Urea uptake by phytoplankton at various stages of nutrient depletion

摘要

Uptake of14C-urea byThalassiosira pseudonanaandSkeletonema costatumgrown in batch culture with NO2−and NO3−as nitrogen sources was measured under three conditions: pre-depletion (when nitrogenous nutrient was present in the culture medium at saturating concentrations), at-depletion (when nitrogenous nutrient could no longer be detected), and several hours post-depletion. Vmax-urea, the initial instantaneous uptake rate, remained constant under all three conditions, and was in excess of uptake rates required for cellular doubling. Variations in uptake under the three conditions were observed, as functions of the length of time over which uptake was observed and the growth rate of the culture. The maximum instantaneous uptake rate was not different for the three conditions; variations in uptake were due to the period of time over which the maximum uptake rate was maintained. The ability of cells to take up urea rapidly, even when adequately nourished by NO2−and NO3−, could be of significance in a low and variable urea-nutrient regime in the natural envi