Modulating effects of pH and dissolved organic carbon on mercury toxicity in Macrobrachium amazonicum: Biochemical...

Mercury (Hg) is a critical contaminant in Amazonian ecosystems due to its methylation and bioaccumulation in aquatic organisms, processes that favour biomagnification along the food chain and constitute an important route of human exposure. Within this context, the Amazon River shrimp Macrobrachium amazonicum represents an ecological link and a relevant fishery resource, supporting its use as a biomodel to assess sublethal Hg effects. This study evaluated how dissolved organic carbon (DOC), isolated from the Rio Negro (Manaus, Brazil), and pH (4 and 7) influence Hg toxicity (220 ng L⁻¹ as Hg, added as HgCl₂) in M. amazonicum after short-term exposure (24 h). Hg exposure induced cytotoxic effects in hemocytes, oxidative and genotoxic damage, and pronounced histopathological alterations in gills and hepatopancreas, while osmoregulatory enzyme activities remained unchanged under the tested conditions. DOC stabilized Hg in the water, maintaining concentrations close to nominal levels, whereas in its absence waterborne Hg concentrations markedly decreased. Despite the lack of detectable muscular bioaccumulation after 24 h, Hg triggered significant biological responses, indicating tissue-level toxicity independent of measurable accumulation. pH strongly modulated both the magnitude and pattern of Hg-induced effects, with acidic conditions enhancing genotoxic damage and histopathological severity, regardless of DOC presence. Overall, Hg toxicity to Macrobrachium amazonicum is strongly modulated by natural water components, with DOC and pH reshaping biological responses rather than preventing toxic effects. The detection of significant effects in the absence of tissue accumulation highlights the importance of integrating early-warning biomarkers and physicochemical modulation into ecotoxicological risk assessments in Amazonian freshwater systems.