Award: OCE-1559179

The goal of this project was to develop a mechanistic, cellular framework for algal host virus interactions using the model eukaryote, Emiliania huxleyi and its associated Coccolithovirus (EhV) and a combination of laboratory-based culture studies and observational and manipulative field studies of natural populations. The premise was to understand the impact of environmental factors on algal host-virus interactions, with a specific focus on the role of light. Given the dependence of phytoplankton on light for growth and productivity, we hypothesized that viruses infecting these hosts would also have a relationship with light given the need for host resources. A major finding of our work was the discovery that light was required for EhV adsorption and entry into host cells and that both the duration of the light period and the irradiance level (i.e. photon flux) were critical for maximum viral replication. We also found that infection by EhV led to a simultaneous decrease in carbon fixation and increase in the pentose phosphate pathway (PPP), the primary pathway for de novo nucleotide synthesis. Given that carbon fixation and the PPP share many of the same enzymes, a reduction in one can result in energy being diverted to the other suggesting that EhVs may divert host resources away from carbon fixation and toward the PPP to synthesize nucleotides needed for viral genome replication. We developed a mathematical model to capture the light-dependent regulation of host-virus interactions that suggests viral replication is supported by a trade-off between recycling and de novo nucleotide synthesis and that light serves to toggle this switch. Most notably, we found light was the most robust and significant driver of active viral replication in natural E. huxleyi populations in the North Atlantic. As part of this project, we also conducted field-based mesocosm experiments in coastal Norwegian waters where E. huxelyi blooms routinely occur. With a team of more than 25 scientists from 8 universities, we enclosed ~23,000 liters of water and the associated microbial communities in floating bags that were treated with different nutrients and shaded to different irradiance levels. Our collective efforts 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 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. Other relevant outcomes from this project include the development of two novel methods for measuring viral production using nanoSIMS and single viron-based stable isotope probing and biorthogonal non-canonical amino acid tagging (BONCAT). We also discovered that light influences the degree of calcification (the biomineral shell that surrounds the cell) in E. huxleyi whereby cultures grown at high light are more heavily calcified than those grown at low light. At the same time, we found that calcification can serve to deter infection making cells more resistant to viral attack. Taken together, our findings led to the unexpected discovery of an interplay between light, calcification, and viral infection that is currently being further explored. This project provided critical support for an underrepresented, female Associate Research Professor in biological oceanography and supported the dissertation work of 2 PhD students. Numerous undergraduate students were involved with the research giving them invaluable hands-on experience. As an effort to communicate science to a wider audience, we worked with Tilapia Film, LLC to produce an educational video on ?Asking Testable Questions? as part of the ?Tools of Science? (ToS; http://toolsofscience.org). The ToS aims to introduce science and engineering practices to students from middle school through college, using real-world examples and practicing scientists. Last Modified: 08/08/2020 Submitted by: Kimberlee Thamatrakoln