THE CHLOROPHYLL-LABELING METHOD - MEASURING SPECIFIC RATES OF CHLOROPHYLL-A SYNTHESIS IN CULTURES AND IN THE OPEN OCEAN

We tested the assumptions underlying the Chl-labeling method of measur ing phytoplankton growth rates. The observed C-14-labeling patterns of Chl a are consistent with the presence of three kinetically important pools, the macrocycle, the phytol sidechain, and at least one precurs or pool. The turnover rates of these pools were determined from a kine tic analysis of their labeling patterns. Turnover rates of Chl a or it s two subunits were not significantly different from zero. The turnove r rate of the precursor of the Chl a macrocycle was a constant multipl e of the specific rate of pigment synthesis (mu(Chl). An equation was derived that predicts mu(Chl a) from the C-14 labeling of Chl a. Rates of C-14 incorporation into Chl a and rates of C fixation are severely unbalanced when algal cultures photoadapt. It is likely that mu(Chl a ) as determined by the Chl-labeling method equals mu(C), the specific rate of carbon synthesis, only when growth is balanced. We demonstrate d that the growth of natural phytoplankton populations can be severely unbalanced when the algae grow under a natural photocycle or when the algae are subjected to shifts in light intensity; we also observed th at Chl a was synthesized in the dark. Thus, with the Chl-labeling meth od it is not possible to determine carbon-based growth rates when alga e photoadapt or when the duration of the incubation covers only part o f the diel light cycle. Chl-labeling experiments should therefore be b ased on 24-h incubations so that the measurements integrate over the d iel photocycle.