Effect of Toxicity of Chromium (VI) Stressors Alone and Combined to High Temperature on the Histopathological, Antioxidation, Immunity, and Energy Metabolism in Fish Phoxinus lagowskii

Fish in aquatic ecosystems are often impacted by environmental stressors like temperature fluctuations and exposure to heavy metals. Chromium (Cr6+) is a known environmental pollutant that poses a threat to aquatic life. Various environmental factors, such as water temperature, have been found to affect the toxicity of dissolved chemicals in aquatic ecosystems. We investigated the toxicity of combinations of different concentrations of hexavalent chromium (Cr6+) with high temperatures in fish. Hematological indices demonstrated changes in white blood cells (WBCs), hematocrit (HCT), red blood cells (RBCs), and hemoglobin (Hb) levels during the exposure. The qualitative and semi-quantitative analyses of different tissues confirmed that higher concentrations of Cr6+ caused more significant damage than lower concentrations, with evident alterations observed in circulatory and regressive aspects. Furthermore, brain acetylcholinesterase levels decreased in both single heavy metal exposure and combined exposure at a high temperature. The activity of antioxidant oxidase and immunological parameters increased in all treatment groups compared with the control group following long-term exposure. A significant and increased effect of Cr6+ in the high-temperature groups was observed on the evaluated biomarkers, suggesting a possible synergistic effect between Cr6+ and increased temperature. The integrated biomarker response (IBR) reported the highest level of stress at 10 mg/L Cr6+ combined with high temperature. The IBR analysis revealed that the highest activity of response enzymes, such as acid phosphatase (ACP), superoxide dismutase (SOD), and glutathione S-transferases (GST), was observed in the liver, whereas the gills displayed alkaline phosphatase (ALP), GST, and SOD activity, and the kidneys demonstrated SOD, ACP, and aspartate aminotransferase (AST) to be most active. Through histopathology, antioxidant enzymes, and metabolism- and immunity-related enzymes, we determined that high temperatures enhance the potential toxicity of Cr6+ in fish. We recommend conducting a thorough assessment of the impact of climate change, particularly temperature fluctuations, when studying the toxic effects of metal pollution, like chromium, in aquatic ecosystems.