Onts, and red and green algae predate the haptophytes, alveolates and stramenopiles that gained photosynthesis via secondary endosymbiosis. But there must be more to the story because green algae did not rival photosynthetic bacteria as primary producers until hundreds of millions of years after chlorophytes first evolved [2]. And the evolutionary introduction of chlorophyll a+c algae into Mesozoic oceans did not by itself insure ecological dominance some Chl a+c clades remain minor participants in the marine carbon cycle, and despite radiationsin several algal clades, cyanobacteria persist as principal primary producers in many open ocean environments. Today, the spatial distribution of phytoplankton mirrors the environmental heterogeneity of surface oceans [3], suggesting the possibility that observed long term trends in phytoplankton composition might find at least partial explanation in the changing nature of marine environments through time (e.g., [4]). In a previous paper [5], we reported an initial set of physiological experiments asking whether seawater chemistry might have favored different photosynthetic clades at different times. Seawater solutions were prepared with [SO42-] that varied from 1 to 30 mM; sulphate was targeted because (1) the limited stoichiometric data available for phytoplankton suggest that modern shelf dominants diatoms, coccolithophorids, and dinoflagellates have higher S:C than green algae or cyanobacteria [6] and (2) geochemical data suggest that seawater [SO42-] has increased through time. Growth rates forPLOS ONE | www.plosone.orgEvolution of Phytoplankton-Grazers Interactionthe cyanobacterial and green algal strains used in this experiment were insensitive to [SO42-], but the algae that have dominated shelf production over the past 100 million years, especially dinoflagellates and coccolithophorids, exhibited higher growth rates with increasing [SO42-], at least up to levels inferred for the late Paleozoic to early Mesozoic oceans in which these groups first evolved.Glycitin In direct competition experiments, using seawater designed to approximate the chemistry of Proterozoic, Paleozoic and modern oceans, diatoms outcompeted other algae in the modern seawater solution, but, consistent with paleontological data, green algae were superior competitors in the “Paleozoic” medium. Such experiments, of course, leave residual uncertainty, as they are necessarily limited to a small number of taxa and are vulnerable to the charge that the biology of living algae owes more to recent physiological adaptation than it does to evolutionary constraint. Further testing is needed to establish differences among taxa with statistical rigor. Yet, the experimental results do fit predictions of the motivating hypothesis and so show that physiological experiments can, in principle, supplement ecological and evolutionary perspectives based on observation and modelling.Givinostat Here we revisit our simple experimental scheme to explore another potentially important aspect of phytoplankton evolution: response to grazing.PMID:27217159 Grazing plays a major role in structuring pelagic ecosystems [7]. Like the physical environment, grazing pressure varies spatially within modern oceans and has changed through time with innovations of phytoplankton grazing, first by protists and then by small metazoans. Here we explore the motivating hypothesis that phylogenetically distinct phytoplankton respond differentially to grazing in ways that might inform the geologica.