A PHYSIOLOGICAL-BASIS FOR ESTIMATING INORGANIC CARBON RELEASE DURING PHOTOSYNTHESIS BY NATURAL PHYTOPLANKTON

A model of inorganic carbon release by unicellular algae is proposed b ased on the involvement of a HCO3/NO3 antiport during growth under non -steady-state conditions, such as nutrient pulsing. In order to test t he model, reductions in inorganic carbon uptake are related to nitrate uptake in axenic cultures of seven species of microalgae during pertu rbations of the nutrient regime under photosynthesizing conditions. Fo r 4 data sets out of 5, 70 to 99% of the variability in reduction in c arbon uptake could be explained by variations in nitrate uptake. Given the specificity of nitrate uptake to phytoplankton, it is suggested t hat such a variable could be used to obtain reliable estimates of alga l respiration in the field in presence of other respiring organisms su ch as zooplankton or bacteria. The model yields values which are consi stent with classical estimates of algal respiration (about 10% of gros s photosynthesis) under conditions of balanced growth only. However, d uring nutrient pulsing such as that occurring near the bottom of the e uphotic zone in stratified aquatic areas, the model indicates that gro wth-related respiration may reach 50% of gross photosynthesis. (C) 199 7 Elsevier Science B.V.