Occurrence of methylated mercury in a terrestrial food chain

Abstract

The presence and transfer of methylated mercury (MeHg) in a terrestrial food chain were determined by radiotracer laboratory experiments and chemical analyses of field-collected organisms. [ 203Hg]MeHg-tagged fescue grass ( Festuca sp.) at 0.65 μg MeHg/g grass (wet wt) was fed to cotton rats ( Sigmodon hispidus). The rats had an assimilation efficiency ( A e) for MeHg of 99% and a biological half-life ( t b) of 9.5 days. Control S. hispidus and Festuca collected from the same populations as the experimental organisms were analyzed for MeHg and found to have, for the S. hispidus, 1.4 ± 0.2 ng/g, 1 SE, and 1.3 ± 0.3 ng/g, 1 SE, in skeletal muscle and liver, respectively; and, for Festuca (composite sample) 0.6 ng/g. Total mercury (ΣHg) for the same tissues was 119.4 ± 39 ng/g, 1 SE, 34.6 ± 8.9 ng/g, 1 SE; and 71 ng/g (composite sample), respectively. Calculations with the experimental and analytical MeHg values indicated that the feral S. hispidus would, at equilibrium, have a whole-body MeHg concentration of 1.89 ng/g. These results, compared with literature reports, indicate that feral rodents have concentration ranges of ∼50 to ∼160 ng/g for ΣHg, and ∼1 to ∼7 ng/g for MeHg. Environmental levels of mercury are likely to increase significantly; further environmental measurements and especially appropriate toxicity tests are warranted to assess the importance of MeHg in terrestrial ecosystems.