Organic composition

Discounting the typical 5-12% ash (possibly up to 80% in the case of some diatoms) content of air-dried plankters, the balancing mass is supposed to be the organic components of the cell, derived from the living protoplasm. In comprising mainly proteins, lipids and condensed carbohydrates, albeit in variable proportions, the elemental composition of the ash-free dry material is dominated by carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), together with smaller amounts of phosphorus (P) and sulphur (S). At least 14 other elements (Ca, Mg, Na, Cl, K, Si, Fe, Mn, Mo, Cu, Co, Zn, B, Va) are consistently recoverable if sufficient analytical rigour is applied (Lund, 1965). It is impressive that, whilst alive, every planktic cell had not only the capability of taking up these elements from extremely dilute media but also the success in doing so. This should be borne in mind when interpreting the relative quantities in which these elements occur in the dry matter of cells, because not all were necessarily equally available relative to demand. Besides falling deficient in one element that is relatively scarce, others that are relatively abundant may tend to accumulate in the cell. In this way, the ratios in which the elements make up the ash-free air-dried algal tissue often give a reliable reflection of the conditions of nutrient availability in the growth medium. Compounded by the special mechanisms that cells may have for taking up and retaining elements from unreliable environmental sources (to be explored in Chapter 4), the absolute quantities of several of the component elements are liable to wide variation.

Not surprisingly, the elemental ratios in natural phytoplankton plankton have for long aroused the interest of physiological ecologists and of biogeochemists and they have been much studied and reported. Absolute quantities do vary substantially, as do the ratios among them. Yet, remarkably, the same data indicate

Table 1.4 The silicon content of some freshwater planktic diatoms


Si (pg cell-1)




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