where (qmax) is the replete cell quota. The larger is the instantaneous cell quota, q, the smaller will be the effective rate of uptake. The multiple allows the cell to accumulate even scarce resources (of, say, phosphorus) from low concentrations so long as the uptake of another (say, nitrogen) is controlling ('limiting') the rate of the deployment of both in the structure of new cell material. An independent increase in the supply of the second resource (nitrogen), however, with no simultaneous alteration in the availability of the first (phosphorus), might very quickly leave the rate of supply of the first as the limiting constraint, when the internal quota is likely to be drawn down. This principle underpins the use of intracellular nutrient ratios to indicate the nutrient status of cells and, thus, the identity of the instantaneously 'limiting factor'. Interspecific differences in the competitive abilities of algae to function at low resource availability are also held to influence the structure of communities.
The terms 'limitation' and 'competition' (in the context of satisfying resource requirements) have been used variously and inconsistently in
The phosphorus relations of phytoplankton cells provide a good example of the ways in which the adaptations for gathering of an essential but frequently scarce resource impinge upon the dynamics of populations and the species structure of natural assemblages. As a component of nucleic acids governing protein synthesis and of the adenosine phosphate transformations that power intracellular transport, phosphorus is an essential requirement of living, functional plankters. As observed earlier (Section 1.5.3), the phosphorus content of healthy, resource-replete, actively growing phytoplankton cells is generally close to 1-1.2% of ash-free dry mass (Round, 1965; Lund, 1965), with a molecular ratio to carbon of around 0.0094 (106 C:P). The minimum cell quota (q0) may vary intespecifically, most probably, between 0.2% and 0.4% of ash-free dry mass (some 320-640 mol C:mol P) but, reportedly, almost an order lower in some species (Asterionella ~0.03% of ash-free dry mass: Rodhe, 1948; Mackereth, 1953), equivalent to molecular C:P ratios of ~4000). Conversely, intracellular storage capacity of phosphorus may allow q to rise in some species to >3% of dry mass (<40 C:P). The interesting deduction is that, as a result of this so-called 'luxury uptake', the cell may contain 8-16 times the minimum quota and that, as a consequence, it is theoretically able to sustain three or possibly four cell doublings without taking up any more phosphorus.
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