Factors controlling the photochemical degradation of methylmercury in coastal and oceanic waters

Abstract

Many studies have recognized abiotic photochemical degradation as an important sink of methylmercury (CH3Hg) in sunlit surface waters, but the rate-controlling factors remain poorly understood. The overall objective of this study was to improve our understanding of the relative importance of photochemical reactions in the degradation of CH3Hg in surface waters across a variety of marine ecosystems by extending the range of water types studied. Experiments were conducted using surface water collected from coastal sites in Delaware, New Jersey, Connecticut, and Maine, as well as offshore sites on the New England continental shelf break, the equatorial Pacific, and the Arctic Ocean. Filtered water amended with additional CH3Hg at environmentally relevant concentrations was allowed to equilibrate with natural ligands before being exposed to natural sunlight. Water quality parameters – salinity, dissolved organic carbon, and nitrate – were measured, and specific UV absorbance was calculated as a proxy for dissolved aromatic carbon content. Degradation rate constants (0.87–1.67 day−1) varied by a factor of two across all water types tested despite varying characteristics, and did not correlate with initial CH3Hg concentrations or other environmental parameters. The rate constants in terms of cumulative photon flux values were comparable to, but at the high end of, the range of values reported in other studies. Further experiments investigating the controlling parameters of the reaction observed little effect of nitrate and chloride, and potential for bromide involvement. The HydroLight radiative transfer model was used to compute solar irradiance with depth in three representative water bodies – coastal wetland, estuary, and open ocean – allowing for the determination of water column integrated rates. Methylmercury loss per year due to photodegradation was also modeled across a range of latitudes from the Arctic to the Equator in the three model water types, resulting in an estimated global demethylation rate of 25.3 Mmol yr−1. The loss of CH3Hg was greatest in the open ocean due to increased penetration of all wavelengths, especially the UV portion of the spectrum which has a greater ability to degrade CH3Hg. Overall, this study provides additional insights and information to better constrain the importance of photochemical degradation in the cycling of CH3Hg in marine surface waters and its transport from coastal waters to the open ocean.

The Global Marine Selenium Cycle: Insights From Measurements and Modeling

“…In experiments using water from Fishers Island Sound, with and without UV exposure (Figure S2b), we found that about 10% of the total photodegradation was due to PAR with the remaining 90% due to UV radiation (Table S3b). We used this result to calculate the depth‐integrated degradation rates for our three types of water (coastal wetland and coastal and offshore waters) by modeling water column light penetration using the HydroLight program (Table S3; DiMento & Mason, ; see SI for further details). Relative degradation rate constants for the different wavelengths of light were based on our data (Table ) and that of Taalba et al () for (CH 3 ) 2 S, which also agreed with wavelength dependence found by others (Bouillon et al, ; Kieber et al, ; Toole et al, , , ).…”

“…The magnitude of (CH 3 ) 2 Se loss due to photodegradation was determined globally by considering the latitudinal variation in solar radiation intensity (Frouin et al, 2012) and ocean surface area (Allen & Gillooly, 2006). Polar seas were corrected for their average ice coverage (DiMento & Mason, 2017). We estimated a global degradation rate of 28.0 Gmol/a, which is used to derive the flux in our box model.…”

A Global Model for Methylmercury Formation and Uptake at the Base of Marine Food Webs

“…Prior work has identified the occurrence of both photochemical and biological degradations of MeHg (DiMento & Mason, 2017;Black et al, 2012;Monperrus et al, 2007). Laboratory studies suggest that the photodegradation rate for MeHg in seawater depends on local incident photon intensity and light attenuation by chlorophyll, dissolved organic matter, and total suspended materials (DiMento & Mason, 2017;Lehnherr et al, 2011;Monperrus et al, 2007;Whalin et al, 2007). Below the photic layer, biotic and other abiotic processes (e.g., reactions with OH radicals) will dominate (Monperrus et al, 2007;Whalin et al, 2007).…”

Microbial generation of elemental mercury from dissolved methylmercury in seawater

“…Whalin et al () reported that the rate constants of demethylation in the water column of Chesapeake Bay were < 1–5 × 10 −6 s −1 where the demethylation was driven by both photochemical and microbial processes. In a lab study using coastal seawater, rate constants attributable solely to photochemistry were reported to range from 10 × 10 −6 s −1 to 19 × 10 −6 s −1 (DiMento and Mason ). DiMento and Mason () also demonstrated that photodegradation of MeHg decreases exponentially with depth, with degradation rates being < 10% of those in surface waters at depths < 5 m in estuaries, and 60 m in open ocean waters.…”

“…In a lab study using coastal seawater, rate constants attributable solely to photochemistry were reported to range from 10 × 10 −6 s −1 to 19 × 10 −6 s −1 (DiMento and Mason ). DiMento and Mason () also demonstrated that photodegradation of MeHg decreases exponentially with depth, with degradation rates being < 10% of those in surface waters at depths < 5 m in estuaries, and 60 m in open ocean waters. Thus, biologically mediated conversion of MeHg would be expected to be greater than photochemical degradation of MeHg in subsurface waters, the absolute depth depending on light penetration in different water columns.…”