TRANSIENT NITRATE UPTAKE AND ASSIMILATION IN SKELETONEMA-COSTATUM CULTURES SUBJECT TO NITRATE STARVATION UNDER LOW IRRADIANCE

Batch cultures of the nitrate-grown marine diatom, Skeletonema costatum, were grown in various levels of nitrate supply, from full sufficiency of nitrate to 96 h of nitrate starvation. All the cultures were maintained at low light intensity (50-mu-E m-2 s-1) to simulate the light regime of natural phytoplankton populations in turbid waters or at the edge of the photic zone. The response of S. costatum cells to perturbations with 10-mu-M nitrate after variable starvation periods was studied. At the deficiency-starvation borderline, the species was able to both increase its uptake rate and store intracellular nitrate pools. Surge, or initially high, uptake is characteristic of this situation. After long starvation periods the cells needed to acclimate to a nitrate environment before being able to utilize it. The time required for this was proportional to the previous starvation period. Time courses of nitrate uptake and reduction were strongly non-linear. Nitrate excretion was high (up to 3-mu-M) under intermediate (24 and 36 h) starvation periods. Differences in the rates and times of uptake, reduction and assimilation produced strong uncoupling of the three processes. The results suggest that there is an adaptation to nitrate utilization under low light, nitrate shortage and discontinuous, sudden nitrate inputs.