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 tricornutum (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 found to exist between mu/[CO2]aq and epsilon(p) (= 1000 (delta C-13CO2 - delta C-13p)/(1000 + delta C-13p), the biological fractionation associated with carbon fixation. The range of [CO2]aq in the continuous culture system was 13-31 mumol kg-1. Measurements of delta C-13CO2 and [CO2]aq in the mixed layer of the equatorial Pacific and estimates of delta 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 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 epsilon(p) and phytoplankton growth rate.