Besides the provision of a full spectrum of inorganic nutrients, successive generations of sub-cultured axenic (bacteria-free) clones of many species of microalgae are known to benefit from organic supplements at low concentrations. In particular, thiamine, biotin and cyanocobal-amine (vitamin B12) have been shown to be essential nutrients to some species of algae at least. The reviews of Provasoli and Carlucci (1974) and Swift (1980) highlight the widespread dependence upon organic micronutrients among algae of the 'red' evolutionary line. The centric diatoms and several pennate species have been shown to require a supply of vitamin B12. Most dinoflag-ellates studied also require vitamin B12, either alone or in combination with thiamine or biotin or both. Among the Haptophyceae, a majority of species tested have a requirement for thiamine, sometimes with B12 as well. Among the Chryso-phyceae, most species require the supply of two or three vitamins.
The biochemical requirement for these substances is general: thiamine is a co-factor in the decarboxylation of pyruvic acid; biotin is a co-factor in the carboxylation and transcarb-oxylation reactions of photosynthesis. Vitamin B12 mediates reactions involving intramolecular recombinations involving C--- C bond cleavages (Swift, 1980). The point is that many bacteria (including the Cyanobacteria), most green algae and higher plants are capable of synthesising these products themselves and are independent of an external supply. As a consequence of their metabolism, however, thiamine, biotin and cyanocobalamine are generally measurable components of the labile DOC fraction in the sea. Moreover, each is present in concentrations (equivalent to 0.5 to 5 ng C L-1: Williams, 1975) sufficient to saturate the demands of individual plankters (said to be in the order 10-12-10-10 mol L-1: Swift, 1980). The demand and supply of organic trace substances seem not to exert any strong ecological outcome on the competitive potential of plankters in the wild.
Although the major ions in lake and sea water (including Ca, Mg, Na, K, Cl) are no less important to planktic cells than are P, N or the micronutri-ents, they are treated in less detail here because their ecological role in regulating species composition and abundance is either trivial or unclear.
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