Carbon biomass, and gross growth rates as estimated from C-14 pigment labelling, during photoacclimation in Prochlorococcus CCMP 1378

The C-14 labelling of chlorophylls and carotenoids is increasingly used to evaluate phytoplanktonic biomass and growth rates in oceanic systems. 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, was subjected to shifts in light intensity. Particulate organic carbon (POC) was monitored by CHN analysis, pigments by HPLC and Prochlorococcus and heterotrophic bacteria concentrations by flow cytometry. Using a combination of HPLC and on-line radioactivity detection, C-14 labelling kinetics of divinyl-chlorophyll a (Dv-chl a) and zeaxanthin were followed. Prochlorococcus changed its Dv-chl a content markedly in response to change in light intensity, but not its zeaxanthin content, which remained nearly constant around 1.07 fg cell(-1) regardless of the irradiance. Pigment synthesis rates were correctly estimated from their C-14 incorporation rates whatever the Light level. From POC measurements and cell concentrations, the Prochlorococcus carbon content was estimated to be 49 fg C cell(-1). Moreover, under both constant and shifted (high to low and vice versa) Light conditions, Prochlorococcus growth rate (as computed from variations in cell. density) was much better estimated from zeaxanthin than from Dv-chl a labelling rates.