The role of viruses in marine phytoplankton mortality

Unicellular algae (phytoplankton) form the basis of the marine foodchain and thus play a critical role in the ocean’s geochemical processes. It is thus somewhat surprising that the mechanisms driving their mortality are still not fully understood. Traditionally, grazing by microzooplankton and sedimentation are considered the most important sources of phytoplankton loss. Other causes of phytoplankton death were more recently emphasized, including viral infection. Despite the awareness of high abundance and diversity of algal viruses, the extent and the factors regulating virally induced mortality remain understudied. This dissertation meaningfully enhanced the understanding of the quantitative significance of viruses as mortality agents for marine phytoplankton in environments with contrasting trophic status (nutrient-rich or eutrophic vs. nutrient-poor or oligotrophic). Viruses were, next to microzooplankton, responsible for the demise of the Phaeocystis globosa spring bloom thriving in the eutrophic southern North Sea. Interestingly, twelve viruses specifically infecting P. globosa harbored a high level of phenotypic diversity despite a close genetic relatedness. The impact of viruses on phytoplankton carbon production was higher in the eutrophic waters than in the oligotrophic environments. In these latter systems, virally induced algal mortality differed geographically, and more importantly, it varied for the different phytoplankton groups investigated. Our results suggest that environmental variables influence the magnitude of virally induced phytoplankton lysis. Laboratory studies highlighted that irradiance strongly and species-specifically affected virus-algal host interactions. The results strengthen the present call for more detailed studies on the role of viruses in the ocean’s functioning and the underlying mechanisms.