After the six elements that each contribute >1% of the ash-free dry mass of phytoplankton cells (in descending order of contribution by mass, C, O, H, N, P, S), all the others figure in relatively small fractions of the cytological structure or participate in its function. Some of these are used in small quantities, despite being normally relatively abundant in the solute content of lake and sea water, where they are constitute some of the major ions (Na, K, Ca, Mg, Cl). Most of the remainder used by plankton cells in small quantities also generally occur naturally at low concentrations. These used to be known as the 'trace elements' but are now more commonly referred to as micronutrients.
Much of the early knowledge of the important part played by micronutrients in the growth and physiological well-being of phytoplankton came not from analysis of lake or sea water but from the attempts to grow algae in prepared artificial media. The use of carefully formulated solutions, contrived and refined through the experimental pragmatism of such pioneers as Chu (e.g. 1942, 1943), Pringsheim (1946), Gerloff and co-workers (1952), Provasoli (see especially Provasoli et al., 1957) and Gorham et al. (1964; see also Stein, 1973) progressively identified the additional 'ingredients' necessary to keep laboratory clones in a healthy, active, vegetative state. More recently, of course, chromatographic applications, atomic-absorption spectroscopy and X-ray microanalysis have helped to confirm and greatly amplify the elemental composition of planktic cells, in field samples as well as in laboratory cultures, even to the specific intracellu-lar locations (Booth et al., 1987; Krivtsov et al., 1999). Yet more recently, a method for measuring the chemiluminescence emitted in reactions between metals and luminol appear to be both precise and sensitive at very low sample concentrations (Bowie et al., 2002).
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