EFFECT OF GROWTH-RATE AND CO2 CONCENTRATION ON CARBON ISOTOPIC FRACTIONATION BY THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM

The carbon isotopic composition (delta(13)C) of the marine diatom Phae odactylum tricornutum was measured over a series of growth rates (mu) in a chemostat system in which both the delta(13)C and the concentrati on of aqueous CO2[CO2(aq)] were measured. CO2(aq) ranged from 0.64 to 35 mu mol kg(-1) and growth rates from 0.5 to 1.4 d(-1). epsilon(p), t he biological fractionation factor associated with carbon fixation, wa s found to be a nonlinear function of mu/CO2(aq), contrary to the pred ictions of a model that assumes that CO2 enters the cell by passive di ffusion. The experimental results suggest that active uptake of bicarb onate does not account for the nonlinearity of the relationship and th at inorganic carbon enters the cell as CO2. The data are very well des cribed by a theoretical model that assumes that P. tricornutum regulat es the CO2 concentration in its cytoplasm so as to minimize the energy required to concentrate CO2 at the site of carboxylation. This is pro bably achieved by active uptake of CO2 or by conversion of bicarbonate to CO2 by an external carbonic anhydrase followed by transport of the CO2 into the cell via either active transport or passive diffusion. B ased on the model and data, mu/CO2(ag) = 0.225 x [(26.8 - epsilon(p))/ (epsilon(p) - 5.5)] kg d(-1) mu mol(-1). This equation accounts for 92 % of the variance in the mu/CO2(aq) data. The model has potential util ity for estimating phytoplankton growth rates in field studies without incubations and has important implications for the estimation of anci ent CO2(aq) from the delta(13)C of preserved organic compounds.