Coccus sp. (except the cells cultured at 30 mM SO42-), and in T. suecica at 30 mM SO42-, the presence of A. tonsa caused a larger increase of the lipid pool than that of Euplotes sp. (p 0.05; Figure 4A, B, C). In all treatments except at 1mM [SO42-], the presence of ciliates resulted in a modest increase in silica. However, when the diatom Thalassiosira weissflogii grew in the presence of copepods, its silica content was a five- to six-fold higher. This occurred despite a concomitant decrease in cell size (p 0.05; Figure 5) and it is thus suggestive of a greater thickness of the frustule. Finally, the overall level of reduction of the cell organic constituents of T. suecica cells, as the ratio of the infrared absorbances of the (-CH3 + -CH2) and H groups, was appreciably lower when the algae were cultured in the presence of A. tonsa, except at 30 mM SO42- (p 0.05; Figure 6A). The same effect was observed when Euplotes sp. was added to cells acclimated to 1 mM and 5 mM SO42-, while at 30 mM the level of reduction of cells was higher (p 0.05; Figure 6). In T. weissflogii and in Synechococcus sp. cells grazers typically had the opposite effect — the reduction index of the algal organic matter was higher than in the cultures without grazers (p 0.05; Figure 6B, C). Interactive effects between sulphate abundance and grazing were limited, but not entirely absent. For example, for unknown reasons, T. suecica showed significant differences in growth rate (Figure 1A) and lipid content (Figure 3) responses toEuplotes at high [SO42-]. These effects were not observed when Acartia was present or when grazers were absent. Tetraselmis also showed significant differences in lipid content when grown with Euplotes at varying sulphate levels. Elemental ratios showed a grazer x chemistry response as well, especially in Synechococcus cultures grown with Euplotes, with C:S and C:P (but not C:N) showing ten to twenty-fold differences between treatments with 5 and 30 mM [SO42-] (Tab. 2). The reasons for these effects remain unclear.Allelopathic activity of Synechococcus sp. and T. weissflogiiWhen Synechococcus sp. was cultured in the presence of A. tonsa, but not of Euplotes sp., the entire copepod population was killed within 24 hours; in the case of T. weissflogii, all copepods died in about 20 days (Tab. 3). Thus, we checked the effect of the spent medium obtained from cultures of these algae that had been exposed to the grazers, and from pure algal cultures (Tab.Sapanisertib 3). The results suggest that the presence of the copepod A. tonsa induced the production and the release into the external medium of anti-grazer compounds by T. weissflogii and Synechococccus sp.Loncastuximab tesirine , independent of [SO42-] (Tab.PMID:23443926 3). Copepod death was also observed in the presence of spent medium obtained from cultures of these algae, but not from media in which pure algal cultures were cultivated (Tab. 3). The toxic effect of such compounds was not detected for the green alga T. suecica, which therefore was used as “safe food” for copepods during the tests with spent media.PLOS ONE | www.plosone.orgEvolution of Phytoplankton-Grazers InteractionFigure 2. Effect of grazers on algal protein content. Protein content of (A) T. suecica, (B) T. weissfloggi, and (C) Synechococcus sp. cells cultured at 1 mM, 5 mM, 10 mM or 30 mM SO42- and in the presence of Euplotes sp. or A. tonsa. Error bars represent standard deviation calculated for at least four independent replicates. Histograms on top of which th.