During the Mesozoic Era, dinoflagellates, coccolithophorids and diatoms became prominent primary producers in the oceans, succeeding an earlier biota in which green algae and cyanobacteria had been proportionally more abundant. This transition occurred during an interval marked by increased sulfate concentration in seawater. To test whether increasing sulfate availability facilitated the evolutionary transition in marine phytoplankton, the cyanobacterium Synechococcus sp., the green alga Tetraselmis suecica and three algae containing chlorophyll a+c (the diatom Thalassiosira weissflogii, the dinoflagellate Protoceratium reticulatum and the coccolithophorid Emiliania huxleyi) were grown in media containing 1, 5, 10, 20, or 30 mm SO42−. The cyanobacterium and the green alga showed no growth response to varying [SO42−]. By contrast, the three chlorophyll a+c algae showed improved growth with higher [SO42−], but only up to 10 mm. The chlorophyll a+c algae, but not the green alga or cyanobacterium, also showed lower C:S with higher [SO42−]. When the same experiment was repeated in the presence of a ciliate predator (Euplotes sp.), T. suecica and T. weissflogii increased their specific growth rate in most treatments, whereas the growth rate of Synechococcus sp. was not affected or decreased in the presence of grazers. In a third experiment, T. suecica, T. weissflogii, P. reticulatum and Synechococcus sp. were grown in conditions approximating modern, earlier Paleozoic and Proterozoic seawater. In these treatments, sulfate availability, nitrogen source, metal availability and Pco2 varied. Monospecific cultures exhibited their highest growth rates in the Proterozoic treatment. In mixed culture, T. weissflogii outgrew other species in modern seawater and T. suecica outgrew the others in Paleozoic water. Synechococcus sp. grew best in Proterozoic seawater, but did not outgrow eukaryotic species in any treatment. Collectively, our results suggest that secular increase in seawater [SO42−] may have facilitated the evolutionary expansion of chlorophyll a+c phytoplankton, but probably not to the exclusion of other biological and environmental factors.