"Marine viruses are an active and important component of the microbial loop because they regulate microbial mortality, production, community structure, and biogeochemical cycling. They mediate genetic exchange between microbes, thus affecting the genetic diversity of microbial communities. Although marine viruses control blooms and shape the evolution of biodiversity in phytoplankton, little is known about the factors governing resistance and susceptibility of their hosts. Better understanding the host-virus interaction of marine phytoplankton is necessary to properly address its impact on phytoplankton dynamics, socio-economic activities, and medical applications.Over the past century, toxic blooms of Prymnesium have devastated increasingly aquaculture and native fish, shellfish, and mollusc populations worldwide. The toxic haptophyte Prymnesium polylepis produces toxins, which have haemolytic properties, that not only kill fish and other filter feeder but also co-existing plankton. Recently, the first virus of P. polylepis was found. The objectives of VINTPRYM are to investigate the interaction and co-evolution of Prymnesium polylepis and its virus and their diversity and population dynamics by field sampling. State-of-the-art RNA-Seq technology will be used to study the infection cycle of P. polylepis and its virus while classical bioassays will be used to measure the toxicity of the alga. Furthermore, environmental samples along the Norwegian coast will be taken to characterize the patterns of abundance and diversity of Prymnesium viruses by using metagenomic anlayses in comparison with PCR amplifications of the major capsid protein. Co-evolutionary aspects will be investigated by sequencing the viral genome and comparing it with the host genome, which is currently sequenced by the hosting group at AWI. VINTPRYM will provide insight into the response of P. polylepis to viral infection in terms of evolution, toxicity, and ecological impact."