Fluctuations in water temperature affected waterborne cadmium toxicity: Hematology, anaerobic glucose pathway, and oxidative stress status of Nile tilapia, Oreochromis niloticus (L.)

•Nile tilapia was exposed to 20, 24, 28, or 32°C and co-exposed to 0.0 or 0.5 mg Cd/L for 8 weeks.•Hematological variables increased significantly by increasing water temperature and they were lower in Cd-exposed fish group.•Hepatic glycolytic and oxidative stress enzymes activities increased with increasing water temperature up to 32°C.•The Cd exposure caused significant increases in the activities of glucolytic pathway especially at high temperature.•Cd-exposed fish showed highest activities of oxidative stress enzymes as compared to Cd-free fish groups.

The increase of water temperature has already been observed because of global climatic changes. Beside fluctuations in temperature, aquatic ecosystems may be stressed by the discharge of heated wastewater and toxic chemicals such as heavy metals. Therefore, this study was conducted to investigate effects of water temperature and/or waterborne cadmium (Cd) on hematology (red blood cells, RBCs; hematocrit, Ht; and hemoglobin, Hb), anaerobic glucolytic metabolism (lactate dehydrogenase, LDH; phosphofructokinase, PFK; and pyruvate kinase, PK) as well as of the pentose pathway (glycose-6-phosphate dehydrogenase, G6PDH), and oxidative defense (sodium dismutase, SOD; catalase, CAT; glutathione peroxidase, GPx and Glutathion transferase, GST) of Nile tilapia. Fish (26.0 ± 0.38 g) were acclimated to 20, 24, 28, or 32 °C and co-exposed to 0.0 or 0.5 mg Cd/L for 8 weeks in triplicates. Two-way ANOVA revealed that all variables were significantly affected by water temperature, Cd exposure, and their interaction. Hematological variables increased significantly by increasing water temperature up to 28 °C, after which they decreased significantly at 32 °C. Additionally, these variables were lower in Cd-exposed fish group than those of Cd-free ones. Irrespective to Cd exposure, there are no significant differences in hematological variables in fish reared at 24 and 28 °C. Hepatic glycolytic and oxidative system enzymes activities increased with increasing water temperature up to 32 °C. In Cd-free fish groups, there are no significant differences in activities of these enzymes in fish reared at 24 and 28 °C. The Cd exposure caused significant increases in the activities of LDH and PK and significant decreases in PFK and G6PDH activities with increasing water temperature compared to Cd-free fish groups. Meanwhile, Cd-exposed fish showed highest activities of oxidative stress enzymes as compared to Cd-free fish groups. The changes in hepatic enzymes suggest that their activities were optimized when fish were reared at 24-28 °C. Furthermore, at 32 °C, the overall increment in the hepatic enzyme activities of Cd-exposed fish seems to be contributed to enhance Cd toxic effects.