Problems in estimating growth rates of marine phytoplankton from short-term(14)C assays

Growth rate estimates (μ') of phytoplankton populations that were sampled from nitrogen-limited continuous cultures and then incubated for short durations in batch culture with added(14)C-HCO3 (-) were significantly different than steady-state growth rates (μ) for 3 of 5 marine phytoplankton species. Two diatoms,Thalassiosira weissflogii andChaetoceros simplex, displayed virtually identical growth rates (μ=μ') over a wide range ofμ, whereasμ' for a third diatom,Phaeodactylum tricornutum, was overestimated by an average of 40% compared toμ. In contrast,μ' was underestimated by the(14)C technique for the two remaining species: up to 40% at a steady-stateμ of 1.0 day(-1) for the chlorophyteDunaliella tertiolecta and up to 100% atμ of 1.4 day(-1) for the haptophytePavlova lutheri. For the latter two species the divergence betweenμ andμ' appeared to increase with increasing steady-stateμ. A simple model of labeled and total carbon flow between the aqueous phase and cellular biomass was constructed to demonstrate that respiration was negligible whenμ=μ', but was significant whenμ'>μ. In the cases in whichμ'<μ, a rapid physiological alteration presumably took place once the steady state was disturbed and cells were placed in the incubation chambers, which perhaps was related to the nutritional state of the cultures at the time of sampling. Questions thus are raised regarding our ability to measure accurately primary productivity from shipboard experiments with confined samples of phytoplankton from nutrient-impoverished waters that probably are less hardy than the laboratory cultures used in these studies.