Light-dependent regulation of coccolithophore host-virus interactions: mechanistic insights and implications for structuring infection in the surface ocean

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Phytoplankton, microscopic photosynthetic algae, form the basis of marine foodwebs and are responsible for producing nearly half the oxygen on the planet, yet represent 1% of Earth's biomass. Steady-state maintenance of a high production to biomass ratio implies that, on average, these organisms grow, die and are replaced once every week. Predatory infection by viruses has emerged as the primary mechanism responsible for the high mortality rates of phytoplankton populations. Despite the importance of viral mortality in structuring marine microbial ecosystems, little is known about the fundamental mechanisms that regulate host-virus interactions. Phytoplankton are inherently dependent on light and photosynthesis. Given the need for host resources, the viruses that infect these organisms must, therefore, also fundamentally depend on light and photosynthesis. This project will explore the relationship between light and viral infection to develop a framework for how light influences viral infection and phytoplankton mortality in the surface ocean. The widespread phytoplankton species Emiliania huxleyi, and its associated virus, Coccolithovirus, has emerged as the prominent model system for investigating algal-viral interactions due to its ecological relevance and collective mechanistic insight from numerous physiological, molecular, biochemical, genomic, and field studies. Laboratory-based culture studies will be used to elucidate the role light plays in mediating infection in E. huxleyi, specifically addressing whether light is required for viral infection as well as identifying the light-regulated host metabolic processes that viruses may co-opt for successful infection and production. These observations will then be extended to natural populations using manipulative, field-based experiments to elucidate the role light plays in structuring infection in the surface ocean. This project provides hands-on training for a Rutgers University undergraduate student, as well as a postdoctoral researcher. To facilitate ocean literacy, researchers will work with the Education and Public Outreach staff and Tilapia Film, LLC to develop an educational video based on research findings and the Next Generation Science Standards. This video, aimed at middle, high school, and undergraduate students, expands on an already successful video series that highlights scientific practices through real research investigations. It will be open access and disseminated through existing connections to the New Jersey Science Teacher Association, the National Science Teachers Association, the National Marine Educators Association, and the National Biology Teachers Association. Predatory infection by viruses is the primary mechanism responsible for the high lysis rates observed in phytoplankton populations. As the most abundant biological entities in aquatic environments, viruses turn over more than a quarter of the photosynthetically-fixed carbon, thereby fueling microbial foodwebs and short-circuiting carbon export to higher trophic levels and the deep sea. Despite its importance, estimates of viral-induced mortality are rarely included in global models of net primary productivity and deep carbon export, in part because we lack a mechanistic understanding of the fundamental factors that regulate host-virus interactions. For viruses infecting obligate photoautotrophs, there is an inherent and fundamental interaction between light and the infection process, as well as a dependence on light-regulated host metabolic processes that may be required for viral replication. Using the model algal host, Emiliania huxleyi and its associated Coccolithovirus, this project addresses the hypotheses that: 1) infection dynamics in E. huxleyi are driven through light-dependent processes, specifically that light mediates viral entry and replication, and that viruses redirect host energy to maximize viral replication, and 2) light increases viral decay relieving hosts of viral pressure. This mechanistic, cellular framework will then be used to elucidate the role light plays in structuring infection in natural coccolithophore populations using manipulative field-based experiments. Given that light is one of the most fundamental, readily, and easily measured features of the ocean, this work will ultimately provide a context for modeling the biogeochemical impact of viral infection in the global ocean.