BGC-01 Mercury biogeochemical cycling in the ocean
Dark Reduction of Mercury by Microalgae-Associated Aerobic Bacteria in Marine Environments
Xiaoyan Zhang* , Laboratory of Environmental Nanotechnology and Health Effect, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
Yingying Guo, Laboratory of Environmental Nanotechnology and Health Effect, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
Guangliang Liu, Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, 33199, United States.
Yanwei Liu, Laboratory of Environmental Nanotechnology and Health Effect, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
Maoyong Song, Laboratory of Environmental Nanotechnology and Health Effect, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
Jianbo Shi, State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
Ligang Hu, State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
Yanbin Li, Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, China
Yongguang Yin, Laboratory of Environmental Nanotechnology and Health Effect, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
Yong Cai, Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, 33199, United States.

Redox transformation of mercury (Hg) is critical for Hg exchange at the air-sea interface and it can also affect the methylation of Hg in marine environments. However, the contributions of microalgae and aerobic bacteria in oxic seawater to Hg2+ reduction are largely unknown. Here, we studied the reduction of Hg2+ mediated by microalgae and aerobic bacteria in surface marine water and microalgae cultures under dark and sunlight conditions. The comparable reduction rates of Hg2+ with and without light suggest that dark reduction by biological processes is as important as photochemical reduction in the tested surface marine water and microalgae cultures. The contributions of microalgae, associated free-living aerobic bacteria, and extracellular substances to dark reduction were distinguished and quantified in 7 model microalgae cultures, demonstrating the associated aerobic bacteria are directly involved in dark Hg2+ reduction. The aerobic bacteria in the microalgae cultures were isolated and a rapid dark reduction of Hg2+ followed by a decrease of Hg0 was observed. The reduction of Hg2+ and re-oxidation of Hg0 were demonstrated in aerobic bacteria Alteromonas spp. by using double isotope tracing (199Hg2+ and 201Hg0). These findings highlight the importance of algae-associated aerobic bacteria in Hg transformation in oxic marine water.