Final Report Summary - VIRUSIGNALLING (The role of cell signalling and infochemicals in marine Algal-Virus interactions) Summary-Phytoplankton are the basis for the oceanic food chain, fixing nearly 50% of the biosphere's net primary production. Viruses that infect phytoplankton blooms were recently discovered as a major force shaping the ecology and evolution of community structure and the cycling of nutrients in the marine environment. In this context, viruses are often viewed as the great ‘engines’ of oceanic biogeochemistry, with their own biogeochemical fate being of considerable importance, given their shear collective mass in the oceans (27-270 Mt carbon; assuming 0.2 fg carbon virus-1 and 109-1010 virus particles l -1). One of the more intriguing aspects of marine viruses illuminated through genomic and metagenomic approaches is the widespread evidence of lateral gene transfer with their hosts, which not only contributes to the diversification and adaptation of phytoplankton in the oceans, but also regulates host-virus infection mechanisms by allowing viruses to manipulate and ‘rewire’ host metabolic pathways for their replication. In the current proposal, we explored the coccolithophore Emiliana huxleyi, a cosmopolitan phytoplankton species, and its specific viruses, as an attractive model system to address fundamental questions regarding the molecular basis of host-virus interactions. Very little is known about the signal transduction pathways mediating phytoplankton cell death by marine viruses, nor of the unique life cycle of these viruses within their phytoplankton host. A wealth of recent genomic information from the host and its lytic virus now provides an unprecedented opportunity to explore cellular pathways triggered during viral infection and to gain insights into the origin of programmed cell death (PCD) in unicellular organism. Genome analysis of E. huxleyi virus 86 (EhV86), the type strain for the Coccolithoviruses, revealed an unexpected cluster of seven putative sphingolipid biosynthetic genes, a pathway never before described in a viral genome and one that is derived from the host’s sphingolipid biosynthetic pathway.We investigate the role of host and viral- induced chemical signals (infochemicals), sphingolipids and reactive oxygen species (ROS) in coccolithphores and their function in the interplay between life cycle, defense and PCD. Ultimately, we aim to gain a better understanding of the chemical co-evolution of the host–virus “arms race” in the marine environment. By incorporating advanced cell biology approaches combined with gene expression studies and identification of novel metabolites (e.g. sphingolipids), this study provides insights on the ecological and evolutionary role of marine viruses in shaping microbial population structure in the marine environment.In our work, we established essential aspects of the molecular mechanisms of host-virus interactions and the signaling role of ROS in regulating host cell fate and viral replication cycles. We analyzed a suite of E. huxleyi strains showing a range of susceptibilities to viral infection by using ROS sensitive fluorescent probes for flow cytometry and imaging, along with biochemical and bioinformatic approaches. We show specificity of ROS production in infected cells that might be a way to produce differential signals influencing cell fate. During viral infection we detect an increase in glutathione levels and ROS production that could be eliminated using an inhibitor of peroxidase activity, eliminating cell death and viral production in infected E. huxleyi cells. This finding revealed the importance of ROS and redox metabolism in the infection viral infection mechanism of E. huxleyi.We further combined high throughput transcriptomic and metabolomic analyses to explore the cellular pathways mediating viral infection. We show that EhV induces profound transcriptome remodeling targeted towards fatty acid synthesis to support viral assembly. A metabolic shift towards production of viral-derived sphingolipids was detected during infection coincided with down-regulation of host de-novo sphingolipid genes and induction of the viral-encoded homologous pathway. The depletion of host specific sterols during lytic infection and their detection in purified virions revealed their novel role in viral life cycle. We identified an essential function of the mevalonate-isoprenoid branch of sterol biosynthesis during infection and propose its down-regulation as antiviral mechanism. We demonstrated how viral replication depends on hijacking host lipid metabolism during host-virus chemical “arms race”. Based on our identification of these novel gene products and metabolites, specific to infection, we propose their potential as novel biomarkers for viral infection in the oceans. This novel tool represents a significant development in assessing the ecological and biogeochemical roles of active infection in the oceans.
