Award: OCE-1459190

Phytoplankton are the central currency in the flow of energy and nutrients through marine food webs, and even small shifts in growth and mortality rates can have large-scale implications for ecosystem structure and biogeochemical cycling. While factors that influence growth are often examined, less is known regarding the regulation of phytoplankton mortality. This project focused on quantifying the competing modes of mortality – microzooplankton grazing and viral mortality – on the bloom-forming coccolithophore, Emiliania huxleyi. In the laboratory, we found that microzooplankton grazing first dominates mortality, followed by viral mortality. Further, we found that microzooplankton do not prefer to ingest virally infected prey. These results suggest that the fate of carbon from these loss processes and their potential roles in carbon export are distinct. We found that viruses can also indirectly influence microzooplankton grazing via the production of transparent exopolymer particles (TEP). After creating a new method for the calibration of TEP, we found that microzooplankton grazers can get entrained in larger TEP aggregates, decreasing grazing rates. In field experiments, including a large mesocosm experiment in Bergen, Norway, we found that mortality due to viral lysis only dominates during a bloom termination event; otherwise microzooplankton grazing is the dominant mechanism of mortality. These findings suggest a complex interplay between nutrient conditions as well as modes and the magnitude of mortality. All together, our research findings have provided a unique mechanistic perspective of microbial interactions, increasing the potential for accurate predictions of phytoplankton population dynamics and biogeochemical cycling. Going forward our activities involve collaborations with trait-based mathematical ecologists in order to contextualize our findings in an ecosystem and biogeochemical model. In addition, this project supported an early-career female PI, and several graduate and undergraduate students. PIs also collaborated with educators of K-12 students on creating modules to better introduce and teach important oceanographic topics such as phytoplankton and climate change. Last Modified: 07/30/2018 Submitted by: Matthew D Johnson