Collaborative Research: Nutrient-mediated interactions between plastic-derived dissolved organic carbon and the biological carbon pump

Plastics are accumulating throughout the Earth system including in the ocean’s subtropical gyres. Studies suggest that plastics are now a major source of dissolved organic material in these regions of the ocean. Therefore, it is important to consider plastic-derived organic material as part of the ocean carbon cycle and understand its effects on ocean biology. This project will test the idea that bacteria in ocean surface waters consume plastic-derived organic matter, and in doing so they compete with phytoplankton for key nutrients like nitrogen and phosphorus. The investigators will utilize a combination of laboratory experiments, field sampling, and numerical modeling to explore this hypothesis. The project will support two graduate students and an undergraduate student researcher. The investigators will share their research with policy makers and with the general public, with a focus on middle and high school aged students. The core hypothesis of this project is that “nutrient-theft” by bacteria when consuming plastic-derived dissolved organic carbon (pDOC) results in sufficient drawdown of nitrogen and phosphorus in surface waters to reduce new production and the strength of the biological carbon pump, especially in the subtropical gyres where microplastic abundance is greatest and nutrients are at a minimum. The team will test this hypothesis using an ecosystem-biogeochemistry model (CESM-MARBL) augmented to include explicit representation of marine bacterial growth, nutrient use, and pDOC all parameterized using novel empirical knowledge gained through shipboard fieldwork, photochemical studies to improve estimates of pDOC photoproduction rates, and bioassays to determine the kinetic parameters of bacterial nitrogen and phosphorus uptake in response to pDOC fertilization. Global ocean model runs will simulate spatiotemporal change in net primary productivity, nitrogen- and phosphorus-based new production, and the strength of the biological carbon pump. Model simulations will be run with no added pDOC, 1948-present pDOC fertilization rates, and with predicted future pDOC fertilization based on business-as-usual increases in surface ocean plastics until 2100. These model simulations will test the core hypothesis by assessing how nutrient theft due to pDOC fertilization is altering ocean biogeochemistry and reducing the strength of the biological carbon pump today and through the 21st century. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.