MODULATION OF HOST ROS METABOLISM IS ESSENTIAL FOR VIRAL INFECTION OF A BLOOM-FORMING COCCOLITHOPHORE IN THE OCEAN

The cosmopolitan coccolithophore Emiliania huxleyi is a unicellular eukaryotic alga, responsible vast blooms in the ocean. These blooms have immense impact on large biogeochemical cycles and are terminated by a specific large dsDNA E. huxleyi virus (EhV, Phycodnaviridae). EhV infection is accompanied by induction of hallmarks of Programmed Cell Death (PCD) and production of Reactive Oxygen Species (ROS). Here we characterized alterations in ROS metabolism and explored its role during infection. Transcriptomic analysis of ROS-related genes predicted an increase in glutathione (GSH) and H₂O₂ production during infection. In accordance, using biochemical assays and specific fluorescent probes we demonstrated the over production of GSH during lytic infection. We also showed that H₂O₂ production, rather than superoxide, is the predominant ROS during the onset of the lytic phase of infection. Using flow cytometryconfocal microscopy and multispectral imaging flow cytometry, we showed that the profound co‑production of H₂O₂ and GSH occurred in the same sub-population of cells but at different subcellular localization. Positively-stained cells for GSH and H₂O₂ were highly infected compared to negatively-stained cells. Inhibition of ROS production by application of a peroxidase inhibitor or an H₂O₂ scavenger inhibited host cell death and reduced viral production. We conclude that viral infection induced remodeling of the host antioxidant network that is essential for a successful viral replication cycle. This study provides insight into viral replication strategy and suggests the use of specific cellular markers to identify and quantify the extent of active viral infection during E. huxleyi blooms in the ocean.