Importance of viral lysis for winter bacterial and cyanobacterial mortality throughout the euphotic zone of the tropical Pacific Ocean

Context The variability in mortality rates induced by grazing and viral activity, as well as their specificity to bacteria and cyanobacteria groups, are likely to influence plankton community structure and carbon cycling in distinct ways. Aims The purpose of this study was to highlight the importance of bacteria and picophytoplankton in the food web dynamics of the tropical Pacific Ocean at different depths. Methods Using a modified seawater dilution method, we measured growth and mortality rates of bacteria and cyanobacteria (Prochlorococcus and Synechococcus spp.) during a winter expedition at the surface, deep chlorophyll maximum (DCM), and 200-m depths in the western tropical oligotrophic Pacific Ocean. Key results Overall, nanoflagellate grazing accounted for 64% of the carbon loss from Synechococcus spp. in surface waters. However, at DCM, viral-mediated carbon loss of Synechococcus spp. was more prominent than grazing-related loss. In most cases, neither viral lysis nor nanoflagellate grazing had a significant effect on Prochlorococcus mortality. Moreover, viral lysis was the primary driver of bacterial mortality throughout the photic zone (~200 m). Conclusions A significant portion of the nutritional requirements of picophytoplankton is likely met through viral lysis, which plays an integral role in the marine nutrient cycle. Implications These estimates need to be verified, but it could suggest that viral lysis supplies a significant portion of the nitrogen required for primary production, highlighting the importance of the viral shunt, particularly in regions with limited nitrogen availability.