I Log/log plot of cell dry mass (Wc) against cell volume (v) for various freshwater phytoplankers (data in Table 3: Cyanobacteria; □, diatoms; o, chlorophytes; others). The equation of the regression is Wc = 0.47 v0 99. Redrawn from Reynolds (1984a).

silicon in lakes and oceans are frequently inadequate to meet the potential demands of unfettered diatom development. On the other hand, the requirement is obligate and within relatively narrow, species-specific ranges and uptake is subject to physiologically definable levels. The biological availability of silicon, its consumption and deployment, as well as the fate of its biogenic polymers, are of special relevance to planktic ecology, as they may well determine the environmental carrying capacity of new diatom production. Thus, they have some selective value for particular types of diatom, or for other types of non-siliceous plankter, when external supplies are substantially deficient.

The cells of all living organisms have a requirement for the small amounts of silicon involved in the synthesis of nucleic acids and proteins (generally <0.1% of dry mass: Sullivan and Volcani, 1981). However, it is the demands of those groups of protistans and poriferans that characteristically employ silicon in skeletal structures - notably diatoms, other chrysophytes radi-olarians and sponges - that impinge most on the geochemical cycling of silicon (Simpson and

Volcani, 1981). In passing, it should be noted that skeletal silica also makes up some 10% of the dry weight of the grasses, whose co-evolution with the mammals and relative abundance during the tertiary period may have been responsible for the long-term fluctuations in the export and availability of the main soluble source of silicon (monosilicic acid: Siever, 1962; Stumm and Morgan, 1996) in the aquatic environments (Falkowski, 2002).

For the moment, our concern is with the cell content of silicon. Most is deposited as a cryptocrystalline polymer of silica ((SiO2)n), resembling opal (Volcani, 1981). The silica contents of several species of planktic diatoms have been derived, either by direct analysis or, indirectly, from the depletion of dissolved silicon by a known specific recruitment of cells by growth. Unlike other elements critical to their survival, diatoms take up scarcely more silicon than is immediately required to form the frus-tules of the next generation (Paasche, 1980; Sullivan and Volcani, 1981). As already indicated, the amounts deposited are generally quite species-specific (Reynolds, 1984a; see also Table 1.4), at least when variability in cell size is taken into account (Lund, 1965; Jaworski et al., 1988). Cell-specific silicon requirements differ considerably among planktic species, reportedly ranging between 0.5% (in the marine Phaeodactylum tricornutum: Lewin et al., 1958) and 37% of dry weight (in some freshwater Aulacoseira spp.: Lund, 1965; Sicko-Goad et al, 1984).

In terms of mass of silicon per cell, broad relationships with the mean volume and with the mean surface area are demonstrable (Fig. 1.9). The regressions reflect the increasing silicon deployment with increasing cell size but their slopes, within the interpretative limits of log/log relationship, suggest that increased cell size is accompanied by a decreasing ratio of silicon : enclosed volume and an increasing ratio of silicon : area. This is in accord with the expectation of Einsele and Grim (1938), among the earliest investigators of the silicon requirements of diatoms, that interspecific variations in deployment are related to differences in shape (surface area-to-volume effects), together with the relative investment in such species-specific features as fenestration, strengthening ribs, bracing struts and spines. Later data on some of the species of diatom considered by Einsele and Grim (1938) suggest that the area-specific silicon content is particularly responsive to increasing size (see Table 1.5).

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