What does 14 C uptake by phytoplankton really measure ? A theoretical modelling approach

Abstract

The patterns of photosynthetic 14C uptake predicted by 4 models, each of which incorporates different assumptions about the fate of C within a few hours of its fixation by phytoplankton, were compared with the patterns of net 12C uptake predicted by the same models. According to all models tested, 14C will accumulate in the cells more rapidly (relative to its abundance) than 12C, until an equilibrium is established between the 14C:12C ratio inside the cells and that in the external medium. Since several hours are required for cells to approach this equilibrium, all models predict that measurements of photosynthesis based on 14C uptake overestimate net photosynthesis in incubations of up to 6-12 h, and that this overestimation will be especially severe close to the compensation point, or at depth in a water column. Experiments showed approximately linear uptake of 14C for up to 4-5 h. The only model predicting this pattern indicated that 14C uptake was proportional to gross photosynthesis, and could not be used to estimate either respiration or net photosynthesis. Other physiological and ecological evidence suggests that the conventional assumption that the 14C-technique measures net photosynthesis should be re-examined.