THE CAROTENOID-LABELING METHOD - MEASURING SPECIFIC RATES OF CAROTENOID SYNTHESIS IN NATURAL PHYTOPLANKTON COMMUNITIES

The physiological basis of the carotenoid-C-14-labeling method for the determination of growth rates (mu, d-1) of specific groups of microal gae was established in the laboratory and the method was tested in the subarctic Pacific and in Chesapeake Bay (USA). C-14-labeling patterns of carotenoids in a variety of algal species grown in batch cultures were described successfully with a simple precursor-pigment model whos e free parameters, the specific rate of carotenoid synthesis (mu(caro) , d-1) and the precursor and the pigment turnover rates (d-1), were de termined by least squares analysis. All xanthophylls except peridinin had turnover rates that did not differ significantly from zero; the tu rnover rate of peridinin was 0.7 mu. Precursor turnover rates varied f rom about 5 mu for fucoxanthin to 36 mu for lutein. We propose to use the precursor-pigment model to calculate mu(caro) from the amount of C -14 incorporated into carotenoids and values of carotenoid precursor t urnover rates, which are assumed to be known a priori. A well-constrai ned estimate of the fucoxanthin precursor turnover rate is presented h ere. It was shown for laboratory cultures that the carotenoid-labeling method is capable of measuring specific rates of carotenoid synthesis and that these rates equal rates of cell growth only when growth is b alanced. We demonstrated that pigment synthesis and carbon fixation ca n be unbalanced in natural phytoplankton populations due to the effect s of light perturbations and growth under a natural photocycle. We rec ommend that labeling experiments last 24 h to average rates of synthes is over the diel photoperiod since rates of carotenoid synthesis and c ell growth can be unbalanced at any time during the photoperiod. The f ield experiments also demonstrated that the carotenoid-labeling method is a powerful tool to study the physiological ecology of natural popu lations of phytoplankton.