Transformations of mercury in the terrestrial isopod Porcellio scaber (Crustacea)

Abstract

The biological cycle of mercury in the terrestrial isopod Porcellio scaber was investigated. Testing the possibility of in vivo Hg 2+ methylation was divided into two methodologically different parts. Firstly, concentrations of total mercury and MeHg in isopods P. scaber and their environment from a Hg-unpolluted area were measured by the use of validated methods (CV AAS, CV AFS). The data obtained show that the percentage of MeHg in leaves, soil and faeces was less than 1%. In contrast, the percentage of MeHg in gut and hepatopancreas was increased to 14 and 77%, respectively, indicating methylation of Hg 2+ in the gut and its further accumulation in glands. To confirm this assumption, the second methodology was applied—a radiotracer technique with 203Hg 2+ of high specific activity. There are few radiotracer techniques for Hg-methylation assays; for our work we chose the method of Czuba et al. which includes alkaline leaching of Hg species, their extraction into dithizone–toluene, followed by specific separation of Hg dithizonates by thin-layer chromatography and gamma counting. All steps of the analytical protocol were checked and optimised by the use of aqueous solutions of 203Hg 2+ and Me 203Hg +. The most important finding was that cleaning-up the extract through a florisil column is not appropriate, because the column retains different percentages of Hg 2+ and MeHg + and consequently affects the accuracy of the final result. This optimised protocol was then applied to Hg transformation studies in the terrestrial isopod P. scaber. Leaching Hg species from P. scaber fed with 203Hg 2+ or Me 203Hg + dosed food was completely efficient only at elevated temperatures. Preliminary results of methylation/demethlytion studies are rather variable but they show that both processes (Hg 2+↔MeHg +) take place in the isopod P. scaber. Additionally, an assessment of the mass balance of Hg in isopods P. scaber exposed to 203Hg 2+ indicates that volatile Hg species are also formed.