The construction and composition of freshwater phytoplankton

The architecture of the cells of planktic algae conforms to a basic model, common to the majority of eukaryotic plants. A series of differentiated protoplasmic structures are enclosed within a vital membrane, the plasmalemma. This membrane is complex, comprising three or four distinct layers. In a majority of algae there is a further, non-living cell wall, made of cellulose or other, relatively pure, condensed carbohydrate polymer, such as pecten. Among some algal groups, the wall may be more or less impregnated with inorganic deposits of calcium carbonate or silica. High-power scanning electron microscopy reveals that these deposits can form a more or less continuous but variably thickened and fenestrated surface (as in the siliceous frustule wall of diatoms) or can comprise an investment of individual scales (like those of the Synurophyceae, made of silica, or of the coccol-ithophorids, made of carbonate). These exoskele-tons are distinctive and species-diagnostic. Some algae lack a polymer wall and are described as 'naked'. Both naked and walled cells may carry an additional layer of secreted mucilage.

The intracellular protoplasm (cytoplasm) is generally a viscous, gel-like suspension in which the nucleus, one or more plastids and various other organelles, including the endoplas-mic reticulum and the mitochondria, and some condensed storage products are maintained. The plastids vary hugely and interspecifically in shape - from a solitary axial cup (as in the Volvocales), numerous discoids (typical of centric diatoms), one or two broad parietal or axial plates (as in Cryptophytes) or more complex shapes (many desmids). All take on the intense coloration of the dominant photosyn-thetic pigments they contain - chlorophyll a and p-carotene and, variously, other chlorophylls and/or accessory xanthophylls. The stored condensates of anabolism are also conspicuously variable among the algae: starch in the chloro-phytes and cryptophytes, other carbohydrates in the euglenoids (paramylon) and the Chryso-phyceae, oils in the Xanthophyceae). Many also store protein in the cytoplasm. The quantities of all storage products vary with metabolism and environmental circumstances.

Intracellular vacuoles are to be found in most planktic algae but the large sap-filled spaces characteristic of higher-plant cells are relatively rare, other than in the diatoms. Osmoregulatory contractile vacuoles occur widely, though not universally, among the planktic algae, varying in number and distribution among the phylogenetic groups.

The prokaryotic cell of a planktic Cyanobac-terium is also bounded by a plasmalemma. It, too, is multilayered but has the distinctive bacterial configuration. The cells lack a membrane-bound nucleus and plastids, the genetic material and photosynthetic thylakoid membranes being unconfined through the main body (stroma) of the cell. The pigments, chlorophyll and accessory phycobilins, colour the whole cell. Glycogen is the principal photosynthetic condensate and pro-teinaceous structured granules may also be accumulated. Many planktic genera contain, potentially or actually, specialised intracellular pro-teinaceous gas-filled vacuoles which may impart buoyancy to the cell.

From an ecological point of view, the ultrastructural properties of phytoplankton cells assume considerable relevance to the resource requirements of their assembly, as well as to adaptive behaviour, productivity and dynamics of populations. Thus, it is important to establish a number of empirical criteria of cell composition that impinge upon the fitness of individual plankters and the stress thresholds relative to light, temperature and nutrient availability. These include methods for assessing the biomass of phytoplankton populations and the environmental capacity to support them.

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