Did sulfate availability facilitate the evolutionary expansion of chlorophyll a+c phytoplankton in the oceans?

9 10 During the Mesozoic Era, dinoflagellates, coccolithophorids and diatoms became prominent 11 primary producers in the oceans, succeeding an earlier biota in which green algae and 12 cyanobacteria had been proportionally more abundant. This transition occurred during an 13 interval marked by increased sulfate concentration in seawater. To test whether increasing 14 sulfate availability facilitated the evolutionary transition in marine phytoplankton, the 15 cyanobacterium Synechococcus sp., the green alga Tetraselmis suecica, and three algae 16 containing chlorophyll a+c (the diatom Thalassiosira weissflogii, the dinoflagellate 17 Protoceratium reticulatum, and the coccolithophorid Emiliania huxleyi) were grown in media 18 containing 1, 5, 10, 20 or 30 mM SO4. The cyanobacterium and the green alga showed no 19 growth response to varying [SO4]. In contrast, the three chlorophyll a+c algae showed 20 improved growth with higher [SO4], but only up to 10 mM. The chlorophyll a+c algae, but not 21 the green alga or cyanobacterium, also showed lower C:S with higher [SO4 In a third experiment, T. suecica, T. weissflogii, P. reticulatum, and Synechococcus sp. 26 were grown in conditions approximating modern, earlier Paleozoic and Proterozoic seawater. In 27 these treatments, sulfate availability, nitrogen source, metal availability and P ]. When the same 22 experiment was repeated in the presence of a ciliate predator (Euplotes sp.), T. suecica and T. 23 weissflogii increased their specific growth rate in most treatments, whereas the growth rate of 24 Synechococcus sp. was not affected or decreased in the presence of grazers. 25 CO2 varied. 28 Monospecific cultures exhibited their highest growth rates in the Proterozoic treatment. In 29 mixed culture, T. weissflogii outgrew other species in modern seawater and T. suecica outgrew 30 the others in Paleozoic water. Synechococcus sp. grew best in Proterozoic seawater, but did not 31 outgrow eukaryotic species in any treatment. Collectively, our results suggest that secular 32 increase in seawater [SO4 35 ] may have facilitated the evolutionary expansion of chlorophyll a+c 33 phytoplankton, but probably not to the exclusion of other biological and environmental factors. 34 36 Corresponding Author: M. Giordano; Tel.: +39 071 220 4652; Email: [email protected] 37 38 39