A b~ Three marine diatoms, Skeletonema costatum, Chaetoceros debilis, and Thalassiosira g r a~d a were grown under no limitation and a m m o n i u m or silicate limitation or starvation. Changes in cell m o r p h o l o g y were d o c u m e n t e d with p h o t o m i c r o g r a p h s of a m m o n i u m and s i l i c a t e-l i m i t e d and n o n-l i m i t e d cells, and c o r r e l a t e d with observed changes in chemical composition. Cultures grown under silicate starvation or limitation showed an increase in p a r t i c u l a t e carbon, nitrogen and phosphorus and c h l o r o p h y l l a per unit cell volume compared to n o n-l i m i t e d cells; p a r t i c u l a t e silica per cell v o l u m e decreased. Si-starved cells were d i f f e r e n t from S i-l i m i t e d cells in that the former c o n t a i n e d more p a r t i c u l a t e carbon and silica per cell volume. The most sensitive indicator of silicate limitation or s t a r v a t i o n was the ratio C:Si, being 3 to 5 times higher than the values for n o n-l i m i t e d cells.

Skeletonema costatum was grown at different steady-state growth rates in ammonium or silicate-limited chemostats. The culture was perturbed from its steady-state condition by a single addition of the limiting nutrient, ammonium or silicate. The transient response was followed by measuring nutrient disappearance of the limiting and non-limiting nutrients at frequent time intervals. The results from a typical perturbation experiment indicate that three distinct modes of uptake of the lim, iting nutrient can be distinguished; surge uptake (Vs), internally controlled uptake (vi), and externally controlled uptake (Ve). An interpretation of these three modes of uptake is given and their relation to control of uptake of the limiting nutrient is discussed. The uptake rates of the non-limiting nutrients were shown to be depressed during the surge of the uptake of the limiting nutrient. Kinetic uptake parameters, K s and Vma x, were obtained from data acquired durin~ the externally controlled uptake segment, v e. The same Vma x value of 0.12 h-l, was obtained under either silicate or ammonium limitation. Estimates of K s were 0.4 ~gat NH4-N 1-I and 0.7 ~g-at Si 1-I. Short-term 15N uptake-rate measurements conducted on nitrogen-limited cultures appear to be a combination of v s or v i, or at lower substrate concentrations v s and v e. It is difficult to separate these different uptake modes in batch or tracer experiments, and ensuing problems in interpretation are discussed.