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Home , Anabaena, Bangia, Caulerpa, Cephaleuros, Chaetomorpha, Chattonella

Phycology Introductory article

Ralph A Lewin, University of California, La Jolla, California, USA Article Contents

Michael A Borowitzka, Murdoch University, Perth, . General Features . Uses The study of is generally called ‘phycology’, from the Greek word phykos meaning . Noxious Algae ‘’. 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 . speaking, the algae comprise all, mainly aquatic, 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 , 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 are also of economic importance. Some in diameter to as long as 60 m. They can be found in kinds are consumed as food by humans. These include almost all aqueous or moist ; in marine and fresh- the red alga (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 and on rocks and the algae and Undaria and the 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 may which are essential for the culture of 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. or kelps such as , Lamin- Furthermore, the -fixing (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 . 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 (Pacific coast of lack true vascular tissue. See also: Algal ecology ), Laminaria and Ascophyllum (Europe), Several species form important symbioses with animals, Durvillea (Australasia) and Laminaria and Undaria plants or fungi. For example, are actually symbiotic (). See also: Algal metabolism; Brown algae associations of specific fungi and algae. A few cyanobac- Many are abundant 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 and carrageenan, which are used in the food Others form symbioses with such as . and cosmetic industries and in microbiological labora- The aquatic fern 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 . An important alga/ symbiosis is duction of about 7000 tonnes. Carrageenan is mainly pro- that between certain reef-forming corals and the dinoflag- duced from (Irish ) and Mastocarpus ellates Symbiodinium spp. There are many other examples stellatus in Europe, and from and Kappaphycus of algal/ symbioses among the , species in the , with an annual production ex- Porifera, Coelenterata and . 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 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

ENCYCLOPEDIA OF 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 dies out, the shellfish tured in Thailand, the USA, China, India, , 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 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 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 in India and other parts of For example, since the blue-green algae (which used to be , where they reduce the need for expensive synthetic called Cyanophyta) are prokaryotic, they have been re- fertilizers in 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 : 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, differentiation the onset of new methods for evaluating phylogenetic re- and sexuality. A few species of Chlorella, , lationships (e.g. electron microscopy, molecular biology) and , 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 aquatic life. The formation of such blooms in lakes and The cyanobacteria are prokaryotic organisms that have no rivers is promoted by nutrient , and increased membrane-limited nuclei, Golgi apparatus, mitochondria, human pollution of water bodies means that they are be- endoplasmic reticulum or . The DNA lies free in the coming more common throughout the world. In fresh wa- , often accompanied by the photosynthetic ters, the culprit is usually a cyanobacterium, Microcystis membranes, the . Like other plants, and unlike sp., while in the sea the main bloom-forming toxic algae are other photosynthetic bacteria, the cyanobacteria can pro- certain diatoms and dinoflagellates, which cause the so- duce oxygen during photosynthesis. Although they have called ‘red tides’. Brevitoxin, produced by certain marine no evident sexual reproduction, genetic recombination by dinoflagellates, is one of the most toxic and complicated transformation or conjugation may occur. The cells are organic compounds produced by any organism. It has only generally blue-green to violet, but some may appear green- recently been synthesized artificially. When such algae are ish, red or black, depending on the relative abundances of consumed by shellfish, these animals can in turn become their pigments, a, phycocyanin and phycoer- poisonous. In many parts of the world the incidence of ythrin in the cells and other dark-coloured substances in

2 Table 1 The main distinguishing features of the presently recognized divisions of algae Main storage Habitats and Main pigments products Cell forms Cell covering number of species Selected genera Prokaryotes Cyanophyta Chlorophyll a Cyanophycean Unicellular, filaments Peptidoglycan wall Freshwater, marine Anabaena (Cyanobacteria) Phycocyanin (a-1,4- and colonial. No with external lipo- and terrestrial Microcystis ‘blue-green algae’ linked glucan), flagellate cells polysaccharide (  2000 species) Nostoc cyanophycin layer. May also have Spirulina (alanine and mucilaginous sheath arginine polymer) Prochlorophyta Chloropyll a Starch-like Unicellular and As for cyanobacteria Marine and Prochloron Chloropyll b polysaccharide filaments. No fresh-water flagellate cells (4 species) Prochlorothrix Glaucophyta Chlorophyll a Starch (outside Unicellular, some Cells surrounded by Freshwater Cyanophora Phycocyanin ) flagellates layer of vesicles con- (  5 species) Glaucocystis taining scales or fibrillar material

Rhodophyta Chlorophyll a Floridean starch Unicellular, filament- of cellulose Mainly marine ‘red algae’ Phycoerythrin (a-1,4-linked ous and thalloid fibrils (Florideo- (  5500 species) Ceramium Phycocyanin glucan) (no flagellate stages) phyceae) or xylan Chondrus fibrils (Bang- Eucheuma iophyceae) in mucilagi- Gelidium nous matrix that may Gracilaria contain agar or Lithophyllum carrageenan. Some Mastocarpus calcified Porphyra Porphyridium Pterocladia Cryptophyta Chlorophyll a Starch (outside Mainly unicellular, Stiff proteinaceous Marine and Chlorophyll c chloroplast) one filamentous periplast, usually freshwater Phycoerythrin and . Two uniequal of rectangular or (  200 species) Phycocyanin flagella polygonal plates Heterokontophyta Chrysophyceae Chlorophyll a Chrysolaminaran Mainly unicellular Naked or with siliceous Mostly freshwater Ochromonas Chlorophyll c (b-1,3-linked or colonial, a few scales on surface (  1000 species) Synura

Fucoxanthin glucan) simple multicellular Phycology or, Two flagella, one smooth, the other hairy

3 continued 4

Table 1 Continued Phycology Main pigments Main storage Cell forms Cell covering Habitats and Selected genera products number of species Xanthophyceae Chlorophyll a Unicellular (coccoid, Mainly of cellulose fi- Mainly freshwater Tribonema Chlorophyll c flagellate or brils, sometimes im- and terrestrial, Vaucheria amoeboid), some pregnated with silica, some marine colonial, uninucleate often in two overlap- (  600 species) and multinucleate ping halves filaments. Two flagella, one smooth, one hairy Eustigmatophyceae Chlorophyll c Unicellular and co- Polysaccharide walls Freshwater and Nannochloropsis Violaxanthin ccoid. One or two marine flagella Bacillariophyceae Chlorophyll a Unicellular and Wall of two siliceous Freshwater and Navicula ‘diatoms’ Chlorophyll c colonial. No flagella valves with bilateral or marine Nitzschia Fucoxanthin except in radial symmetry (  100 000 species) Skeletonema spermatozoids Thalassiosira Raphidophyceae Chlorophyll a Unicellular. Two No cell walls Freshwater and Chattonella Chlorophyll c flagella, one smooth, marine Fibrocapsa Fucoxanthin other hairy (  40 species) (marine spp.) Diadoxanthin, Vaucheriaxanthin Heteroxanthin (freshwater spp.) Dictyochophyceae Chlorophyll a Unicellular with one Mucilaginous envelope Marine (2 species) Dictyocha Chlorophyll c hairy flagellum Fucoxanthin Phaeophyceae ‘brown Chlorophyll a Multicellular. Two Wall of cellose Marine (  2000 Ascophyllum algae’ Chlorophyll c laterally inserted microfibrils, stiffened Species) Dictyota Fucoxanthin flagella in reproduc- by calcium alginate Durvillea tive cells in a mucilaginous Ecklonia matrix Ectocarpus Laminaria Macrocystis Nereocystis Sargassum Undaria Dinophyta Chlorophyll a Starch (outside Predominatly Cell surrounded by Mainly marine Alexandrium ‘dinoflagellates’ Chlorophyll c chloroplast) unicellular, some layer of flat, polygonal (  2000 species) Amphidinium and lipid coccoid filamentous. vesicles, which may be Ceratium Two flagella, one empty or, more usual- Gymnodinium transverse in a ly, contain cellulose Noctiluca furrow, the other plates Symbiodinium longitudinal

Haptophyta Chlorophyll a Chrysolaminaran Unicellular with 2 Cell covered with Mainly marine Chrysochromulina (Prymnesiophyta) Chlorophyll c flagella. Each cell cellulose scales and/or (  500 species) Emiliania Haptophyceae also has a calcified scales Pleurochrysis ‘haptonema’ (coccoliths) Euglenophyta Chlorophyll a Paramylon Unicellular with Cell surrounded by Mainly freshwater Astasia ‘euglenoids’ Chlorophyll b (b-1,3-linked single flagellum proteinaceous strips (>800 species) Euglena Euglenophyceae glucan) wound helically around the cell ‘green algae’ Chlorophyll a Starch Unicellular: Covered with organic Marine and freshwa- Pyramimonas Chlorophyll b (in ) 1–8 flagella scales composed ter (  180 species) mainly of 3-deoxy- manno-2-octulosonic acid Coccoid, unicellular cells with Mainly freshwater, Chlamydomonas or colonial glycoprotein envelope, but also terrestrial Chlorella flagellates, multcel- non-flagellate cells and marine forms Dunaliella lular or multinucle- with polysaccharide (  1000 species) Oedogonium ate filaments. walls, which may Oocystis Usually with 2–4 contain some cellulose Volvox flagella Unicellular, multicel- Wall with microfibrils Almost all marine Acrosiphonia lular or multinucle- of polymers of a (  270 species) Enteromorpha ate filamentous. 2 or range of sugars Monostroma 4 flagella in repro- embedded in a complex Ulva ductive cells mucilaginous matrix Cladophorophyceae Multinucleate fila- Wall mainly of cellulose Mainly marine ments. Reproduc- (  420 species) tive cells with 2–4 Valonia flagella Bryopsidiophyceae Mainly giant multinu- Wall mainly of cellulose Mainly marine Caulerpa cleate cells. Repro- (  150 species) Codium Phycology ductive cells with Halimeda 2–4 flagella Udotea continued 5 6

Table 1 Continued Phycology Main pigments Main storage Cell forms Cell covering Habitats and Selected genera products number of species Dasycladophyceae Giant multinucleate Wall mainly of a fibrillar Marine and almost cells. Biflagellate re- b-1,4-linked mannan all tropical productive cells (  50 species) Trentepohliophyceae Uninucleate filaments. Polysaccharide walls Subaerial Reproductive cells with outer layer of (  80 species) Cephaleuros with 2–4 flagella sporopollenin Pleurastrophyceae Coccoid, sarcinoid or Walls with sporopollenin Mainly subaerial Myrmecia filamentous. Repro- like substances Trebouxia Clorophyta ‘green ductive cells with 2 algae’ flagella Klebsormidiophyceae Coccoid, sarcinoid Walls mainly of Freshwater and Coleochaete or filamentous crystalline cellulose moist subaerial Reproductive cells with 2 flagella Coccoid or filament- Walls mainly of crystal- Only freshwater Closterium (unicellular known ous. No flagellate line cellulose (  6000 species) Mougeotia as ‘desmids’) stages Spirogyra Complex thalli with Walls of crystalline Freshwater and Chara (‘stoneworts and multinucleate cells. cellulose brackish waters Nitella brittleworts’) Biflagellate male (  90 species) gametes Phycology

Table 2 Comparison of the classification system used in this latter cells are ‘’, in which elemental nitrogen is article with that used by Corliss for the Protozoa fixed into ammonium. Most of the filamentous forms can move by gliding over the surface of a substratum. Some This article Corliss planktonic species have gas , which help them to Eubacteria float. See also: Bacterial cells; Bacterial intracellular Division Cyanophyta membranes; Cyanobacterial heterocysts; Photosynthesis Class Cyanophyceaea and respiration in cyanobacteria Class Prochlorophytaa The cyanobacteria have a long fossil record. Some have Kingdom Eukaryota Empire Eukaryota been found mineralized in rocks over 3 billion years old. Kingdom Protozoa These algae are thought to have been the dominant forms Division Euglenophyta of life in the Proterozoic era (between 2.5 and 2 billion years Division Dinophyta Phylum Dinozoa ago). Certain kinds of cyanobacteria occur as fossils in Division Heterokontophyta Kingdom layered structures called stromatolites, and living stroma- Class Chrysophyceae Phylum Chrysophyta tolites of similar construction can still be seen in Shark Bay, Class Xanthophyceae Western Australia. See also: Algae: phylogeny and Class Eustigmatophyceae evolution; Fossil record Class Bacillariophyceae Phylum Diatomae The Prochlorophyta differ from the Cyanobacteria Class Raphidophyceae Phylum Raphidophyta mainly in their pigments and the fact that their thylakoids Class Dictyochophyceae Phylum Dictyochae are paired or stacked rather than single. Recent molecular Class Phaeophyceae Phylum Phaeophyta data indicate a close relationship between these two groups Division Cryptophyta Phylum Cryptophyca of prokaryotic algae, and it now seems hardly justifiable to Division Haptophyta Phylum Haptomonada retain the prochlorophytes as a separate division. Pro- Kingdom Plantae chloron, the first of the prochlorophytes to be described, Division Glaucophyta Phylum Glaucophyta grows symbiotically in ascidians, while the two genera dis- Division Rhodophyta Phylum Rhodophyta covered more recently are free living. Filaments of Pro- Division Chlorophyta chlorothrix occur in lakes, while the tiny Prochlorococcus Class Prasinophyceae has been found to be very abundant in oceanic . Class Chlorophyceae Class Cladophorophyceaeb Eukaryotic algae Class Bryopsidophyceae Class Dasycladophyceae Division Rhodophyta (red algae) Class Trentepohliophyceaeb c Most red algae are multicellular, mainly marine, growing Class Pleurastrophyceae attached to intertidal or subtidal rocks. A few are unicel- Class Klebsormidiophyceae lular (e.g. Porphyridium), while others are filamentous. The Class Ulvophyceae Phylum Ulvophyta filaments may be simple, branched or closely packed and Class Charophyceaed Phylum d held together by a gelatinous matrix to form diverse and Class Zygnematophyceae often spectacular thallus shapes. In the Florideophyceae aRecent molecular data indicate the Cyanophyta and the Pro- the individual cells in each filament, and sometimes also chlorophyta should be merged. those of neighbouring filaments, are connected by ‘pit b Recent data suggest that these two classes should be merged with plugs’. The cell walls are generally composed of cellulose Ulvophyceae. fibrils (but in Porphyra and Bangia the fibrillar material is a cThis class is polyphyletic and is likely to be distributed among sev- eral other classes of the Chlorophyta. polymer of xylose) embedded in an amorphous material dThis Charophyceae and the Zygnematophyceae are sister taxa to typically composed of sulfated galactose residues. Some the higher plants (the Streptophyta) and should probably not be red algae (e.g. in the Corallinales) have cell walls indurated included with the Chlorophyta. with calcium carbonate. See also: Algal cell walls The life history of most of the red algae is unique in that it has three phases: (1) a haploid free-living , (2) their extracellular coverings. Their cells, with diameters a diploid phase, the carposporophyte, which is parasitic on ranging from about 0.2 to 200 mm, may exist singly, some- the female gametophyte and (3) a second diploid phase, the times in a slimy matrix, or in filaments of indefinite length free-living tetrasporophyte. with or without a polysaccharide sheath. Some cyanobac- teria form large colonies or extensive mats. In one subclass the filaments may comprise two kinds of cells, some being Division Heterokontophyta pigmented and photosynthetic, whereas others have thick The Heterokontophyta include organisms whose flagellate walls with clear contents and internal knob-like projections cells have one long ‘hairy’ flagellum generally directed for- at the ends where they connect to adjacent cells. These ward during swimming, and a shorter smooth flagellum

7 Phycology that points backwards. The hairy flagellum has two rows of Diatoms are common members of the freshwater and lateral hairs called mastigonemes, composed of glycopro- marine , while many species are epiphytic or tein. In this division the chloroplast is enclosed not only by benthic. Their abundance means that they are significant its own double membrane, but also by a part of the endo- producers of organic matter, accounting for some 20–25% plasmic reticulum. This division includes a morphologi- of the total primary production in the ocean. cally diverse group of organisms ranging from small naked unicells and silica-walled diatoms to the complex multi- Division Dinophyta () cellular brown algae, as well as several classes of colourless The dinoflagellates are mainly unicellular, although there protozoans and even some organisms hitherto classed as are a few filamentous forms. The nucleus has a unique fungi. See also: Algal chloroplasts; Algal flagella; Algal structure, with the chromosomes almost always highly taxonomy: historical overview; Diatoms condensed in a sort of prophase. The cells swim with two dissimilar flagella, a transverse flagellum, usually coiled in Class Phaeophyceae (brown algae) a groove around the body of the alga, and a longitudinal The brown algae, mostly seaweeds, are among the most flagellum directed backwards that serves as a kind of rud- conspicuous algae especially in temperate areas, where der. The cell of many species is covered by a definite they flourish in the intertidal regions and may also form number of polygonal plates composed of cellulose. The extensive beds offshore. In the tropics, the brown alga free-living cells are normally haploid. After pairing (which Sargassum is abundant in shallow reef waters and can be is rarely observed), the diploid zygotes become thick- found free-floating in the Sargasso Sea. Brown algae range walled , which can survive for long periods in dark in size from microscopic filamentous forms such as Ecto- and cold conditions so that many of them can overwinter. carpus to large kelps up to 60 m in length. The structure and See also: Algal flagella differentiation of the thallus may be complex. For example, About 90% of the dinoflagellates are marine, being par- kelps such as Laminaria and Ecklonia have a holdfast, ticularly abundant in warmer waters. Most are planktonic, which attaches to a rocky bottom, a stipe (stem) and upper although benthic forms occur. Many are phagotrophic, leafy parts, the laminae or blades, where most of the and can catch and eat plankton cells even larger than photosynthesis occurs. Several genera such as Sargassum themselves. Some form symbioses with a wide range of and Fucus, and the kelps Macrocystis and Nereocystis, invertebrates, especially reef-building corals, where the have gas-filled bladders that aid in holding up the thallus cells (called zooxanthellae) live within the cells of their an- when submerged. See also: Biogeography of marine algae; imal hosts, to which they provide photosynthetically fixed Brown algae carbon compounds. This additional nutrition contributes As in the red algae, the cell wall consists of cellulose to the spectacular success of corals in shallow tropical wa- fibrils and amorphous polysaccharides, which in brown ters, where they may form extensive reefs. Some jellyfish, algae are characteristically alginates. These are polymers and the giant clam Tridacna, also harbour symbiotic zoo- of two sugar acids, D-mannuronic acid and L-guluronic xanthellae. See also: Algal symbioses acid, which make some of the brown algae economically Dinoflagellates can form large blooms known as red or valuable. brown tides. Some are phosphorescent, producing a lovely show along warm marine shores and lagoons, but a few are Class Bacillariophyceae (diatoms) very toxic as mentioned above. See also: Harmful algal blooms cells are characterized by having lacy silicified walls, called frustules, consisting of two, often overlapping, Division Haptophyta sections somewhat like Petri dishes. They exhibit a wide range of shapes and ornamentations with bumps and The are unicellular flagellates with paired, spines, and many openings that allow the exchange of nu- yellow-brown chloroplasts and a peculiar anterior fila- trients and waste products with the external environment. ment, the haptonema, which sometimes serves for attach- They may be single-celled or colonial, and are either bilat- ing the cells to a substrate or for catching prey. Among the erally symmetrical (the Pennales) or radially symmetrical most extensively studied is Emiliania, often common in (the Centrales). The inelastic nature of the silicified lateral marine phytoplankton. Many haptophytes have more or walls is responsible for the fact that on each division the less calcified scales: the chalk downs and the white cliffs cells of most species become a little smaller, in average of Dover are the accumulated fossil deposits of their dimensions, until eventually the cells of a population tend coccoliths. See also: Phytoplankton to die out unless they undergo some kind of reproduction during which they regain their original maximum size. Division Chlorophyta (green algae) Unlike many other unicellular algae, they are typically The Chlorophyta show a great diversity in form, life his- diploid. See also: Algal calcification and silicification; tory and , and this is reflected in the many classes Diatoms currently recognized (Table 2). Green algae of one kind or

8 Phycology another have successfully colonized almost all possible wet thesize and live autotrophically. In that plastids have their habitats. They are common in marine and fresh waters, own DNA, separate from that of the , and can some genera grow in soil, on trees and on rocks, while a few reproduce autonomously (no cell can produce chloroplasts species have adapted to flourish in extreme habitats such as de novo), they resemble prokaryotes. This observation led snow (e.g. Chlamydomonas nivalis) or salt lakes (e.g. Dun- Merezhkovsky to suggest, some 100 years ago, that plastids aliella salina). Almost all freshwater green algae are ha- originally evolved from intracellular symbionts, although for ploid, with only the zygote being diploid, whereas many many years this idea was not taken seriously. However, a kinds of marine algae such as Ulva have free-living haploid variety of recent techniques and discoveries, combined with and diploid stages. Most Caulerpales have only diploid molecular biological data, have shown almost conclusively thalli. See also: Algal ecology; Green algae that his hypothesis, which he called , is prob- The Chlorophyta have a wide range of flagellar arrange- ably correct. This means that, when we try to construct ments and modes of nuclear and . Like the red phylogenetic trees indicating how the various kinds of algae and brown algae, their cell walls have a structural, fibrillar have evolved, we must consider separately the evolutionary component, generally of cellulose, embedded in an amor- pathways of the nuclear (eukaryotic) and the chloroplastic phous matrix. The walls also contain some protein, as do (prokaryotic) components. In fact, recent evidence indicates those of higher plants. that, perhaps by ingesting or otherwise incorporating a Most chlorophytes have flagellate stages in their life his- cyanophyte-like cell, a heterotrophic, eukaryotic cell could tory. However, in the Zygnematophyceae (which include have become the first alga. It might have been a red alga, Spirogyra and the desmids) it seems that the ability to form since the plastids of rhodophytes have almost the same pig- flagellated cells has been totally lost, sexual reproduction ments as those of cyanophytes. Development of the ability to involving the conjugation of amoeboid gametes. Several produce a second, accessory chlorophyll could have given interesting evolutionary trends are found in the Chlor- rise to an ancestral green alga. Scientists are still speculating ophyta. In one branch of the Chlorophyceae (Volvocales) as to whether the different pigment systems of algae could the trend has been to form larger and larger colonies of have evolved like this, after the first symbiogenesis. Alter- flagellated cells, culminating in the spherical colonies of the natively, pigment diversification may have preceded the freshwater green alga Volvox. The marine Bryopsidophy- postulated ingestion of several different ancestors, ceae, on the other hand, have large multinucleate giving rise to the ancestors of red, brown, green and other (coenocytic) tubes that branch and intertwine to form the algal classes. These are challenging questions. More re- thallus, as in Halimeda and Udotea, or form a creeping search, and specifically more molecular–biological analyses, stolon from which upright photosynthetic, more or less will probably provide answers before long. See also:Algal -like shoots arise, as in Caulerpa. The coenocytic con- chloroplasts; Algal photosynthesis; Algal pigments; Endo- dition has also arisen in several other chlorophyte classes, symbionts; chloroplasts and other plastids among which the Dasycladales are probably the oldest and The situation has further complications. Electron-mi- most remarkable. Their giant vegetative cells, which may croscopic evidence indicates that viable plastids can be reach 20 cm in length, have only a single nucleus until handed on, transferred by a second incorporation from one shortly before reproduction. For this reason, Acetabularia kind of to another. Some euglenoids and crypto- has been extensively investigated in experimental studies of phytes apparently obtained their chloroplasts by ingesting interrelations between nucleus and cytoplasm. See also: and incorporating plastids from cells of a eukaryotic green Algae: phylogeny and evolution; Protozoan sexuality alga. Other kinds of ingesting cells may already possess and The Chlorophyta have a long fossil record, the retain their own kind of plastid, and in fact dinoflagellates Prasinophyceae dating back at least 600 million years. with two dissimilar kinds of plastids in each cell have al- The Bryopsidophyceae, Dasycladophyceae and Charo- ready been found, indicating that such transfers might phyceae are at least 400–500 million years old, and evi- have occurred more than once in evolution. See also: dence suggests that all the higher plants, from mosses and Molecular evolution: introduction liverworts to the angiosperms, evolved from green algae The branched genealogical trees of algae are apparently (possibly the Charophyceae) between 400 and 500 million more entangled by anastomoses than those of any other years ago. See also: Geological time: principles class of organisms. Phycology can be complicated. That is what makes the study of algae so interesting. Evolution Further Reading Not all algae are photosynthetic. About half of the dino- Berger S and Kaever MJ (1992) Dasycladales. Stuttgart: Thieme Verlag. flagellates, many of the euglenoids, and about half a dozen Bold HC and Wynne MJ (1978) Introduction to the Algae. New York: chlorophytes and diatoms, are colourless, and live hetero- Prentice-Hall. trophically like protozoa or fungi. However, the cells of most Borowitzka MA and Borowitzka LJ (eds) (1988) Micro-algal Biotech- other algae have chloroplasts that enable them to photosyn- nology. Cambridge: Cambridge University Press.

9 Phycology

Clayton MN and King RJ (eds) (1990) Biology of Marine Plants. Lewin RA (ed.) (1993) Origins of Plastids: Symbiogenesis, Pro- Melbourne: Longman Cheshire. chlorophytes, and the Origins of Chloroplasts. New York and London: Fritsch FE (1935, 1945) Structure and Reproduction of the Algae, vols. I Chapman & Hall. and II. Cambridge: Cambridge University Press. Lobban CS and Harrison PJ (1994) Seaweed Ecology and Physiology. Harris EH (1988) The Chlamydomonas Source-book. San Diego: Cambridge: Cambridge University Press. Academic Press. Round FE, Crawford R and Mann DG (1990) The Diatoms: Biology and Lee RE (1989) Phycology, 2nd edn. Cambridge: Cambridge University Morphology of the Genera. Cambridge: Cambridge University Press. Press. van den Hoek C, Mann DG and Jahns HM (1995) Algae. An Introduction Lembi CA and Waaland RJ (eds) (1988) Algae and Human Affairs. to Phycology. Cambridge: Cambridge University Press. Cambridge: Cambridge University Press.

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