Dark Reduction of Mercury by Microalgae-Associated Aerobic Bacteria in Marine Environments.

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 Hg reduction are largely unknown. Here, we studied the reduction of Hg mediated by microalgae and aerobic bacteria in surface marine water and microalgae cultures under dark and sunlight conditions. The comparable reduction rates of Hg 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 that the associated aerobic bacteria are directly involved in dark Hg reduction. The aerobic bacteria in the microalgae cultures were isolated and a rapid dark reduction of Hg followed by a decrease of Hg was observed. The reduction of Hg and re-oxidation of Hg were demonstrated in aerobic bacteria spp. using double isotope tracing (Hg and Hg). These findings highlight the importance of algae-associated aerobic bacteria in Hg transformation in oxic marine water.