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In this article, we present new laboratory results examining the relative importance of HCO3 transport and extracellular carbonic anhydrase (eCA) in 17 marine diatom species. We observed significant variability in both HCO3 transport and eCA expression across a range of diatom species with different cell morphologies. All species we examined took up HCO3 through a direct transport mechanism, with the fraction of HCO3 transport ranging from 40 to 95% of total C uptake. eCA expression also varied significantly, with catalytic enhancement factors ranging approximately 10-fold among species. There was a significant positive correlation between HCO3 transport and eCA expression among the test species. However, neither HCO3 transport nor eCA expression was significantly correlated to cell growth rates or surface area to volume ratios. We did observe weak positive trends between the ratio of C demand:supply and HCO3 utilization/eCA expression, but these were not statistically significant. We are thus unable to provide a mechanistic explanation for the apparent variability in HCO3 transport and eCA expression in marine diatoms. This variability may, nonetheless, have important implications for the physiological ecology of oceanic diatoms.