ABSTRACT: The large temporal and spatial variability in carbon isotope fractionation of marine phytoplankton (e_p) is thought to reflect differences in environmental conditions. Meaningful interpretation of this variability requires an understanding of the processes responsible for phytoplankton isotope fractionation. While numerous factors have been suggested to potentially influence e_p, recent theoretical and experimental evidence has emphasized the primary role of phytoplankton growth rate (µ) and CO₂ concentration ([CO₂aq]) in controlling e_p. Experimental examination of the relationship of e_p with µ and [CO₂aq] in studies using different experimental approaches, however, has yielded inconsistent results. Here we directly compare new and previously published data on e_p as a function of CO₂ concentration and growth rate for the marine diatom Phaeodactylum tricornutum. When grown under nitrogen-deficient conditions (nitrate-limited chemostat), e_p of P. tricornutum decreases with increasing growth rate. In contrast, under N-replete conditions e_p values are considerably lower at comparable growth rates and CO₂ concentrations and are largely insensitive to a 3-fold increase in growth rate due to increasing photon flux density. In both experimental approaches, e_p shows a relatively small CO₂ sensitivity in the range of CO₂ concentrations naturally occurring in the ocean (8 to 25 µmol kg^-1). Below ca 5 µmol CO₂ kg^-1, a strong decline in e_p with decreasing [CO₂aq] is observed. The apparent difference in e_p responses between nitrate-limited and light-controlled cultures of P. tricornutum suggests a principal difference in carbon acquisition for different growth-rate-limiting resources. A mechanistic explanation is proposed and potential implications for the interpretation of phytoplankton carbon isotope fractionation are discussed.

KEY WORDS: d¹³C · Isotope fractionation · CO₂ · Phaeodactylum tricornutum · Growth limitation

Published in MEPS Vol. 193 (2000) on February 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2000