The carotenoid-labeling method: measuring specific rates of carotenoid synthesis in natural phytoplankton communities

The physiological basis of the carotenoid-14C-labeling method for the determination of growth rates (μ, d−1) of specific groups of microalgae was established in the laboratory and the method was tested in the subarctic Pacific and in Chesapeake Bay (USA). 14C-labeling patterns of carotenoids in a variety of algal species grown in batch cultures were described successfully with a simple precursor-pigment model whose free parameters, the specific rate of carotenoid synthesis (μcaro, d−1) and the precursor and the pigment turnover rates (d−1), were determined by least squares analysis. All xanthophylls except peridinin had turnover rates that did not differ significantly from zero; the turnover rate of peridinin was 0.7 μ. Precursor turnover rates varied from about 5 μ for fucoxanthin to 36 μ for lutein. We propose to use the precursor-pigment model to calculate μcaro from the amount of 14C incorporated into carotenoids and values of carotenoid precursor turnover rates, which are assumed to be known a priori. A well-constrained estimate of the fucoxanthin precursor turnover rate is presented here. 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 balanced. We demonstrated that pigment synthesis and carbon fixation can be unbalanced in natural phytoplankton populations due to the effects of light perturbations and growth under a natural photocycle. We recommend that labeling experiments last 24 h to average rates of synthesis over the diel photoperiod since rates of carotenoid synthesis and cell growth can be unbalanced at any time during the photoperiod. The field experiments also demonstrated that the carotenoid-labeling method is a powerful tool to study the physiological ecology of natural populations of phytoplankton.