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taxonomic listings in Table 1.1 are deliberately conservative.

Although the life forms of the plankton include acellular microorganisms (viruses) and a range of well-characterised Archaea (the halobac-teria, methanogens and sulphur-reducing bacteria, formerly comprising the Archaebacteria), the most basic photosynthetic organisms of the phytoplankton belong to the Bacteria (formerly, Eubacteria). The separation of the ancestral bacteria from the archaeans (distinguished by the possession of membranes formed of branched hydrocarbons and ether linkages, as opposed to the straight-chain fatty acids and ester linkages found in the membranes of all other organisms: Atlas and Bartha, 1993) occurred early in microbial evolution (Woese, 1987; Woese et al., 1990).

The appearance of phototrophic forms, distinguished by their crucial ability to use light energy in order to synthesise adenosine triphos-phate (ATP) (see Chapter 3), was also an ancient event that took place some 3000 million years ago (3 Ga BP (before present)). Some of these organisms were photoheterotrophs, requiring organic precursors for the synthesis of their own cells. Modern forms include green flexibacteria (Chlo-roflexaceae) and purple non-sulphur bacteria (Rhodospirillaceae), which contain pigments similar to chlorophyll (bacteriochlorophyll a, b or c). Others were true photoautotrophs, capable of reducing carbon dioxide as a source of cell carbon (photosynthesis). Light energy is used to strip electrons from a donor substance. In most modern plants, water is the source of reductant electrons and oxygen is liberated as a by-product (oxygenic photosynthesis). Despite their phyletic proximity to the photoheterotrophs and sharing a similar complement of bacteriochloro-phylls (Beja et al., 2002), the Anoxyphotobac-teria use alternative sources of electrons and, in consequence, generate oxidation products other than oxygen (anoxygenic photosynthesis). Their modern-day representatives are the purple and green sulphur bacteria of anoxic sediments. Some of these are planktic in the sense that they inhabit anoxic, intensively stratified layers deep in small and suitably stable lakes. The trait might be seen as a legacy of having evolved in a wholly anoxic world. However, aerobic, anoxy-genic phototrophic bacteria, containing bac-terichlorophyll a, have been isolated from oxic marine environments (Shiba et al., 1979); it has also become clear that their contribution to the oceanic carbon cycle is not necessarily insignificant (Kolber et al, 2001; Goericke, 2002).

Nevertheless, the oxygenic photosynthesis pioneered by the Cyanobacteria from about 2.8 Ga before present has proved to be a crucial step in the evolution of life in water and, subsequently, on land. Moreover, the composition of the atmosphere was eventually changed through the biological oxidation of water and the simultaneous removal and burial of carbon in marine sediments (Falkowski, 2002). Cyanobacterial photosynthesis is mediated primarily by chlorophyll a, borne on thylakoid membranes. Accessory

Table 1.1 Survey of the organisms in the phytoplankton

Domain: BACTERIA

Division: Cyanobacteria (blue-green algae)

Unicellular and colonial bacteria, lacking membrane bound plastids. Primary photosynthetic pigment is chlorophyll a, with accessory phycobilins (phycocyanin, phycoerythrin). Assimilation products, glycogen, cyanophycin. Four main sub-groups, of which three have planktic representatives. Order: CHROOCOCCALES

Unicellular or coenobial Cyanobacteria but never filamentous. Most planktic genera form mucilaginous colonies, and these are mainly in fresh water: Picophytoplanktic forms abundant in the oceans.

Includes: Aphanocapsa, Aphanothece, Chroococcus, Cyanodictyon, Gomphosphaeria, Merismopedia, Microcystis, Snowella, Synechococcus, Synechocystis, Woronichinia Order: OSCILLATORIALES

Uniseriate-filamentous Cyanobacteria whose cells all undergo division in the same plane. Marine and freshwater genera.

Includes: Arthrospira, Limnothrix, Lyngbya, Planktothrix, Pseudanabaena, Spirulina, Trichodesmium, Tychonema Order: NOSTOCALES

Unbranched-filamentous Cyanobacteria whose cells all undergo division in the same plane and certain of which may be facultatively differentiated into heterocysts. In the plankton of fresh waters and dilute seas.

Includes: Anabaena, Anabaenopsis, Aphanizomenon, Cylindrospermopsis, Gloeotrichia, Nodularia Exempt Division: Prochlorobacteria Order: PROCHLORALES

Unicellular and colonial bacteria, lacking membrane-bound plastids. Photosynthetic pigments are chlorophyll a and b, but lack phycobilins. Includes: Prochloroccus, Prochloron, Prochlorothrix Division: Anoxyphotobacteria

Mostly unicellular bacteria whose (anaerobic) photosynythesis depends upon an electron donor other than water and so do not generate oxygen. Inhabit anaerobic sediments and (where appropriate) water layers where light penetrates sufficiently Two main groups:

Family: Chromatiaceae (purple sulphur bacteria) Cells able to photosynthesise with sulphide as sole electron donor. Cells contain bacteriochlorophyll a, b or c.

Includes: Chromatium, Thiocystis, Thiopedia. Family: Chlorobiaceae (green sulphur bacteria) Cells able to photosynthesise with sulphide as sole electron donor. Cells contain bacteriochlorophyll a, b or c. Includes: Chlorobium, Clathrocystis, Pelodictyon.

Domain: EUCARYA

Phylum: Glaucophyta

Cyanelle-bearing organisms, with freshwater planktic representatives. Includes: Cyanophora, Glaucocystis. Phylum: Prasinophyta

Unicellular; mostly motile green algae with 1-16 laterally orapically placed flagella, cell walls covered with fine scales and plastids containing chlorophyll a and b. Assimilatory products mannitol, starch.

CLASS: Pedinophyceae Order: PEDINOMONADALES

Small cells, with single lateral flagellum. Includes: Pedinomonas CLASS: Prasinophyceae Order: CHLORODENDRALES

Flattened, 4-flagellated cells.

Includes: Nephroselmis, Scherffelia (freshwater); Mantoniella, Micromonas (marine) Order: PYRAMIMONADALES

Cells with 4 or 8 (rarely 16) flagella arising from an anterior depression. Marine and freshwater:

Includes: Pyramimonas Order: SCOURFIELDIALES

Cells with two, sometimes unequal, flagella. Known from freshwater ponds. Includes: Scoufiieldia Phylum: Chlorophyta (green algae)

Green-pigmented, unicellular, colonial, filamentous, siphonaceous and thalloid algae. One or more chloroplasts containing chlorophyll a and b. Assimilation product, starch (rarely, lipid). CLASS: Chlorophyceae

Several orders of which the following have planktic representatives: Order: TETRASPORALES

Non-flagellate cells embedded in mucilaginous or palmelloid colonies, but with motile propagules.

Includes: Paulschulzia, Pseudosphaerocystis Order: VOLVOCALES

Unicellular or colonial biflagellates, cells with cup-shaped chloroplasts.

Includes: Chlamydomonas, Eudorina, Pandorina, Phacotus, Volvox (in fresh waters); Dunaliella, Nannochloris (marine) Order: CHLOROCOCCALES

Non-flagellate, unicellular or coenobial (sometimes mucilaginous) algae, with many planktic genera.

Includes: Ankistrodesmus, Ankyra, Botryococcus, Chlorella, Coelastrum, Coenochloris, Crucigena, Choricystis, Dictyosphaerium, Elakatothrix, Kirchneriella, Monorophidium, Oocystis, Pediastrum, Scenedesmus, Tetrastrum Order: ULOTRICHALES

Unicellular or mostly unbranched filamentous with band-shaped chloroplasts. Includes: Geminella, Koliella, Stichococcus Order: ZYGNEMATALES

Unicellular or filamentous green algae, reproducing isogamously by conjugation. Planktic genera are mostly members of the Desmidaceae, mostly unicellular or (rarely) filmentous coenobia with cells more or less constricted into two semi-cells linked by an interconnecting isthmus. Exclusively freshwater genera. Includes: Arthrodesmus, Closterium, Cosmarium, Euastrum, Spondylosium, Staurastrum, Staurodesmus, Xanthidium

Phylum: Euglenophyta

Green-pigmented unicellular biflagellates. Plastids numerous and irregular; containing chlorophyll a and b. Reproduction by longitudinal fission. Assimilation product, paramylon, oil. One Class, Euglenophyceae, with two orders. Order: EUTREPTIALES

Cells having two emergent flagella, of approximately equal length. Marine and freshwater species.

Includes: Eutreptia Order: EUGLENALES

Cells having two flagella, one very short, one long and emergent.

Includes: Euglena, Lepocinclis, Phacus, Trachelmonas Phylum: Cryptophyta

Order: CRYPTOMONADALES

Naked, unequally biflagellates with one or two large plastids, containing chlorophyll a and c2 (but not chlorophyll b); accessory phycobiliproteins or other pigments colour cells brown, blue, blue-green or red; assimilatory product, starch. Freshwater and marine species.

Includes: Chilomonas, Chroomonas, Cryptomonas, Plagioselmis, Pyrenomonas,

Rhodomonas Phylum: Raphidophyta

Order: RAPHIDOMONADALES (syn. CHLOROMONADALES)

Biflagellate, cellulose-walled cells; two or more plastids containing chlorophyll a; cells yellow-green due to predominant accessory pigment, diatoxanthin; assimilatory product, lipid. Freshwater.

Includes: Gonyostomum Phylum: Xanthophyta (yellow-green algae)

Unicellular, colonial, filamentous and coenocytic algae. Motile species generally subapically and unequally biflagellated; two or many more discoid plastids per cell containing chlorophyll a. Cells mostly yellow-green due to predominant accessory pigment, diatoxanthin; assimilation product, lipid. Several orders, two with freshwater planktic representatives. Order: MISCHOCOCCALES

Rigid-walled, unicellular, sometimes colonial xanthophytes.

Includes: Goniochloris, Nephrodiella, Ophiocytium Order: TRIBONEMATALES

Simple or branched uniseriate filamentous xanthophytes.

Includes: Tribonema Phylum: Eustigmatophyta

Coccoid unicellular, flagellated or unequally biflagellated yellow-green algae with masking of chlorophyll a by accessory pigment violaxanthin. Assimilation product, probably lipid.

Includes: Chlorobotrys, Monodus Phylum: Chrysophyta (golden algae)

Unicellular, colonial and filamentous. often uniflagellate, or unequally biflagellate algae. Contain chlorophyll a, q and c2, generally masked by abundant accessory pigment, fucoxanthin, imparting distinctive golden colour to cells. Cells sometimes naked or or enclosed in an urn-shaped lorica, sometimes with siliceous scales. Assimilation products, lipid, leucosin. Much reclassified group, has several classes and orders in the plankton.

CLASS: Chrysophyceae Order: CHROMULINALES

Mostly planktic, unicellular or colony-forming flagellates with one or two unequal flagella, occasionally naked, often in a hyaline lorica or gelatinous envelope.

Includes: Chromulina, Chrysococcus, Chrysolykos, Chrysosphaerella, Dinobryon, Kephyrion, Ochromonas, Uroglena Order: HIBBERDIALES

Unicellular or colony-forming epiphytic gold algae but some planktic representatives.

Includes: Bitrichia CLASS: Dictyochophyceae Order: PEDINELLALES

Radially symmetrical, very unequally biflagellate unicells orcoenobia.

Includes: Pedinella (freshwater); Apedinella, Pelagococcus, Pelagomonas, Pseudopedinella (marine) CLASS: Synurophyceae Order: SYNURALES

Unicellular or colony-forming flagellates, bearing distinctive siliceous scales. Includes: Mallomonas, Synura Phylum: Bacillariophyta (diatoms)

Unicellular and coenobial yellow-brown, non-motile algae with numerous discoid plastids, containing chlorophyll a, q and c2, masked by accessory pigment, fucoxanthin. Cell walls pectinaceous, in two distinct and overlapping halves, and impregnated with cryptocrystalline silica. Assimilatory products, chrysose, lipids. Two large orders, both conspicuously represented in the marine and freshwater phytoplankton. CLASS: Bacillariophyceae Order: BIDDULPHIALES (centric diatoms)

Diatoms with cylindrical halves, sometimes well separated by girdle bands. Some species form (pseudo-)filaments by adhesion of cells at their valve ends.

Includes: Aulacoseira, Cyclotella, Stephanodiscus, Urosolenia (freshwater); Cerataulina, Chaetoceros, Detonula, Rhizosolenia, Skeletonema, Thalassiosira (marine)

Order: BACILLARIALES (pennate diatoms)

Diatoms with boat-like halves, no girdle bands. Some species form coenobia by adhesion of cells on their girdle edges.

Includes: Asterionella, Diatoma, Fragilaria, Synedra, Tabellaria (freshwater); Achnanthes, Fragilariopsis, Nitzschia (marine) Phylum: Haptophyta

CLASS: Haptophyceae

Gold or yellow-brown algae, usually unicellular, with two subequal flagella and a coiled haptonema, but with amoeboid, coccoid or palmelloid stages. Pigments, chlorophyll a, q and c2, masked by accessory pigment (usually fucoxanthin). Assimilatory product, chrysolaminarin. Cell walls with scales, sometimes more or less calcified. Order: PAVLOVALES

Cells with haired flagella and small haptonema. Marine and freshwater species. Includes: Diacronema, Pavlova

Order: PRYMNESIALES

Cells with smooth flagella, haptonema usually small. Mainly marine or brackish but some common in freshwater plankton.

Includes: Chrysochromulina, Isochrysis, Phaeocystis, Prymnesium Order COCCOLITHOPHORIDALES

Cell suface covered by small, often complex, flat calcified scales (coccoliths). Exclusively marine.

Include: Coccolithus, Emiliana, Florisphaera, Gephyrocapsa, Umbellosphaera

Phylum: Dinophyta

Mostly unicellular; sometimes colonial, algae with two flagella of unequal length and orientation. Complex plastids containing chlorophyll a, q and c2, generally masked by accessory pigments. Cell walls firm, or reinforced with polygonal plates. Assimilation products: starch, oil. Conspicuously represented in marine and freshwater plankton. Two classes and (according to some authorities) up to 11 orders. CLASS: Dinophyceae

Biflagellates, with one transverse flagellum encircling the cell, the other directed posteriorly. Order: GYMNODINIALES

Free-living, free-swimming with flagella located in well-developed transverse and sulcal grooves, without thecal plates. Mostly marine.

Includes: Amphidinium, Gymnodinium, Woloszynskia Order: GONYAULACALES

Armoured, plated, free-living unicells, the apical plates being asymmetrical. Marine and freshwater:

Includes: Ceratium, Lingulodinium Order: PERIDINIALES

Armoured, plated, free-living unicells, with symmetrical apical plates. Marine and freshwater.

Includes: Glenodinium, Gyrodinium, Peridinium Order: PHYTODINIALES

Coccoid dinoflagellates with thick cell walls but lacking thecal plates. Many epiphytic for part of life history. Some in plankton of humic fresh waters.

Includes: Hemidinium CLASS: Adinophyceae Order: PROROCENTRALES

Naked or cellulose-covered cells comprising two watchglass-shaped halves. Marine and freshwater species.

Includes: Exuviella, Prorocentrum

pigments, called phycobilins, are associated with these membranes, where they are carried in granular phycobilisomes. Life forms among the Cyanobacteria have diversified from simple coc-coids and rods into loose mucilaginous colonies, called coenobia, into filamentous and to pseu-dotissued forms. Four main evolutionary lines are recognised, three of which (the chroococ-calean, the oscillatorialean and the nostocalean; the stigonematalean line is the exception) have major planktic representatives that have diversified greatly among marine and freshwater systems. The most ancient group of the surviving groups of photosynthetic organisms is, in terms of individuals, the most abundant on the planet.

Links to eukaryotic protists, plants and animals from the Cyanobacteria had been supposed explicitly and sought implicitly. The discovery of a prokaryote containing chlorophyll a and b but lacking phycobilins, thus resembling the pigmentation of green plants, seemed to fit the bill (Lewin, 1981). Prochloron, a symbiont of salps, is not itself planktic but is recoverable in collections of marine plankton. The first description of Prochlorothrix from the freshwater phytoplankton in the Netherlands (Burger-Wiersma et al., 1989) helped to consolidate the impression of an evolutionary 'missing link' of chlorophyll-a- and -b-containing bacteria. Then came another remarkable finding: the most abundant picoplankter in the low-latitude ocean was not a Synechococcus, as had been thitherto supposed, but another oxyphototrophic prokaryote containing divinyl chlorophyll-a and -b pigments but no bilins (Chisholm et al., 1988, 1992); it was named Prochlorococcus. The elucidation of a bio-spheric role of a previously unrecognised organism is achievement enough by itself (Pinevich et al., 2000); for the organisms apparently to occupy this transitional position in the evolution of plant life doubles the sense of scientific satisfaction. Nevertheless, subsequent investigations of the phylogenetic relationships of the newly defined Prochlorobacteria, using immuno-logical and molecular techniques, failed to group Prochlorococcus with the other Prochlorales or even to separate it distinctly from Synechococcus (Moore et al., 1998; Urbach et al., 1998). The present view is that it is expedient to regard the Prochlorales as aberrent Cyanobacteria (Lewin, 2002).

The common root of all eukaryotic algae and higher plants is now understood to be based upon original primary endosymbioses involving early eukaryote protistans and Cyanobacteria (Margulis, 1970, 1981). As more is learned about the genomes and gene sequences of microorganisms, so the role of 'lateral' gene transfers in shaping them is increasingly appreciated (Doolit-tle et al., 2003). For instance, in terms of ultrastructure, the similarity of 16S rRNA sequences, several common genes and the identical pho-tosynthetic proteins, all point to cyanobacterial origin of eukaruote plastids (Bhattacharya and Medlin, 1998; Douglas and Raven, 2003). Pragmatically, we may judge this to have been a highly successful combination. There may well have been others of which nothing is known, apart from the small group of glaucophytes that carry cyanelles rather than plastids. The cyanelles are supposed to be an evolutionary intermediate between cyanobacterial cells and chloro-plasts (admittedly, much closer to the latter). Neither cyanelles nor plastids can grow independently of the eukaryote host and they are apportioned among daughters when the host cell divides. There is no evidence that the handful of genera ascribed to this phylum are closely related to each other, so it may well be an artificial grouping. Cyanophora is known from the plankton of shallow, productive calcareous lakes (Whitton in John et al, 2002).

Molecular investigation has revealed that the seemingly disparate algal phyla conform to one or other of two main lineages. The 'green line' of eukaryotes with endosymbiotic Cyanobacteria reflects the development of the chlorophyte and euglenophyte phyla and to the important offshoots to the bryophytes and the vascular plant phyla. The 'red line', with its secondary and even tertiary endosymbioses, embraces the evolution of the rhodophytes, the chrysophytes and the haptophytes, is of equal or perhaps greater fascination to the plankton ecologist interested in diversity.

A key distinguishing feature of the algae of the green line is the inclusion of chlorophyll b among the photosynthetic pigments and, typically, the accumulation of glucose polymers (such as starch, paramylon) as the main product of carbon assimilation. The subdivision of the green algae between the prasinophyte and the chlorophyte phyla reflects the evolutionary development and anatomic diversification within the line, although both are believed to have a long history on the planet (~1.5 Ga). Both are also well represented by modern genera, in water generally and in the freshwater phyto-plankton in particular. Of the modern prasino-phyte orders, the Pedinomonadales, the Chloro-dendrales and the Pyramimonadales each have significant planktic representation, in the sense of producing populations of common occurrence and forming 'blooms' on occasions. Several modern chlorophyte orders (including Oedogoniales, Chaetophorales, Cladophorales, Coleochaetales, Prasiolales, Charales, Ulvales a.o.) are without modern planktic representation. In contrast, there are large numbers of volvocalean, chloro-coccalean and zygnematalean species in lakes and ponds and the Tetrasporales and Ulotrichales are also well represented. These show a very wide span of cell size and organisation, with flagellated and non-motile cells, unicells and filamentous or ball-like coenobia, with varying degrees of mucilaginous investment and of varying consistency. The highest level of colonial development is arguably in Volvox, in which hundreds of networked biflagellate cells are coordinated to bring about the controlled movement of the whole. Colonies also reproduce by the budding off and release of near-fully formed daughter colonies. The desmid members of the Zygnematales are amongst the best-studied green plankters. Mostly unicellular, the often elaborate and beautiful architecture of the semi-cells invite the gaze and curiosity of the microscopist.

The euglenoids are unicellular flagellates. A majority of the 800 or so known species are colourless heterotrophs or phagotrophs and are placed by zoologists in the protist order Euglenida. Molecular investigations reveal them to be a single, if disparate group, some of which acquired the phototrophic capability through secondary symbioses. It appears that even the phototrophic euglenoids are capable of absorbing and assimilating particular simple organic solutes. Many of the extant species are associated with organically rich habitats (ponds and lagoons, lake margins, sediments).

The 'red line' of eukaryotic evolution is based on rhodophyte plastids that contain phycobilins and chlorophyll a, and whose single thylakoids lie separately and regularly spaced in the plastid stroma (see, e.g., Kirk, 1994). The modern phylum Rhodophyta is well represented in marine (especially; mainly as red seaweeds) and freshwater habitats but no modern or extinct plank-tic forms are known. However, among the interesting derivative groups that are believed to owe to secondary endosymbioses of rhodophyte cells, there is a striking variety of planktic forms.

Closest to the ancestral root are the cryp-tophytes. These contain chlorophyll c2, as well as chlorophyll a and phycobilins, in plastid thylakoids that are usually paired. Living cells are generally green but with characteristic, species-specific tendencies to be bluish, reddish or olive-tinged. The modern planktic representatives are exclusively unicellular; they remain poorly known, partly because thay are not easy to identify by conventional means. However, about 100 species each have been named for marine and fresh waters, where, collectively, they occur widely in terms of latitude, trophic state and season.

Next comes the small group of single-celled flagellates which, despite showing similarities with the cryptophytes, dinoflagellates and euglenophytes, are presently distinguished in the phylum Raphidophyta. One genus, Gonyostomum, is cosmopolitan and is found, sometimes in abundance, in acidic, humic lakes. The green colour imparted to these algae by chlorophyll a is, to some extent, masked by a xanthophyll (in this case, diatoxanthin) to yield the rather yellowish pigmentation. This statement applies even more to the yellow-green algae making up the phyla Xanthophyta and Eustigmatophyta. The xanthophytes are varied in form and habit with a number of familiar unicellular non-flagellate or biflagellate genera in the freshwater plankton, as well as the filamentous Tribonema of hard-water lakes. The eustigmatophytes are unicellular coc-coid flagellates of uncertain affinities that take their name from the prominent orange eye-spots.

The golden algae (Chrysophyta) represent a further recombination along the red line, giving rise to a diverse selection of modern unicellular, colonial or filamentous algae. With a distinctive blend of chlorophyll a, c1 and c2, and the major presence of the xanthophyll fucoxanthin, the chrysophytes are presumed to be close to the Phaeophyta, which includes all the macro-phytic brown seaweeds but no planktic vegetative forms. Most of the chrysophytes have, in contrast, remained microphytic, with numerous planktic genera. A majority of these come from fresh water, where they are traditionally supposed to indicate low nutrient status and productivity (but see Section 3.4.3: they may simply be unable to use carbon sources other than carbon dioxide). Mostly unicellular or coenobial flagellates, many species are enclosed in smooth protective loricae, or they may be beset with numerous delicate siliceous scales. The group has been subject to considerable taxonomic revision and reinterpretation of its phylogenies in recent years. The choanoflagellates (formerly Craspedo-phyceae, Order Monosigales) are no longer considered to be allied to the Chrysophytes.

The last three phyla named in Table 1.1, each conspicuously represented in both limnetic and marine plankton - indeed, they are the main pelagic eukaryotes in the oceans - are also remarkable in having relatively recent origins, in the mesozoic period. The Bacillariophyta (the diatoms) is a highly distinctive phylum of single cells, filaments and coenobia. The characteristics are the possession of golden-brown plastids containing the chlorophylls a, c1 and c2 and the accessory pigment fucoxanthin, and the well-known presence of a siliceous frustule or exoskeleton. Generally, the latter takes the form of a sort of lidded glass box, with one of two valves fitting in to the other, and bound by one or more girdle bands. The valves are often patterned with grooves, perforations and callosities in ways that greatly facilitate identification. Species are ascribed to one or other of the two main diatom classes. In the Biddulphiales, or centric diatoms, the valves are usually cylindrical, making a frustule resembling a traditional pill box; in the Bacillariales, or pennate diatoms, the valves are elongate but the girdles are short, having the appearance of the halves of a date box. While much is known and has been written on their morphology and evolution (see, for instance, Round et al., 1990), the origin of the siliceous frustule remains obscure.

The Haptophyta are typically unicellular gold or yellow-brown algae, though having amoeboid, coccoid or palmelloid stages in some cases. The pigment blend of chlorophylls a, c1 and c2, with accessory fucoxanthin, resembles that of other gold-brown phyla. The haptophytes are distinguished by the possession of a haptonema, located between the flagella. In some species it is a prominent thread, as long as the cell; in others it is smaller or even vestigial but, in most instances, can be bent or coiled. Most of the known extant haptophyte species are marine; some genera, such as Chrysochromulina, are represented by species that are relatively frequent members of the plankton of continental shelves and of mesotrophic lakes. Phaeocystis is another haptophyte common in enriched coastal waters, where it may impart a visible yellow-green colour to the water at times, and give a notoriously slimy texture to the water (Hardy, 1964).

The coccolithophorids are exclusively marine haptophytes and among the most distinctive microorganisms of the sea. They have a characteristic surface covering of coccoliths - flattened, often delicately fenestrated, scales impregnated with calcium carbonate. They fossilise particularly well and it is their accumulation which mainly gave rise to the massive deposits of chalk that gave its name to the Cretaceous (from Greek kreta, chalk) period, 120-65 Ma BP. Modern coc-colithophorids still occur locally in sufficient profusion to generate 'white water' events. One of the best-studied of the modern coccolithophorids is Emiliana.

The final group in this brief survey is the dinoflagellates. These are mostly unicellular, rarely colonial biflagellated cells; some are relatively large (up to 200 to 300 ^m across) and have complex morphology. Pigmentation generally, but not wholly, reflects a red-line ancestry, the complex plastids containing chlorophyll a, c1 and c2 and either fucoxanthin or peridinin as accessory pigments, possibly testifying to tertiary endosymbioses (Delwiche, 2000). The group shows an impressive degree of adaptive radiation, with naked gymnodinioid nanoplankters through to large, migratory gonyaulacoid swimmers armoured with sculpted plates and to deep-water shade forms with smooth cellulose walls such as Pyrocystis. Some genera are non-planktic and even pass part of the life cycle as epiphytes. Freshwater species of Ceratium and larger species of Peridinium are conspicuous in the plankton of certain types of lakes during summer stratification, while smaller species of Peridinium and other genera (e.g. Glenodinium) are associated with mixed water columns of shallow ponds.

Figure 1.1

Non-motile unicellular phytoplankters. (a) Synechococcus sp.; (b) Ankyra judayi; (c) Stephanodiscus rotula; (d) Closterium cf. acutum. Scale bar, 10 |im. Original photomicrographs by Dr H. M. Canter-Lund, reproduced from Reynolds (1984a).

Figure 1.1

Non-motile unicellular phytoplankters. (a) Synechococcus sp.; (b) Ankyra judayi; (c) Stephanodiscus rotula; (d) Closterium cf. acutum. Scale bar, 10 |im. Original photomicrographs by Dr H. M. Canter-Lund, reproduced from Reynolds (1984a).

The relatively recent appearance of diatoms, coccolithophorids and dinoflagellates in the fossil record provides a clear illustration of how evolutionary diversification comes about. Although it cannot be certain that any of these three groups did not exist beforehand, there is no doubt about their extraordinary rise during the Mesozoic. The trigger may well have been the massive extinctions towards the end of the Permian period about 250 Ma BP, when a huge release of volcanic lava, ash and shrouding dust from what is now northern Siberia brought about a world-wide cooling. The trend was quickly reversed by accumulating atmospheric carbon dioxide and a period of severe global warming (which, with positive feedback of methane mobilisation from marine sediments, raised ambient temperatures by as much as

10-11 °C). Life on Earth suffered a severe setback, perhaps as close as it has ever come to total eradication. In a period of less than 0.1 Ma, many species fell extinct and the survivors were severely curtailed. As the planet cooled over the next 20 or so million years, the rump biota, on land as in water, were able to expand and radiate into habitats and niches that were otherwise unoccupied (Falkowski, 2002).

Dinoflagellate fossils are found in the early Triassic, the coccolithophorids from the late Tri-assic (around 180 Ma BP). Together with the diatoms, many new species appeared in the Jurassic and Cretaceous periods. In the sea, these three groups assumed a dominance over most other forms, the picocyanobacteria excluded, which persists to the present day.

Figure 1.2

Planktic unicellular flagellates. (a) Two variants of Ceratium hirundinella; (b) overwintering cyst of Ceratium hirundinella, with vegetative cell for comparison; (c) empty case of Peridinium willei to show exoskeletal plates and flagellar grooves; (d) Mallomonas caudata; (e) Plagioselmis nannoplanctica; (f) two cells of Cryptomonas ovata; (g) Phacus longicauda; (h) Euglena sp.; (j) Trachelomonas hispida. Scale bar, 10 |m. Original photomicrographs by Dr H. M. Canter-Lund, reproduced from Reynolds (1984a).

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