The time courses of uptake, distribution, and excretion of radioactive PO, were determined for 11 species of freshwater algae. Uptake of PO, by algae was rapid, reaching a steady-state asymptote within an hour and maintaining that maximum for at least 10 h. Uptake could be described as a biphasic exponential function. The rate constants for uptake decreased with cell volume raised to a power of--0.25, whereas the uptake rates per cell increased with cell size to a power of-0.75, suggesting that small cells were much more efficient at sequestering PO,. Excretion of PO, by algae was fast during the first lo-20 min; thereafter, there was a slower prolonged loss of PO, for the remaining period. A two-compartment model could be convincingly fitted directly to the excretion time-course data. Both rate constants and flux rates of PO, excretion and intercompartmental exchanges showed a similar size-dependency to those of PO, uptake. The net uptake of PO, by algae increased proportionately to cell volume raised to a normalized power of 0.75. The allometric compartment model yielded estimates for net PO, uptake rates one order of magnitude higher than those determined by means of the Michaelis-Menten model. Phosphate uptake is thought to be the principal pathway of P accumulation by algae (Lean 1973). Because PO, uptake is mediated by an algal enzyme system, the rate parameter
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