Holm and Armstrong (1981)

a The original data come from the works cited, as recalculated to a common scale of cell-carbon specificity by Reynolds (1988a).

phosphorus. Species might be relatively velocity-adapted, in which high rates of cellular growth and replication (r ) are matched by suitably rapid rates of nutrient uptake (VUmaJ, or else, they may be more storage-adapted, in which rapid, opportunistic uptake rates exceed relatively slow rates of deployment in growth, thereby permitting a net accumulation of an intracellular reserve of phosphorus. These adaptations are said to be distinguished by differences in the species-specific ratio (VUmax/r'max). Species may also show a tendency to be more or less affinity-adapted according to the species-specific ratio, VUmax /KU; as suggested, high affinity is imparted by a low KU requirement.

Sommer's terminology is helpful but the derived measures are themselves subject to interexperimental variability, even for the same species. The values noted in Table 4.2 are those used in the construction of Fig. 4.5. To an extent, VUmax is necessarily greater than the rate of deployment of phosphorus in new cell material, supposing that this corresponds to rmax (at 20 °C) and that the cell quota remains constant. From this, it may also be deduced that the uptake rate, VU, has to be markedly under-saturated for a considerable time before r' can be said to be P-limited. The most helpful adaptation to enable algae to deal with chronically low external MRP concentrations is a very low half-saturation coefficient. However, it is clearly relevant for such algae to be able still to function on a relatively low internal phosphorus quota. Davies' (1997) recent investigations of the cell-phosphorus-related growth kinetics of natural Asterionella populations during the spring-bloom period in the English Lakes are illustrative. Plotting cell-increase rates against the corresponding cell P quotas at the time of their sampling, Davies (1997) was able to fit a single, statistically significant Michalis-Menten-type curve, that suggested growth rate was fully saturated by cell quotas of 5-10 pg P (cell)-1 (or, roughly, 0.023-0.045 mol cell P (mol cell C)-1) and half-saturated at about 0.7 pg P (cell)-1 (i.e. ~0.003 mol cell P (mol cell C)-1). Plotting cell phosphorus as a function of the MRP concentration in the lake water at the time of collection, she showed that maintenance of a quota of this magnitude was possible at an external concentration of round 0.75 |g P L-1 (0.024 |imol P L-1). Plainly then, good growth is still possible in the face of external depletion so long as the cell quota is maintained. In contrast, at the very minimum cell quota (e.g. of Mackereth, 1953) corresponding to ~0.0003 mol cell P (mol cell C)-1, growth is quite impossible.

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