COMPARISON OF CO(2)-C-14 AND CO(2)-C-12 UPTAKE AND RELEASE RATES IN LABORATORY CULTURES OF PHYTOPLANKTON

The accuracy of the C-14 method of estimating photosynthetic rates was evaluated by comparing inorganic C-14 and C-12 uptake and release rat es in three species of microalgae. Net photosynthesis was defined as t he net decrease in inorganic C-12 concentration during a given light p eriod. The axenic algae cultures were incubated in a continuous flow-c hamber system and the experiments conducted as time-course studies in light and dark. All experiments were characterized by higher C-14 upta ke rates than C-12 during the first part of the light period, but in m any of the experiments the C-12 uptake approached the C-12 net photosy nthesis by time. Slight differences between species were observed but the overall interpretation of the results suggests, that the average r ate of C-14 uptake in standard incubations (2-4 h), will not provide a simple measure of either net or gross photosynthesis. Earlier propose d tracer flow models did not describe the observed carbon flow. The re lative over-estimation of C-12 net photosynthesis by the C-14 method d epended on the ratio of C-12 respiration to net photosynthesis. The re sults suggest that the C-14 labelling of the respiratory pool(s) was a time-dependent process, which did not necessarily require any dark pe riod, and that dark respiration may continue at a higher rate in light than in darkness.