Award: OCE-1355720

Mercury (Hg) is a toxic trace element that bioaccumulates into marine foodwebs imposing a health threat to humans through the consumption of seafood. Mercury exists in different oxidation states that control how it binds to other molecules and dictates its ability to move back and forth between the atmosphere and ocean. Interconversion of Hg between the Hg(II) and Hg(0) oxidation states represents the primary control on the amount of Hg present in the ocean because Hg(0) is a gas and can therefore leave the ocean through gas exchange and thereby "detoxify" the ocean. The interconversion between these two forms currently lies in a state of delicate balance. Accordingly, this research focused on identifying new chemical and biological controls on the cycling of Hg within the ocean to help inform global and ecosystem models used to predict the availability of Hg to marine organisms and therefore ultimately to people. Using a combination of field measurements and laboratory experiments, this research showed that the conversion of aqueous Hg(II) to gaseous Hg(0) is primarily driven by reactions independent of light. In fact, these previously unappreciated "dark" reactions appear to control the concentration of Hg(0) in the surface ocean and its release to the atmosphere. This research further revealed that microorganisms within the ocean are important producers of reactive forms of oxygen that have the potential to cycle Hg under some conditions. These reactive oxygen species (ROS) are formed by all microorganisms, including bacteria, algae, and diatoms. This biological production of ROS is also agnostic to light and occurs throughout the ocean. As these ROS are highly reactive the formation of these molecules has important implications to the health and chemistry of the ocean beyond the cycling of Hg. Identification of these new processes allowed us to refine current models for the cycling of both oxygen and mercury within the ocean. Further study of these processes and incorporation of these reactions into global models will improve our predictive capabilities of the cycling of mercury and other redox-sensitive metals in response to changing climate conditions. This project supported the education and research of three graduate students within the MIT-WHOI Joint Program, two postdoctoral scientists at WHOI, and one postdoctoral scientist at University of Santa Cruz. Further, a local high school teacher participated in one of the research cruises and was trained in scientific research for a year at WHOI. The PI also gave yearly presentations at the local elementary school educating K-4 students about how science is done at sea. Data collected on two research cruises as part of this project were deposited in the publically available database operated by the Biological and Chemical Oceanography Data Management Office (BCO-DMO). Last Modified: 02/07/2020 Submitted by: Colleen Hansel