Abstract: The carbon isotopic composition of algal organic matter preserved in marine sediments provides a window into the evolution of the global carbon cycle through geologic time, including variations in atmospheric CO2 levels. Traditional models for interpreting marine isotope records assume that these archives largely reflect kinetic isotopic discrimination by the carbon-fixing enzyme RubisCO. However, recent measurements in our laboratory and in the literature appear to contradict this assumption, indicating that significant questions remain regarding the mechanistic underpinning of algal isotopic signatures. Here, we present experimental results from chemostat cultures of eukaryotic algae performed under different growth rates and [CO2(aq)] conditions. We also present a revised model framework for explaining carbon isotope fractionation in ecologically prominent eukaryotic phytoplankton groups. One implication of our work is that RubisCO is not predicted to exert significant control on carbon isotope signals in nutrient-limited regimes, but becomes more influential as light begins to limit algal growth, creating taxon-specific isotopic predictions. By developing a mechanistic understanding of marine photosynthetic carbon isotope fractionation, we may begin to reexamine Phanerozoic isotope records and the importance of different growth conditions through geologic time.