CARBON BIOMASS, AND GROSS GROWTH-RATES AS ESTIMATED FROM C-14 PIGMENTLABELING, DURING PHOTOACCLIMATION IN PROCHLOROCOCCUS CCMP 1378

The C-14 labelling of chlorophylls and carotenoids is increasingly use d to evaluate phytoplanktonic biomass and growth rates in oceanic syst ems. Rigorous testing of the technique in the laboratory, however, is necessary prior to its application in the field. A Mediterranean clone of Prochlorococcus, a photosynthetic prokaryote which is an important component of the autotrophic biomass in oligotrophic environments, wa s subjected to shifts in light intensity. Particulate organic carbon ( POC) was monitored by CHN analysis, pigments by HPLC and Prochlorococc us and heterotrophic bacteria concentrations by flow cytometry. Using a combination of HPLC and on-line radioactivity detection, C-14 labell ing kinetics of divinyl-chlorophyll a (Dv-chl a) and zeaxanthin were f ollowed. Prochlorococcus changed its Dv-chl a content markedly in resp onse to change in light intensity, but not its zeaxanthin content, whi ch remained nearly constant around 1.07 fg cell(-1) regardless of the irradiance. Pigment synthesis rates were correctly estimated from thei r C-14 incorporation rates whatever the Light level. From POC measurem ents and cell concentrations, the Prochlorococcus carbon content was e stimated to be 49 fg C cell(-1). Moreover, under both constant and shi fted (high to low and vice versa) Light conditions, Prochlorococcus gr owth rate (as computed from variations in cell. density) was much bett er estimated from zeaxanthin than from Dv-chl a labelling rates.