Exposure to Mercury at Trace Concentrations Leads to Collapse of the Hepato-Nephrocitic System in Two Neotropical Species of Bumblebee

This research aimed to investigate the effects of exposure to safe concentrations of mercury on the morpho-physiology of cells of the hepato-nephrocitic system of two species of neotropical bees. Workers of the neotropical bumblebee species Bombus morio and B. atratus were exposed to 0.2 ppb mercury for 48 h by ingestion. Bioassays were performed according some direction of OECD (2017) for bioassays in B. terrestris. The mercury concentration used is allowed for all types of water bodies for the Brazilian Environmental Council standards. The results showed that exposure severely impacted the fat body cells (oenocytes and trophocytes) and pericardial cells, either morphology and HSP70 expression, of both species and may represent a threat for these neotropical bee species. We observed that B. atratus is more sensitive to mercury exposure, although the two species have a very close phylogenetic relationship. To the best of our knowledge, this is the first study to compare the expression pattern of HSP70 in the fat body and pericardial cells of neotropical bumblebees under normal and mercury-exposure conditions. Both species expressed the protein in all studied cells under normal and exposed conditions; however, mercury exposure led to overexpression of HSP70 in B. morio workers only, with reduced expression observed in B. atratus following exposure. We conclude that 0.2 ppb of mercury severely impacts both species, being B. atratus more susceptible.