The large temporal and spatial variability in carbon isotope fractionation of marine phytoplankton (E,,) is thought to reflect differences in environmental conditions. Meaningful interpretation of this variab&ty requires an understanding of the processes responsible for phytoplankton isotope fractionation. While numerous factors have been suggested to potent~ally influence %, recent theoretical and experimental evidence has emphasized the primary role of phytoplankton growth rate ( p ) and CO2 concentration ([CO2aqj) in controlling E,,. Experimental examination of the relationship of E, with p and [C02aq] in studies using different experimental approaches, however, has ylelded inconsistent results. Here we directly compare new and previously published data on c, as a function of CO2 concentration and growth rate for the marine diatom Phaeodactylum tncornutum. When grown under nitrogen-deficient conditions (nitrate-limited chemostat), E,, of P. tn'cornutum decreases with increasing growth rate. In contrast, under N-replete conditions E, 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, shows a relatively small CO2 sensitivity in the range of CO2 concentrations naturally occurring in the ocean (8 to 25 p 0 1 kg-'). Below ca 5 pm01 CO2 kg-', a strong decline in E,, with decreasing [C02aq] is observed. The apparent difference in E, 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.
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