The known micronutrients, as they are now understood, include several metals whose availability in natural waters may vary between deficiency and toxic concentrations. Some (barium, vanadium) are required in such trivial amounts that their specific inclusion in artificial culture media is considered unnecessary; their presence as impurities in other laboratory-grade chemicals or among the solutes that leach from the containing glassware suffices for most practical purposes. On the other hand, iron, manganese, zinc, copper, molybdenum and cobalt are necessary additions to culture media (Huntsman and Sunda, 1980), even if the information about their deployment and effects is difficult to interpret. For instance, the cellular content of manganese (Mn) ranks next to that of iron. The finite requirement for its central role in re-reducing P+80 in photosynthesis (see Section 3.2.1) is usually fulfilled by the amounts present in lakes, which may be sufficiently abundant to bring about external deposition on the cell walls. In (unspecified) excesses of manganese ions are supposed to inhibit algal growth. Although there are occasional references to growth being stimulated by the addition of manganese (Goldman, 1964), there is little evidence to suggest that the metal is ever a significant growth-regulating factor. Similar conclusions apply to zinc, copper and cobalt, insofar as each participates vitally in one or more enzymic or cytochrome reactions. In solution at concentrations >10 nmol L-1 each is seriously toxic to a majority of algae. Copper sulphate is still widely used as an algicide (although, in many countries, its use in waters eventually supplied for drinking is banned) and is effective at concentrations of 0.3-1.0 mg CuSO4 L-1 (2-6 |imol Cu L-1). Toxicity varies interspecifically among algae and in relation to the organic content of the water (Huntsman and Sunda, 1980). Possible toxic effects of redox-sensitive metal species may be magnified in relevant habitats, including lakes subject to seasonal deep-water anoxia, where bioavailable species may be recycled (Achterberg et al., 1997).
Clear incidences of regulation (or 'limitation') of algal activity through deficiency of these elements are scarce. In contrast, both molybdenum and (especially) iron are known to fulfil, on occasions, this key limiting role in pelagic systems. The case for molybdenum has been made on a number of occasions (Goldman, 1960; Dumont, 1972). In the best-known case, additions of a few micrograms of molybdenum per litre to water from Castle Lake (in an arid part of California) were sufficient to promote, quite strikingly, the growth and the attainment of a higher standing biomass of phytoplankton, where previously, despite the presence of adequate levels of bioavailable P and DIN, activities had been severely constrained (Goldman, 1960). In a later investigation of the same lake, molybdenum addition was shown to stimulate carbon fixation and nitrogen uptake rates, especially when nitrate dominated the nitrogen sources (Axler et al., 1980). Molybdenum is specifically involved in the nitrogen metabolism of the cell, participating as a co-factor in the action of nitrate reductase and (in Cyanobacteria) nitrogenase, and in the intracellular transport of nitrogen (Rueter and Peterson, 1987). The cell requirement is estimated to be about 1/50 000 of that for P (~0.2 |imol Mo (mol cell C)-1), which can be accumulated from external concentrations of the order of 10-11 M. According to Steeg et al. (1986), Mo deficiency nevertheless results in symptoms of nitrogen limitation, including heterocyst formation among members of the Nostocales, even though rates of nitrogen fixation are themselves seriously impaired.
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