Marine diatoms grown in chemostats under silicate or ammonium limitation. II. Transient response of Skeletonema costatum to a single addition of the limiting nutrient

Abstract

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 nutrients ammonium or silicate. The transient response was followed by measuring nutrient disappearance of the liliting perturbation experiment indicate that three distinct modes of uptake of the limiting nutrient can be distinguished; surge uptake (V _s ), internally controlled uptake (V _i ), and externally controlled uptake (V _e ). 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 V _max, were obtained from data acquired during the externally controlled uptake segment, V _e . The same V _max value of 0. 12 h^-1, was obtained under either silicate or ammonium limitation. Estimates of K _s were 0.4 μg-at NH₄-N l^-1 and 0.7 μg-at Si l^-1. 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.