Phytoplankton Taxonomy

Phytoplankton taxonomy is based on cell morphology (shape), surface ornamentation, colour, and food reserves. Most often identification is through a compound or inverted light microscope at 250-400x magnification. For some species it is necessary to observe details through an electron microscope before a species can be correctly identified. Bacillariophyceae (Diatoms) Unicellular alga with siliceous frustules; each half of the frustule consists of a valve and a connecting or girdle band; there are sometimes numerous intercalary bands. Valves are marked in various ways by pores, poroids, knobs, areolae, and ribs. Cytoplasm concentrated near the outer areas of cell while inner region contains a large vacuole. One to several platelike or many disk-shaped chromatophores usually of a yellowish or greenish-brown colour. Storage products are chrysolaminaran and oil droplets. The class Bacillariophyceae is divided into two main orders: Centrales and Pennales Centrales (Centric diatoms) Have concentric or radiating sculpture around a point or points No raphe or pseudoraphe is present and spontaneous movement does not occur By far the most important in the plankton Common marine genera: Skeletonema, Chaetoceros, Thalassiosira, Pennales (Pennate diatoms) Sculpture arranged with relation to a longitudinal line Often possess a raphe or pseudoraphe; many species are capable of spontaneous movement. More important in the benthos (sediments). Common marine pelagic genera: Pseudonitzschia, Thalassionema, Navicula Pseudonitzschia delicatissima - produces domoic acid. Pseudonitzschia multiseries - produces domoic acid. Dinophyceae (Dinoflagellates) Most possess a transverse girdle (cingulum) and a longitudinal furrow (sulcus). Cingulum and sulcus meet on the ventral Species are either thecate (armoured) or athecate (naked) depending on whether they have cellulose plates in addition to the cell membrane. Starch and oil as food reserves (starch stains darkly with LugolÕs iodine) The class Dinophyceae is divided into a number of orders: Prorocentrales Lack furrows Flagella inserted near apical end of cell Theca = 2 large valves with small platelets near flagellar insertion. Platelets may have spines. Dinophysiales Has both cingulum and sulcus. Cingulum is displaced towards apical end of cell. Wings (called lists) line both furrows; wing along left side is especially well-developed and supported by three ribs. Most species are strongly compressed; therefore, usually seen in lateral view. Identification is by size and shape of cell, ornamentation of plates, morphology of left sulcal list. Gymnodiniales Naked therefore does not retain morphology upon preservation. Identification is by size and general shape, position and path of cingulum and sulcus, apical groove, stripes on surface, number of chloroplasts, position and shape of nucleus. Gymnodinium and Gyrodinium separated by relative displacement of cingulum; displacement is > 1/5 cell length in Gyrodinium. Peridiniales Toxic genus = Alexandrium Identification by plate arrangement (squash cells carefully to remove cell contents) Distinctive pore plate at apex (resembles a pork chop) Some species form distinctive chains (e.g., Alexandrium catenella) Noctilucales Main phase is a large naked cell, often with a tentacle. Common marine genera: Noctiluca, Kofoidinium Pyrocystales Main phase is a non-motile coccoid cell and reproduction is by a gonyaulacoid or gymnodinoid motile cell. Common marine genera: Pyrocystis Raphidophyceae (Chloromonads) Less than 20 described species Chloroplasts discoid, yellow to yellow brown Biflagellate: anterior flagellum pulls cells forward, posterior flagellum is trailing. Outer membrane disappears when preserved with LugolÕs iodine, resembles a raspberry. Prymnesiophyceae (Prymnesiophytes/Haptophytes) Covered with fine organic scales; composed of calcium carbonate in coccolithophores Usually two golden brown chloroplasts Biflagellate with one haptonema Approximately 50 species of Chrysochromulina alone. Diverse generic array of coccolithophores Dictyophyceae (Silicoflagellates) Numerous discoid golden-brown chloroplasts (in photosynthetic genera) Single anterior flagellum Exterior siliceous skeleton composed of tubular elements Very few species (3) Euglenopyceae (Euglenoids) Pliable, green biflagellates Common marine genera: Eutreptiella, Euglena Prasinophyceae (Prasinophytes) Primitive precursors to higher green algae 1-8 flagella, equal or unequal in length, deep flagellar pit common Single, bowl-shaped chloroplast occupying most of interior space of cell Principal soluble photosynthetic product is mannitol Starch stains reddish-purple or reddish-brown with Lugol's Common marine genera: Pyramimonas, Tetraselmis, Micromonas, Heteromastix, Nephroselmis Cryptophyceae (Cryptomonads) Tear drop shaped cells, flattened dorso-ventrally Two flagella, equal or subequal in length, covered in hairs. Rectangular or hexagonal surface pattern evident with electron microscope. Usually one or two chloroplasts; broad range of pigmentation (red, blue, olive-yellow, brown, green); some genera colourless. Photosynthetic forms store starch that stains darkly when preserved with Lugol's iodine. Gullet at flagellar base is lined with ejectosomes. Common marine genera: Cryptomonas, Rhodomonas, Plagioselmis, Chroomonas, Hemiselmis Chrysophyceae (Chrysophytes) Golden-brown flagellates; usually 1 or 2 chloroplasts per cell, some with 6. Usually two unequal flagella inserted at oblique angle to each other; larger flagellum directed forward when swimming, smaller flagellum directed towards posterior of cell. Some are naked, some have cell coverings of either scales, loricas, or cell walls. Common marine genera: Ochromonas, Apedinella, Pseudopedinella, Dinobryon Chlorophyceae (Chlorophytes) 2 or 4 flagella of equal length, anteriorly inserted in cell (swims with flagella forward). Most species have large, cup-shaped chloroplast Common marine genera: Chlamydomonas, Dunaliella