A simple carbon-exchange model accounted for the kinetics of I4C uptake and release by the diatom 73alassiosira pseudonana in nitrogen-limited chernostat culture. The model treats the cells as consisting of 2 pools of carbon; an exchanging pool which carries out photosynthesis, respiration and excretion, and a synthetic pool which does not exchange, but accumulates carbon from the exchanging pool. The model fitted well to observed I4C kinetics over a 10-fold range of growth rates and was demonstrably superior to 2 alternate models which have been prevalent in the theory and application of 14C methodology in primary production studies. The exchanging pool was small (4 to 15 % of cell carbon) and rapidly cycled (90 % turnover time of 1 to 12 h) in all steady-state cultures, but was larger (21 %) and more slowly cycled (15 h) in a chemostat deprived of its limiting nitrogen supply for 24 h. In all cultures, the observed kinetics indicated that usual I4C estimates of phytoplankton production should be close to net production rates, but that short-term 14C estimates of excretion should be too low for slow-growing populations.
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