Selenium Alters the Tissue Distribution of Inorganic but Not Methylmercury in Zebrafish Larvae

Mercury toxicity is strongly influenced by its chemical speciation, with interactions between mercury and selenium species either mitigating or exacerbating toxic effects. Here, synchrotron-based X-ray fluorescence imaging was used to examine the influence of l-selenomethionine on the tissue distributions of inorganic mercury and methylmercury in zebrafish larvae. Inorganic mercury accumulated predominantly in the pronephric ducts and liver, whereas methylmercury was distributed more uniformly throughout the organism. Administration of l-selenomethionine substantially altered the distribution of inorganic mercury and increased selenium accumulation within the pronephric ducts. Subcellular-resolution imaging revealed strong spatial correlations among mercury, selenium, and sulfur following inorganic mercury exposure, revealing mercury-, selenium-, and sulfur-rich domains within the pronephric ducts. In contrast, l-selenomethionine had comparatively little influence on methylmercury distribution, although combined exposure increased tissue sulfur levels. These findings demonstrate fundamentally different interactions of selenium with inorganic and organometallic mercury species in vivo and emphasize the importance of both chemical form and subsequent metabolism in determining mercury–selenium interactions.