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 measuring phytoplankton growth rates. The observed 14 C‐labeling patterns of Chl a are consistent with the presence of three kinetically important pools, the macrocycle, the phytol sidechain, and at least one precursor pool. The turnover rates of these pools were determined from a kinetic analysis of their labeling patterns. Turnover rates of Chl a or its two subunits were not significantly different from zero. The turnover rate of the precursor of the Chl a macrocycle was a constant multiple of the specific rate of pigment synthesis (µ Chl ). An equation was derived that predicts µ Chla from the 14 C labeling of Chl a. Rates of 14 C incorporation into Chl a and rates of C fixation are severely unbalanced when algal cultures photoadapt. It is likely that µ Chl a as determined by the Chl‐labeling method equals µ C , the specific rate of carbon synthesis, only when growth is balanced. We demonstrated 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 that Chl a was synthesized in the dark. Thus, with the Chl‐labeling method it is not possible to determine carbon‐based growth rates when algae photoadapt or when the duration of the incubation covers only part of the diel light cycle. Chl‐labeling experiments should therefore be based on 24‐h incubations so that the measurements integrate over the diel photocycle.