Biochemical and Histological Assessment in Macrobrachium amazonicum Exposed to Mercury under pH and Dissolved Organic Carbon Variations from the Rio Negro

Abstract

Mercury contamination in Amazonian blackwater rivers, such as the Rio Negro, represents an environmental threat due to the toxicity and bioaccumulation potential of this metal. This study evaluated the short-term exposure (24 h) effects of the Amazonian shrimp Macrobrachium amazonicum (n = 10; weight: 0.12 ± 0.002 g; length: 5.4 ± 0.13 cm) to HgCl₂ (220 ng L⁻¹, the limit established by Brazilian legislation) under four experimental conditions (pH 7, pH 7 + DOC, pH 4, and pH 4 + DOC). Total mercury concentration was measured in water and shrimp muscle. Gill histopathology, lipid peroxidation (LPO), and enzymatic activities (Na⁺/K⁺-ATPase, V-type H⁺-ATPase, and carbonic anhydrase) were analyzed. The presence of DOC increased mercury persistence in water (regardless of pH); however, no bioaccumulation effects were detected in muscle tissue (0.0013 ± 0.000095 mg kg⁻¹). Despite the short exposure, mercury induced gill lesions (hypertrophy, inflammation, and hemorrhage), without significantly altering lipid peroxidation or the activity of the osmoregulatory enzymes studied. These results demonstrate that mercury, even at concentrations allowed by law and during short-term exposure, can cause histopathological alterations, reinforcing the need for further studies to understand its medium- and long-term effects.