companies. See also: Nitrogen and general biology of the main groups are given below. fixation; Photosynthesis and respiration in cyanobacteria See also: Algal pigments; Algal taxonomy: historical Several green algae, selected largely for the ease with overview; Prokaryotic systematics: a theoretical overview which they can be cultured in the laboratory, have been used The systematic positions of the various groups of algae for investigating fundamental processes such as photosyn- have undergone many changes in the last century, and with thesis, light perception, flagellar motility, cell differentiation the onset of new methods for evaluating phylogenetic re- and sexuality. A few species of Chlorella, Chlamydomonas, lationships (e.g. electron microscopy, molecular biology) Volvox and Euglena, in particular, have contributed much more changes are being proposed. At present there is no to our basic knowledge of such phenomena. complete consensus. Furthermore, differences between the Botanical and Zoological Codes of Nomenclature mean that sometimes different names may be applied to the same Noxious Algae algal groups. The classification scheme used here follows that of van den Hoek et al. (1995) for the most part. For Although most algae are beneficial, some produce power- convenience, Table 2 compares it with some other classifi- ful toxins, and when they form ‘blooms’ they can cause cations. See also: Protozoan taxonomy and systematics major environmental problems by sickening or killing fish, birds, cattle and other animals drinking the water. More- Prokaryotic algae over, when the cells die and decay they may use up so much of the oxygen in the water that they cause more deaths of Cyanobacteria (blue-green algae) and Prochlorophyta aquatic life. The formation of such blooms in lakes and The cyanobacteria are prokaryotic organisms that have no rivers is promoted by nutrient pollution, and increased membrane-limited nuclei, Golgi apparatus, mitochondria, human pollution of