Effects of wetland salinization on mercury cycling in low-lying coastal wetlands: A microcosm experiment incubating coastal wetland soils with water of different salinities

Low-lying coastal wetlands are known as significant hotspots for the production of highly toxic methylmercury (MeHg). Extensive salinization of these low-lying coastal wetlands occurs due to sea level rise but the impacts on mercury (Hg) biogeochemical cycles are poorly understood. In particular, seawater contains abundant sulfate, a nutrient often found to be limiting for anaerobic sulfate reduction/Hg methylation in freshwater wetlands. In this study, we collected freshwater wetland soils, freshwater and seawater from a coastal wetland in South Carolina. We conducted sealed microcosm experiments in the laboratory by incubating freshwater wetland soil and freshwater mixed with different portions of seawater (resultant salinities: 0, 1, and 5 psu). All microcosms turned to be anoxic quickly upon the start, we filtered the water samples after 3 and 9 weeks, and analyzed for total Hg (THg) and MeHg. Results showed that THg and MeHg levels exhibited the trend: 3-weeks data: 1 psu (THg: 9.6 ng/L; MeHg: 6.1 ng/L) > 5 psu (THg: 5.6 ng/L; MeHg: 3.8 ng/L) > 0 psu (THg: 4.01 ng/L; MeHg: 0.71 ng/L), but after 9-weeks: 5 psu (THg: 11.5 ng/L; MeHg: 7.0 ng/L) > 1 psu (THg: 6.5 ng/L; MeHg: 2.3 ng/L) > 0 psu (THg: 3.6 ng/L; MeHg: 0.26 ng/L). The results suggest that salinization of wetland soils in the short term would significantly increase dissolution of THg (inorganic Hg) from soils, and more importantly greatly stimulate MeHg production in the soil slurry. In order to separate the effects of "non-sulfate" sea salt and sulfate alone, we performed a microcosm experiment in which we added seawater to the freshwater to result in 1 psu, and separately added reagent-grade sulfate to the freshwater to result in the same level of sulfate as in 1 psu treatment. This latter experiment showed that the same amount of sulfate addition as in the 1 psu water produced considerably lower dissolved THg (42% lower) and MeHg (55% lower), suggesting that factors other than sulfate in seawater (e.g., chloride) may contribute to dissolution of THg from soils and/or to microbial MeHg production. We concluded that wetland salinization should increase the dissolution of THg or inorganic Hg, and Hg methylation with increased salinities at least in the short-term. This study has implications on Hg bioavailability in coastal wetlands as threatened by global climate change.