SIZE‐DEPENDENT PHOSPHORUS UPTAKE KINETICS AND CELL QUOTA IN PHYTOPLANKTON1

Abstract

The allometric equation, y = aX b, described the interspecific variation of phosphate uptake kinetics and cell quota with phytoplankton cell size and showed that smaller cells are superior in uptake rate to large. Species‐specific measurements, made by track autoradiography in phosphorus deficient cultures of communities from a phosphorus‐limited lake, revealed that eight different species did not differ significantly in the Michaelis‐Menten half‐saturation constant, K m. However, both saturated uptake rates (V max) and the initial slope of the uptake curve (V max:K m) decreased per unit biomass with increasing cell size. Biomass‐specific cell phosphorus quotas also decreased with increasing cell volume, but less rapidly than did V max or V max: K m. Comparable data from the literature showed that marine species were superior in phosphorus uptake to freshwater species of similar size, but allometric variation of kinetics appeared to exist within both groups. Together with a variable internal stores model of phosphorus‐limited growth, the allometric relationships of uptake kinetics and quotas predicted competition to favor smaller cells, with a differential in growth rate diminishing as competitive intensity increased.