Carbon exchange and 14C tracer methods in a nitrogen-limited diatom, Thalassiosira pseudonana

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Abstract
A simple C-exchange model accounted for the kinetics of 14C uptake and release by the diatom T. pseudonana in N-limited chemostat culture. The model treats the cells as consisting of 2 pools of C; an exchanging pool which carries out photosynthesis, respiration and excretion, and a synthetic pool which does not exchange, but accumulates C from the exchanging pool. The model fitted well to observed 14C 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 (4-15% of cell C) and rapidly cycled (90% turnover time of 1-12 h) in all steady-state cultures, but was larger (21%) and more slowly cycled (15 h) in a chemostat deprived of its limiting N supply for 24 h. In all cultures, the observed kinetics indicated that usual 14C 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.