Dependence of phytoplankton carbon isotopic composition on growth rate and [CO2)aq: Theoretical considerations and experimental results

Abstract

The carbon isotopic composition of the marine diatom Phaeodactylum tricornutum (δ13Cp) was measured over a series of growth rates (μ) in a continuous culture system in which both δ133CCO2 and [CO2]aq were determined. In accord with theory, a linear relationship was found to exist between μ/ [CO2]aq and ɛp (≡1000(δ13CCO2 − δ13Cp)/1000 + d δ13Cp), the biological fractionation associated with carbon fixation. The range of [CO2]aq in the continuous culture system was 13–31 μmol kg−1. Measurements of δ13CCO2 and [CO2]aq in the mixed layer of the equatorial Pacific and estimates of δ13Cp obtained from the δ13C of chlorophyll a combined with the regression line fit to the P. tricornutum data give phytoplankton growth rates that are in excellent agreement with those estimated via other techniques. Measurement of ɛp and [CO2]aq in the field can provide an estimate of in situ phytoplankton growth rates without the potential artifacts associated with incubation methodologies. These findings also suggest that accurate estimations of ancient CO2(aq) concentrations will require knowledge of both ɛp and phytoplankton growth rate.