Speaker: Heather H. Kim, Ph.D., Biogeochemical Modeling Laboratory, Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution

Title: “Modeling Marine Heterotrophic Bacteria: Implications for the Ocean Carbon Cycle”

Abstract: Marine heterotrophic Bacteria play an important role in the ocean carbon cycle by utilizing, respiring, and remineralizing the flux of organic carbon exported from the surface to deep ocean. Bacteria account for a large fraction of heterotrophic community respiration, serving as a major sink for organic carbon and a source for CO2. Despite their importance in climate, the vast majority of ocean biogeochemical models do not simulate bacteria explicitly. This presentation will demonstrate how mechanistic modeling can be done for bacteria at multiple scales. For regional-scale dynamics, we developed and utilized 1-D variational data assimilation models to investigate a potential link between bacterial traits and ecosystem functions for the marine ecosystem along the West Antarctic Peninsula. The results show that both taxonomic and physiological traits reflect the variability in bacterial carbon demand, net primary production, and particle sinking fluxes, and imply a potential shift from low nucleic acid cells to high nucleic acid cells under climate change. For global-scale dynamics, we deployed a 3-D coupled ocean physical-biogeochemical model to quantify the response of bacteria to century-scale climate change (2015-2100). The results show that on a global scale, bacterial carbon biomass declines by ~10% and bacterial respiration increases by ~15% by end of the century. More importantly, different mechanisms are responsible for the bacterial responses in different oceanic regions. In the Southern Ocean, the increase in dissolved organic carbon (DOC) stocks drives the increase in DOC uptake rates of bacteria, while in the Arctic and tropical oceans, the increase in temperature drives the increase in DOC uptake rates of bacteria. These patterns suggest that bacteria affect the functioning of the future biological carbon pump, potentially resulting in important feedback to the Earth’s climate system.