Visualizing active viral infection reveals diverse cell fates in synchronized algal bloom demise

Marine viruses are the most abundant biological entity in the ocean and are considered as major evolutionary drivers of microbial life (Suttle, 2007). Yet, we lack quantitative approaches to assess their impact on the marine ecosystem. Here, we provide quantification of active viral infection in the bloom forming single celled phytoplankton Emiliania huxleyi infected by the large virus EhV, using high-throughput single molecule mRNA in situ hybridization of both virus and host transcripts. In natural samples, viral infection reached only 25% of the population despite synchronized bloom demise exposing co-existence of infected and non-infected subpopulations. We prove that photosynthetically active cells chronically release viral particles through non-lytic infection, and that viral-induced cell lysis can occur without viral release, thus challenging major assumptions regarding the life cycle of giant viruses. We could also assess active infection in cell aggregates, linking viral infect...