Photoreduction of Inorganic Mercury in Surface Seawater

Gaseous mercury (Hg(0)) supersaturation and evasion from surface seawater are commonly attributed to a combination of biotic and photochemical reduction of divalent mercury (Hg(II)). This aqueous reduction and subsequent Hg(0) loss constrain methylmercury production, thereby limiting its bioavailability to marine food webs. Despite the pivotal role of Hg(II) reduction in the global biogeochemical mercury cycle, the mechanism and relative importance of Hg(II) photoreduction in seawater remain poorly understood. To address this gap in knowledge, we conducted shipboard Hg stable isotope photoreduction experiments using natural seawater collected from the North Pacific Subtropical Gyre. Our results indicate that Hg(II) photoreduction in surface seawater is largely controlled by indirect photolysis of HgCl2. This represents a mechanism that contrasts with freshwater systems, where direct photolysis of Hg(II) dominates. Furthermore, by comparing experimental results with the stable isotopic composition of Hg in seawater and marine particles, we conclude that dissolved Hg(II) photoreduction in seawater is largely ineffective at producing gaseous Hg(0). These findings point to microbial Hg(II) reduction as the likely driver of Hg(0) supersaturation and evasion in seawater. We recommend further investigations into this phenomenon, particularly the potential for Hg(II) photoreduction mediated by pico- and phytoplankton.