INFLUENCE OF PREY AND NUTRITIONAL STATUS ON THE RATE OF NITROGEN UPTAKE BY PRYMNESIUM PARVUM (HAPTOPHYTE)

We studied how the specific nitrogen (N) uptake rates of nitrate (NO3-), urea, and the amino acids, glutamic acid and glycine, by Prymnesium parvum were affected by (1) the change from N-deficient status to N-sufficient status of the P. parvum cells, (2) presence of prey from a natural Baltic Sea plankton community, and (3) the composition of prey as affected by additions of terrestrial originated dissolved organic matter (DOM) or inorganic nutrients. Nitrogen-deficient P. parvum (16 mu M NO3- and 4 mu M PO4-, molar N:P ratio of 4:1) were mixed with a natural Baltic plankton community and given PO43- and (1) NO3- (control) or (2) high molecular weight DOM, > 1 kDa concentrated from sewage effluent (+DOM), in a molar N:P ratio of 9-10:1. With additions of N-15-enriched substrates, rates of N uptake from NO3-, urea, and the amino acids glycine and glutamic acid were measured every 24 h for 72 h. Initial N-deficient P. parvum were highly toxic (3.7 +/- 0.9 x 10(-4) mg Sap equiv/cell) and toxic allelochemicals were released into the medium causing the natural plankton community to lyse. Rates of N uptake differed between the "control" and the "+DOM" treatments over time; total (sum of the N substrates measured) absolute uptake rates (rho(cell), fmol N/cell/h) at ambient culture conditions were significantly higher (ANOVA, p < 0.05) in the more toxic "control" treatments compared with the "+DOM" treatments after 48 h. In the "control" treatment, the total rho(cell) increased significantly (ANOVA, p < 0.01) from time 0 to 48 h, while in the "+DOM" treatment there was no significant increase. Released organic nutrients from the lysed plankton cells may have increased uptake rates of amino acids and urea by P. parvum. All uptake rates declined in all treatments by 72 h. Total dissolved N uptake rates at ambient culture conditions were estimated to make up about 10% of the N P. parvum are potentially capable of ingesting from particulate prey.