A physiological basis for estimating inorganic carbon release during photosynthesis by natural phytoplankton

A model of inorganic carbon release by unicellular algae is proposed based on the involvement of a HCO3NO3 antiport during growth under non-steady-state conditions, such as nutrient pulsing. In order to test the model, reductions in inorganic carbon uptake are related to nitrate uptake in axenic cultures of seven species of microalgae during perturbations of the nutrient regime under photosynthesizing conditions. For 4 data sets out of 5, 70 to 99% of the variability in reduction in carbon uptake could be explained by variations in nitrate uptake. Given the specificity of nitrate uptake to phytoplankton, it is suggested that such a variable could be used to obtain reliable estimates of algal respiration in the field in presence of other respiring organisms such as zooplankton or bacteria. The model yields values which are consistent with classical estimates of algal respiration (about 10% of gross photosynthesis) under conditions of balanced growth only. However, during nutrient pulsing such as that occurring near the bottom of the euphotic zone in stratified aquatic areas, the model indicates that growth-related respiration may reach 50% of gross photosynthesis.