CARBON-SPECIFIC PHYTOPLANKTON GROWTH-RATES - A COMPARISON OF METHODS

Measurements of biomass and growth rate of two axenic algal cultures were carried out using three different methodological approaches: the specific C-14-labelling of chlorophyll a, [H-3]adenine incorporation into DNA and net organic carbon assimilation. Time-course experiments revealed that the specific activities of chlorophyll a were significantly higher than the specific activity of total algal carbon in six of seven experiments. When the specific activity of chlorophyll a is used to calculate the carbon biomass and growth rate, the carbon biomass of the algae will thus be underestimated and the specific growth rate will be too high. Determination of growth rates from incorporation of [H-3]adenine gave lower values than those obtained from net organic carbon assimilation and from C-14 incorporation into chlorophyll a. Problems with adenine saturation are suggested. When [H-3]adenine is used to measure growth rates in dense algal cultures, additions of > 1-mu-M [H-3]adenine are often required to maximally label the extracellular and intracellular adenine pols and hence DNA.