| Summary: | Methylmercury (MeHg) is a toxin that poses health risks to humans and wildlife, primarily through consumption of seafood (Sunderland and Mason, 2007). Most MeHg bioaccumulation by marine fish likely occurs in the upperof seafood (Sunderland and Mason, 2007). Most MeHg bioaccumulation by marine fish likely occurs in the upper 200 m of the ocean, which is mostly oxygenated (Mason et al., 2012). However, mechanisms of Hg methylation in oxic seawater remain unknown, since the hgcAB gene cluster, which encodes proteins for MeHg production, had been found exclusively in anaerobic microbes (Gilmour et al., 2013; Parks et al., 2013). Recent work, however, has shown that hgc genes are widespread in oxic seawater, including in the icroaerophilic nitrifier, Nitrospina (Tada et al., 2020; Tada et al., 2021; Gionfriddo et al., 2016; Bowman et al., 2020). Here, we show that potential MeHg production rates in Western Tropical North Atlantic Ocean surface waters, within and near the Amazon River plume, were correlated positively and strongly to nitrification rates and Nitrospina-specific 16S gene expression. Potential Hg methylation and nitrification rates were highest at the most saline and least turbid stations, indicating that sediment particles and nutrient-rich, riverine discharges were not the primary factors promoting either process. These novel results in oxic seawater provide further evidence that Hg methylation is linked to abundant, nitrifying microbes and may help explain marine MeHg distributions.
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