Abstract
Giant viruses infect many unicellular eukaryotes, including algae that form massive oceanic blooms. Despite the major impact of viruses on the marine ecosystem, the ability to quantify and assess active viral infection in nature remains a major challenge. We applied single-cell RNA sequencing, to profile virus and host transcriptomes of 12,000 single algal cells from a coccolithophore bloom. Viral infection was detected already at early exponential bloom phase, negatively correlating with the bloom intensity. A consistent percent of infected coccolithophores displayed the early phase of viral replication for several consecutive days, indicating a daily turnover and continuous virocell-associated metabolite production, potentially affecting the surrounding microbiome. Linking single-cell infection state to host physiology revealed that infected cells remained calcified even in the late infection stage. These findings stress the importance of studying host-virus dynamics in natural populations, at single-cell resolution, to better understand virus life cycle and its impact on microbial food webs.