Effect of growth rate and CO2 concentration on carbon isotopic fractionation by the marine diatom Phaeodactylum tricornutum

Abstract

The carbon isotopic composition (δ13C) of the marine diatom Phaeodactylum tricornutum was measured over a series of growth rates (μ) in a chemostat system in which both the δ13C and the concentration of aqueous CO2 [CO2(aq)] were measured. CO2(aq) ranged from 0.64 to 35 μnol kg-1 and growth rates from 0.5 to 1.4 d-1. ε(p), the biological fractionation factor associated with carbon fixation, was found to be a nonlinear function of μ/CO2(aq), contrary to the predictions of a model that assumes that CO2 enters the cell by passive diffusion. The experimental results suggest that active uptake of bicarbonate does not account for the nonlinearity of the relationship and that inorganic carbon enters the cell as CO2: The data are very well described by a theoretical model that assumes that P. tricornutum regulates the CO2 concentration in its cytoplasm so as to minimize the energy required to concentrate CO2 at the site of carboxylation. This is probably 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. Based on the model and data, μCO2(aq) = 0.225 X [(26.8 - ε(p))/(ε(p) - 5.5)] kg d-1 μmol-1. This equation accounts for 92% of the variance in the μCO2(aq) data. The model has potential utility for estimating phytoplankton growth rates in field studies without incubations and has important implications for the estimation of ancient CO2(aq) from the δ13C of preserved organic compounds.