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 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 that 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. using double isotope tracing (¹⁹⁹Hg²⁺ and ²⁰¹Hg⁰). These findings highlight the importance of algae-associated aerobic bacteria in Hg transformation in oxic marine water.