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

摘要

During the Mesozoic Era, dinoflagellates, coccolithophorids and diatoms became prominent primary producersin the oceans, succeeding an earlier biota in which green algae and cyanobacteria had been proportionally moreabundant. This transition occurred during an interval marked by increased sulfate concentration in seawater. Totest whether increasing sulfate availability facilitated the evolutionary transition in marine phytoplankton, thecyanobacterium Synechococcus sp., the green alga Tetraselmis suecica and three algae containing chlorophylla+c (the diatom Thalassiosira weissflogii, the dinoflagellate Protoceratium reticulatum and the coccolithophoridEmiliania huxleyi) were grown in media containing 1, 5, 10, 20, or 30 mM SO_4~(2-). The cyanobacterium and thegreen alga showed no growth response to varying [SO_4~(2-)]. By contrast, the three chlorophyll a+c algae showedimproved growth with higher [SO_4~(2-)], but only up to 10 mM. The chlorophyll a+c algae, but not the green algaor cyanobacterium, also showed lower C:S with higher [SO_4~(2-)]. When the same experiment was repeated in thepresence of a ciliate predator (Euplotes sp.), T. suecica and T. weissflogii increased their specific growth rate inmost treatments, whereas the growth rate of Synechococcus sp. was not affected or decreased in the presenceof grazers. In a third experiment, T. suecica, T. weissflogii, P. reticulatum and Synechococcus sp. were grown inconditions approximating modern, earlier Paleozoic and Proterozoic seawater. In these treatments, sulfate availability,nitrogen source, metal availability and PCO2 varied. Monospecific cultures exhibited their highest growthrates in the Proterozoic treatment. In mixed culture, T. weissflogii outgrew other species in modern seawaterand 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 inseawater [SO_4~(2-)] may have facilitated the evolutionary expansion of chlorophyll a+c phytoplankton, butprobably not to the exclusion of other biological and environmental factors.