Synchrotron-based X-ray analysis of solid-state mercury speciation serves as a useful complement to traditional analytical methods for mercury by providing detailed mercury speciation (e.g., coordination with protein structures) to more accurately assess risks to human and ecosystem health. As described above, GUA and LAH have distinctly different mercury sources, but both are mining-impacted, mineralogically based, proximal sources in freshwater. In contrast, an extensive survey of fish throughout the Western United States suggested the importance of atmospheric sources to most freshwater systems. Although it is unclear if atmospheric, hydrothermal, or benthic sources of mercury dominate the environment where previously examined marine fish were caught, these marine sources are clearly different from those for GUA and LAH. It is generally assumed that fish accumulate mercury in their tissues predominantly as methylmercury, but notable exceptions have reported significant inorganic mercury in tissues.

Piscivorous fish, all exceeding 300 mm in total length, were collected by angling in GUA, or by electroshocking in LAH in August 2005. The USEPA's human consumption threshold for mercury in fish is 0.3 mg·g^-1. Total mercury in our fish samples consistently exceeded that threshold and ranged from 1.74 to 8.28 mg·g^-1. The mean (4.05 mg·g^-1) was an order of magnitude greater than the mean mercury concentration reported for piscivores of the Western United States (0.26 mg·g^-1). In these mining-affected watersheds, extraordinary concentrations of mercury were common even for lower-trophic level organisms such as zooplankton and planktivorous fish (Kuwabara et al., 2005). Bioaccumulation factors (BAF; the ratio of methylmercury concentration in the fish tissue (mg·kg^-1) over the dissolved methylmercury concentration in the water (mg·L^-1) revealed log BAF values between 7.3 and 8.0, higher than reported for other Hg-contaminated aquatic systems (6.0 and 6.8). These higher BAFs suggested efficient methylmercury trophic transfer at both GUA and LAH.

Hg L_III XANES spectra from our piscivores were compared to ten model compounds. While spectral features of CH₃HgS(Cys) showed a striking resemblance to our fresh fish samples at pre- and post-edge regions, features of other model compounds were clearly dissimilar to our fish samples. Regardless of bass species or sampling locations, the predominant mercury species in their muscle tissues was CH₃HgS(Cys), containing linear two-coordinate Hg with methyl and cysteinyl sulfur donors. Our findings were consistent with previous work on commercial marine species (Harris et al., 2003) which reported the presence of CH₃HgS(Cys).

XANES results demonstrated that mercury was accumulated almost exclusively as methylmercury-cysteine complexes in the muscle tissues of piscivorous freshwater fish from two mining-impacted reservoirs. This result, consistent with observations for several marketed marine fish species, suggested that speciation of bioaccumulated mercury at high trophic levels was consistent over a wide range of ionic strengths and mercury sources. The dominance of methylmercury cysteine complexes in muscle tissues of fish obtained from such contrasting environments and exposure conditions suggested that a generic toxicological model for the consumption of fish could be applicable over a wide range of ecologic settings. Chloro-complexes of methylmercury have long been used as the mercury species of toxicological concern for trophic transfer. This work highlights the importance of describing processes that intra- or extracellularly transform mercury coordination between toxicologically labile and inert mercury complexes.

Primary Citation
Kuwabara, J.S., Arai, Yuji, Topping, B.R., Pickering, I. J., and George, G.N. (2007). Mercury speciation in piscivorous fish from mining-impacted reservoirs: Environmental Science and Technology 41, 2745-2749.

References