Environmental assessment of the Ariyankuppam River, India: chromium and lead accumulation in Mugil cephalus and Azolla pinnata-mediated toxicity mitigation in Labeo rohita.

Heavy metals are persistent environmental pollutants that pose long-term risks to aquatic ecosystems and public health because of their non-biodegradable nature. This study was carried out under two complementary components, namely, an environmental assessment of the Ariyankuppam River, Puducherry, India and a controlled laboratory exposure experiment. Water, sediment, and Mugil cephalus samples were collected to assess the concentrations of heavy metals via inductively coupled plasma mass spectrometry (ICP-MS) in the field investigation. The major toxic metal found in water samples was chromium (Cr) with concentration of 0.058 ± 0.003 mg L⁻1 and in sediments (15.616 ± 6.672 mg kg⁻1), while lead (Pb) was consistently detected in all environmental matrices, indicating contamination and accumulation in fish tissues. Geochemical analyses using XRD, EDX, and XPS were used to identify crystalline and mineral phases associated with Cr and Pb in the sediment matrix. laboratory study, Labeo rohita were exposed to sublethal concentrations of Cr (5 mg L⁻1) and Pb (11 mg L⁻1) for 96 h. Heavy metal exposure significantly increased catalase (CAT) activity in gill tissues and decreased CAT activity in muscle tissues (p < 0.05), indicating tissue-specific oxidative stress responses. Alterations in protein band patterns were found by SDS-PAGE, while histopathological examination demonstrated marked damage to the gill, liver, and muscle tissues. Treatment with Azolla pinnata restored CAT activity toward control levels, reduced protein band alterations, and alleviated histopathological damage. Overall, the findings demonstrate the acute toxicity of Cr and Pb and highlight the potential of A. pinnata to mitigate heavy metal-induced toxicity under controlled laboratory conditions. Further long-term field studies are needed to evaluate its practical application in freshwater ecosystems.