Award: OCE-1537951

This project combined controlled, laboratory experiments, mathematical models and ocean observations to explore the robustness and efficacy of Emiliania huxleyi-EhV interactions under different environmental conditions and nutrient regimes. Our working hypothesis was that nutrient availability and E. huxleyi host physiological state critically influence the nature and success of infection by EhVs, ultimately regulating the fate of photosynthetically fixed carbon toward the production of aggregates, which enhance sinking flux and coupling to grazing, as opposed to lysis and bacterial respiration in the upper ocean. Our work united empirical, experimental work with theoretical ecology and mathematical modeling around a few central themes: 1) determine the nature of infection in the E. huxleyi-EhV system under natural host-virus densities and environmental conditions; 2) elucidating mechanisms of host nutrient stress and susceptibility to infection; and 3) understanding the ecological and biogeochemical significance of host susceptibility to infection in different nutrient regimes. Our strategy for communicating our novel approach and findings to a wider audience (both scientific and societal) used an information technology platform and novel educational instructional videos (‘Tools of Science’) on the process of science to inform and show young people (especially those from underrepresented groups), scientists, and other interested members of the general public, about the value of models in scientific practice using viral ecology as a case study. These videos are publically available and include topics on: Asking Testable Questions, Collaborations, Sampling, Proxies, and Mathematical Models. Lab-based experiments examined the impact of cell density, physiological state, N and P availability, biochemical capabilities, physical mixing regimes, and both organic and inorganic particles (calcium carbonate biominerals) interactions on infection dynamics of E. huxleyi cells and viruses. We found that all of these factors critically controlled the nature and outcome of virus infection in the oceans. Our lab-based experimental work revealed some fundamental surprises in these phytoplankton-virus interactions, most significant of which was that host-virus interactions are not lethal at natural host densities and instead more of temporary symbiotic-like nature, which breaks down as cells become stressed. Also, our theoretical modeling work revealed that this stable, non-lethal type of infection may be because encounter rates in the oceans at natural cell and virus densities are far too low to support classic views of viruses as lethal mortality drivers. Rather, viruses must remain with cells in order to even have a chance at successful infection and propagation. This is a fundamental departure in our view of how viruses work in the oceans. We also found that host-derived calcium carbonate biominerals serve to mediate host virus interactions in both antagonistic and beneficial ways to the host, the former being successful infection and execution and the latter representing a form of resistance. Relevant field campaigns include previous open-ocean cruises in the North Atlantic and international, field-based mesocosm experiments in coastal Norwegian waters. Our efforts here were broadly focused on understanding the biotic and abiotic factors that influence algal host-virus interactions and the implications these interactions have on nutrient biogeochemistry, microzooplankton grazing, and the vertical structuring of phytoplankton communities. We specifically tested hypotheses regarding: 1) the role of light in structuring infection in the mixed layer, 2) the relative impact, and possible interactions, of viral infection and microzooplankton grazing on E. huxleyi, 3) the role of nutrient stress on E. huxleyi susceptibility to EhV infection, 4) the interplay between calcification and EhV infection, and 4) the impact of EhV infection on carbon flux. Taken together our lab-, field- and theoretical-based work has revealed heretofore unappreciated aspects on the controls of virus infection in the oceans, which help us quantify their impact on the Earth’s carbon cycle. Last Modified: 01/03/2020 Submitted by: Kay D Bidle