Size and shape

Apart from the issue of suspension, there is a further set of constraints that resists large size among phytoplankters. Autotrophy implies a requirement for inorganic nutrients that must be absorbed from the surrounding medium. These are generally so dilute and so much much less concentrated than they have to be inside the plankter's cell that uptake is generally against a very steep concentration gradient that requires the expenditure of energy to counter it. Once inside the cell, the nutrient must be translocated to the site of its deployment, invoking diffusion and transport along internal molecular pathways. Together, these twin constraints place a high premium on short internal distances: cells that are absolutely small or, otherwise, have one or two linear dimensions truncated (so that cells are flattened or are slender) benefit from this adaptation. Conversely, simply increasing the diameter (d) of a spherical cell is to increase the constraint for, though the surface area increases in proportion to d2, the volume increases with d3. However, distortion from the spherical form, together with surface convolution, provides a way of increasing surface in closer proportion to increasing volume, so that the latter is enclosed by relatively more surface than the geometrical minimum required to bound the same volume (a sphere). In this respect, the adaptive requirements for maximising entrainability and for enhancing the assimilation of nutrients taken up across the surface coincide.

It is worth adding, however, that nutrient uptake from the dilute solution is enhanced if the medium flows over the cell surface, displacing that which may have already become depleted. Movement of the cell relative to the adjacent water achieves the similar effect, with measurable benefit to uptake rate (Pasciak and Gavis, 1974; but see discussion in Section 4.2.1). It may be hypothesised that it is advantageous for the plankter not to achieve isopycnic suspension in the water but to retain an ability to sink or float relative to the immediate surroundings, regardless of the rate and direction of travel of the latter, just to improve the sequestration of nutrients.

These traits are represented and sometimes blended in the morphological adaptations of specific plankters. They can be best illustrated by the plankters themselves and by examining how they influence their lives and ecologies. The wide ranges of form, size, volume and surface area are illustrated by the data for freshwater plankton presented in Table 1.2. The list is an edited, simplified and updated version of a similar table in Reynolds (1984a) which drew on the author's own measurements but quoted from other compilations (Pavoni, 1963; Nalewajko, 1966; Besch et al., 1972; Bellinger, 1974; Findenegg, Nauwerck in Vollenweider, 1974; Willén, 1976; Bailey-Watts, 1978; Trevisan, 1978). The sizes are not precise and are often variable within an order of magnitude. However, the listing spans nearly eight orders, from the smallest cyanobacterial unicells of ~1 |im3 or less, the composite structures of multicellular coenobia and filaments with volumes ranging between 103 and 105 |im3, through to units of >106 |im3 in which cells are embedded within a mucilaginous matrix. Indeed, the list is conservative in so far as colonies of Microcystis of >1 mm in diameter have been observed in nature (author's observations; i.e. up to 109 | m3 in volume). Because all phytoplankters are 'small' in human terms, requiring good microscopes to see them, it is not always appreciated that the nine or more orders of magnitude over which their sizes range is comparable to that spanning forest trees to the herbs growing at their bases. Like the example, the biologies and ecologies of the individual organisms vary considerably through the spectrum of sizes.

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