Acclimation of nitrate uptake by phytoplankton to high substrate levels

A literature review of data on nitrate uptake by phytoplankton suggests that nitrate levels above 20 mu mol N.L-1 generally stimulated uptake rates in cultured unicellular algae and natural phytoplankton communities. This phenomenon indicates that phytoplankton cells acclimate to elevated nitrate levels by increasing their uptake capacity in a range of concentrations previously considered to be saturating. Cyanobacteria and flagellates were found to present a considerable capacity for acclimation, with low (0.1-2 mu mol N.L-1) half-saturation values (K-s) at low (5-20 mu mol N.L-1) substrate levels and high (1-80 mu mol N.L-1) K-s values at high (30-100 mu mol N.L-1) substrate levels. However, some diatom genera (Rhizosolenia, Skeletonema, Thalassiosira) also appeared to possess a low affinity nitrate uptake system (K-s between 18 and 120 mu mol N.L-1), which can help resolve the paradox of their presence in enriched seas. It follows that present models of nitrate uptake can severely underestimate the effects of high nitrate concentrations on phytoplankton dynamics and development. A more adequate approach would be to consider the possibility of multiphasic uptake involving several phase transitions as nitrate concentrations increased. Because it is a nonlinear phenomenon featuring strong thresholds, this effect appears to override that of other variables, such as irradiance, temperature, and cell size. Within the present context of eutrophication and for a range of concentrations that is becoming more and more ecologically relevant, equations are tentatively presented as a first approach to estimate K-s from ambient nitrate concentrations.