The uptake of nitrate or ammonium (at a concentration of 10 μmol I) by marine phytoplankton was studied in relation to cell size. Initial specific nitrate uptake rates by small (35000 μm) and large (130000 μm) cells of the diatom did not differ significantly. However, the larger cells maintained a high uptake rate 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, (7474 μm) and (98 (μm), had lower initial specific nitrate uptake rates and lower intracellular pools. Transient-state ammonium uptake did not result in accumulation of large 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 in the range of smaller species. Furthermore, by functioning as a storage reservoir it reduces inhibition of the uptake rate by cytoplasmic accumulated nutrients. The effect of the latter mechanism is that larger algal species are better at nitrate uptake under fluctuating conditions. These results imply that, in nitrogen-controlled marine systems, resource competition under fluctuating nutrient concentrations can only lead to a shift towards larger phytoplankton species if nitrate rather than ammonium is the main nitrogen source. From theoretical considerations it is argued that the maximum growth rate of algae is determined by nutrient assimilation properties rather than by photosynthetic capacity.


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