Elucidating Algal Host-Virus Dynamics in Different Nutrient Regimes - Mechanistic Interactions and Biogeochemical Impact

Marine phytoplankton, photosynthetic microscopic organisms that float with the oceans currents, account for ~50% of the Earth's primary productivity. When there are sufficient nutrients and light to sustain their growth, phytoplankton thrive and produce large-scale blooms in the world oceans that can be seen from Earth-observing satellites. Coccolithophores are arguably one of the most dominant and globally distributed phytoplankton. Their dual ability to produce calcium carbonate cell walls and to use carbon dioxide for photosynthesis make them a key component of the oceanic carbon cycle and marine ecosystems. As such, water column processes that impact the fate of this cellular carbon are of critical importance. Emiliania huxleyi is a globally widespread, cosmopolitan coccolithophore that forms blooms in all but the polar oceans. These blooms are routinely terminated by virus infection (Coccolithoviruses), which results in cell death and the release of organic matter into the upper ocean. At the same time, infection triggers the production and release of a sticky mucus-like gel which serves to aggregate free floating cells (and even viruses) into larger particles that have very high sinking rates into the deep ocean. Hence, viruses play multifaceted roles in determining whether phytoplankton carbon sinks to the deep ocean and is sequestered away from the atmosphere or is recycled in the upper ocean free to exchange with the atmosphere. Ultimately, factors that impact the interactions between phytoplankton cells and viruses are likely to affect the direction of carbon flow in the oceans. This project uses a well-characterized, laboratory-based coccolithophore-virus system (E. huxleyi and Coccolithoviruses) to elucidate the basic mechanisms that underlie host-virus interactions at the levels of adsorption, replication and production. Proposed work will manipulate nutrient supply to understand its impact on mechanisms of infection and to better interpret population changes in different oceanic regimes. A key tenet is to investigate the role of mucus-like gels and calcium carbonate cell walls, both of which are produced under nutrient stress, as important first order drivers in host-virus interactions. Experimental work will be integrated into mathematical models as a tool to extrapolate our findings and postulate how, to first order, viruses control the fate of phytoplankton populations in the ocean. Research concepts and findings will be relayed to broader audiences by developing an online educational software tool and web app (via the Rutgers University Mobile App Development group) that focuses on the use of mathematical modeling in marine science. It will be designed to meet national requirements for the Next Generation Science Standards (NGSS) for 15-16 year olds. Students will learn about patterns of ocean productivity, articulate how and why ocean ecosystems are sensitive to environmental change, and understand the role of viruses in ecosystem structure. To ensure large-scale distribution of the app, with a particular aim to reach underrepresented students and to address the NGSS, Rutgers will host workshops to familiarize the teachers with the science, the scientists, and effective use of the app and associated lessons. The investigators will work with external evaluators to assess the effectiveness of these activities and deliverables. Research activities will also be communicated to the general public by interactions with the 'Liquid Living' display at the San Francisco Exploratorium and the annual 'Nautical Night' at the MIT museum in Boston, MA.

Phytoplankton are the basis of marine food webs and are responsible for approximately half of global net primary production. As highly abundant infectious entities in the oceans, marine viruses can cause the demise of phytoplankton blooms and drive the release of dissolved and particulate organic matter (DOM and POM), which stimulates microbial activity, facilitates bacterial re-mineralization, enhances nutrient recycling and respiration, as well as short-circuits carbon transport to higher trophic levels. At the same time, enhanced production and release of "sticky" colloidal cellular components, such as transparent exopolymer particles (TEP), during viral lysis can cause particle aggregation and enhance carbon export. As yet, the dynamics of phytoplankton infection by viruses and the balance between these diametrically opposed ecosystem pathways has not been fully characterized under different physicochemical conditions. An enhanced mechanistic and quantitative understanding of host-virus interactions can critically inform and constrain ecosystem models and allow researchers to ascertain and quantify its ecological and biogeochemical impacts on large spatial scales. This collaborative project aims to bridge existing gaps in our mechanistic and quantitative understanding of viruses as agents of phytoplankton mortality and their impact on biogeochemical processes. The ability of ecosystem models to predict carbon flow in marine systems is limited, in part, by a lack of appropriate information regarding the nutrient sensitivity of fundamental infection parameters: viral adsorption rates onto/into hosts, virus replication efficiency and latent period, and the production of infectious viruses and their excretion into the surrounding medium. Using lab-based experiments with a coccolithophore host-virus model system, as well as extensive datasets from virus infected natural coccolithophore blooms in the North Atlantic, this project aims to elucidate the impact of nutrient limitation and host cell fitness on virus infection and to what degree the dependence of viral infection on nutrient supply impacts large scale biogeochemistry and biogeography of a globally significant phytoplankton species. Our interdisciplinary approach combines grounded molecular- and flow cytometry-based diagnostic techniques, with the development of a mathematical model of infection, to understand the primary mechanisms underlying observed host-virus dynamics. We will embed the mathematical model of infection dynamics into a global ecosystem model, so we may understand the ecological impact of phytoplankton infection by viruses, and its dependence on nutrient supply, on large spatial scales.