EFFECT OF PHYTOPLANKTON CELL-SIZE ON TRANSIENT-STATE NITRATE AND AMMONIUM UPTAKE KINETICS

The uptake of nitrate or ammonium (at a concentration of 10 mu mol l(- 1)) by marine phytoplankton was studied in relation to cell size. Init ial specific nitrate uptake rates by small (35000 mu m(3)) and large ( 130000 mu m(3)) cells of the diatom Ditylum brightwellii did not diffe r significantly. However, the larger cells maintained a high uptake ra te for a longer time. Therefore, they accumulated nitrate in a higher biomass-specific pool than the smaller cells. In the dark, this effect was even more pronounced. Two smaller diatom species, Lauderia boreal is (7474 mu m(3)) and Thalassiosira pseudonana (98 mu m(3)), had lower initial specific nitrate uptake rates and lower intracellular pools. Transient-state ammonium uptake did not result in accumulation of larg e intracellular pools of ammonium. Theoretically, and on the basis of the presented results, we stress the dualistic functional role of the vacuole. A large vacuole is an effective way for larger algal species to possess a minimum cell nutrient quota/cell surface ratio which is i n the range of smaller species. Furthermore, by functioning as a stora ge reservoir it reduces inhibition of the uptake rate by cytoplasmic a ccumulated nutrients. The effect of the latter mechanism is that large r algal species are better at nitrate uptake under fluctuating conditi ons. These results imply that, in nitrogen-controlled marine systems, resource competition under fluctuating nutrient concentrations can onl y lead to a shift towards larger phytoplankton species if nitrate rath er than ammonium is the main nitrogen source. From theoretical conside rations it is argued that the maximum growth rate of algae is determin ed by nutrient assimilation properties rather than by photosynthetic c apacity.