Unveiling the hidden threat: Molecular, cellular and behavioral effects of dietborne inorganic mercury and methylmercury in Dreissena polymorpha

Methylmercury (MeHg) bioaccumulation in food webs has been recognized as a significant health risk for over 50 years, yet most studies focus on high concentrations of waterborne inorganic mercury (IHg). This study investigates the effects of dietborne mercury (Hg) exposure at environmentally realistic IHg and MeHg levels on a freshwater food chain. Freshwater mussels, Dreissena polymorpha, were fed with microalgae previously contaminated with 2 and 20 fg IHg or MeHg per cell for 4 d. Filtration behavior, Hg bioaccumulation, histopathology, antioxidant enzyme activity, and gene expression related to defense and energy metabolism were measured across gills, digestive glands, and other soft tissues (rests) for 1, 2, and 4 d. While all microalgae were filtered at the end of feeding, only MeHg exposure led to a reduced filtration at the beginning of feeding. Bioaccumulation factors were higher for MeHg than IHg, particularly in gills. Dietborne MeHg also caused more fibrosis and structural changes in gills than IHg, in line with bioaccumulation. Necrosis, tubular atrophy, and hemocyte infiltration were observed in the digestive gland. Both IHg and MeHg triggered oxidative stress, as evidenced by significant changes in antioxidant enzyme activities and increased lipid peroxidation levels. MeHg exposure significantly upregulated the sod gene in rests and modulated cs and aox genes involved in energy metabolism in gills and digestive gland, depending on exposure duration. The findings aligned with established Hg toxicity targets but demonstrated notable differences in response depending on Hg forms and tissue type, emphasizing the importance of Hg speciation and tissue type in assessing toxicity.