Structure and replication cycle of a virus infecting climate-modulating algaEmiliania huxleyi

Abstract

AbstractThe globally distributed marine algaEmiliania huxleyiproduces reflective calcite disks (coccoliths) that increase the albedo of ocean water and thus reduce the heat absorption in the ocean, which cools the Earth’s climate. The population density ofE. huxleyiis restricted by nucleocytoplasmic large DNA viruses, includingE. huxleyivirus 201 (EhV-201). Despite the impact ofE. huxleyiviruses on the climate, there is limited information about their structure and replication. Here we show that the dsDNA genome inside the EhV-201 virion is protected by an inner membrane, capsid, and outer membrane decorated with numerous transmembrane proteins. The virions are prone to deformation, and parts of their capsids deviate from the icosahedral arrangement. EhV-201 virions infectE. huxleyiby using their fivefold vertex to bind to a host cell and fuse the virus’s inner membrane with the plasma membrane. Whereas the replication of EhV-201 probably occurs in the nucleus, virions assemble in the cytoplasm at the surface of endoplasmic reticulum-derived membrane segments. Genome packaging initiates synchronously with the capsid assembly and completes through an aperture in the forming capsid. Upon the completion of genome packaging, the capsids change conformation, which enables them to acquire an outer membrane by budding into intracellular vesicles. EhV-201 infection induces a loss of surface protective layers fromE. huxleyicells, which allows the continuous release of virions by exocytosis. Our results provide insight into how EhVs bypass the surface protective layers ofE. huxleyiand exploit the organelles of an infected cell for progeny assembly.