DEPENDENCE OF PHYTOPLANKTON CARBON ISOTOPIC COMPOSITION ON GROWTH-RATE AND [CO2](AQ) - THEORETICAL CONSIDERATIONS AND EXPERIMENTAL RESULTS

The carbon isotopic composition of the marine diatom Phaeodactylum tri cornutum (deltaC-13p) was measured over a series of growth rates (mu) in a continuous culture system in which both delta C-13Co2 and [CO2]aq were determined. In accord with theory, a linear relationship was fou nd to exist between mu/[CO2]aq and epsilon(p) (= 1000 (delta C-13CO2 - delta C-13p)/(1000 + delta C-13p), the biological fractionation assoc iated with carbon fixation. The range of [CO2]aq in the continuous cul ture system was 13-31 mumol kg-1. Measurements of delta C-13CO2 and [C O2]aq in the mixed layer of the equatorial Pacific and estimates of de lta C-13p obtained from the delta C-13 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 epsilon(p) and [CO2]aq in the field c an provide an estimate of in situ phytoplankton growth rates without t he potential artifacts associated with incubation methodologies. These findings also suggest that accurate estimations of ancient CO2 (aq) c oncentrations will require knowledge of both epsilon(p) and phytoplank ton growth rate.