SPECIES-DIFFERENCES IN ACCUMULATION OF NITROGEN POOLS IN PHYTOPLANKTON

The changes in the intracellular concentrations of nitrate, ammonium, free amino acids, protein, DNA, RNA and total N were measured in batch cultures of 7 spp. of marine phytoplankton [Chaetoceros gracilis, Isochrysis galbana, Skeletonema costatum, Thalassiosira pseudonona, T. nordenskioldii, Dunaliella tertiolecta, Amphidinium carterae] as they progressed from being N sufficient to being N starved. After several days of N starvation, either nitrate or ammonium was added to the cultures and the measurements were continued for 10-36 h. By this means it was possible to assess the long-term and short-term changes in cellular N compounds and how they relate to phytoplankton N uptake and growth. Considerable species differences were observed in the amounts and kinds of N compounds which were stored and the degree to which the utilization of these compounds could support growth if the external N supply is low or variable. Despite the species variation, the results suggest concentrations or ratios of a number of intracellular N compounds can be used to assess the N deficiency and/or growth rate of natural phytoplankton populations.