• ocean acidification;
  • Cyanobacteria ;
  • Synechococcus ;
  • cyanophage


Increased anthropogenic CO2 emissions are expected to cause a drop in oceanic pH of c. 0.4 units within this century. According to current assessments, the consequences of this are limited for oceanic Cyanobacteria, and absent for viruses. We investigated the effect of pH on the life history of cyanophage S-PM2 and its host, Synechococcus sp. WH7803, at current pH concentrations and at predicted future concentrations. We identified significant negative effects of decreasing pH on Synechococcus growth rate, with profound negative implications for S-PM2 biogenesis and its infection cycle. The duration of the S-PM2 eclipse period increased significantly with decreasing pH. In contrast, the latent period was shorter at pH 7.6 than at pH 8, coinciding with a reduction in S-PM2 burst size from 20.1 ± 3.2 progeny phages per cell at pH 8 to 5.68 ± 4.4 progeny phages per cell at pH 7.6. At pH 7, there was no detectable progeny release. The extracellular stage of S-PM2 was insensitive to pH changes, but sensitive to light, with significant loss in infectivity (0.35–0.38 day−1) at relatively low irradiances (> 130 μmol photon m−2 s−1). Overall, the results suggest that pH has significant influence on cyanobacterial growth with important implications for the interactions between Cyanobacteria and their viruses.