In this article, we present new laboratory results examining the relative importance of HCO−3 transport and extracellular carbonic anhydrase (eCA) in 17 marine diatom species. We observed significant variability in both HCO−3 transport and eCA expression across a range of diatom species with different cell morphologies. All species we examined took up HCO−3 through a direct transport mechanism, with the fraction of HCO−3 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 HCO−3 transport and eCA expression among the test species. However, neither HCO−3 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 HCO−3 utilization/eCA expression, but these were not statistically significant. We are thus unable to provide a mechanistic explanation for the apparent variability in HCO−3 transport and eCA expression in marine diatoms. This variability may, nonetheless, have important implications for the physiological ecology of oceanic diatoms.