PIGMENT TURNOVER IN THE MARINE DIATOM THALASSIOSIRA-WEISSFLOGII .1. THE 14CO2-LABELING KINETICS OF CHLOROPHYLL-A

The turnover of chlorophyll a (chl a) was investigated in the diatom Thalassiosira weissflogii (Grunow)Fryxell and Hasle using a new method based on the incorporation Of C-14 into chl a. The alga was maintained in its exponential growth phase under continuous light; C-14 was supplied as bicarbonate. The time course of label accumulation into the tetrapyrrole ring and the phytol side chain was determined for time periods equivalent to 1-2 cell doublings. The labeling kinetics of the tetrapyrrole ring and the phytol side chain were described satisfactorily by a simple precursor-pigment model with two free parameters, the precursor turnover rate and the pigment turnover rate, both having dimensions of time-1. The model was fit to the experimental data to determine the values of these two free parameters. The turnover rates of the tetrapyrrole ring and the phytol side chain were not significantly different, ranging from 0.01 to 0.1 per day. These rates are equivalent to turnover times ranging from days to weeks. Growth rate-normalized turnover rates did not vary with irradiance (7.5-825-mu-E . m-2 . s-1). The precursor turnover rates of the tetrapyrrole ring and the phytol side chain differed by an order of magnitude. These results indicate that chl a is not degraded significantly in cultures of T. weissflogii grown under continuous light. Neither irradiance nor growth rate affected growth rate-normalized chlorophyll turnover rates. Our results are inconsistent with the hypothesis that steady-state cellular concentrations of chl a are maintained by a dynamic equilibrium between rates of synthesis and degradation.