Cellular energy budget in tropical freshwater fish following exposure to sublethal concentrations of cadmium

Abstract

Under stressful conditions (toxicity), organisms often try to detoxify by mobilizing available energy sources with costs to various metabolic functions, such as growth or reproduction. Cellular energy allocation (CEA) is a methodology used to evaluate the energetic status and which relates with organisms’ overall condition and response to toxic stress. It consists of the integration of the energy reserves available (Ea) and energy consumption (Ec). The effects of different sublethal concentrations (0.828, 0.0828, and 0.00828 mg/l) of cadmium (Cd) was evaluated on the total energy budget of African catfish (Clarias gariepinus) juveniles over 56-day period of exposure. A total of 180 C. gariepinus were exposed under the static renewal assay, and parameters measured were the total energy reserves available (protein, carbohydrate and lipid budgets) and the energy consumption (based on electron transport system activity assay) being further integrated to obtain the CEA. The Bradford method, Phenol-sulphuric acid method and Bligh and Dyer method were used to evaluate the protein, carbohydrate, and lipid contents, respectively of the test animals. Significant changes (p<0.05) in energy reserves and energy consumption were observed upon Cd exposure. Among the three energy reserves obtained, carbohydrate offered the least energy fraction (0.23%), and followed by protein (20.27%). The highest energy fraction was offered by lipid (79.50%). The effect of cadmium brought about a tremendous decrease in Carbohydrate (from 0.04 kj/g in day 7 to 0.01 kj/g in day 28). Lipid always stepped up (from 9.84 kj/g on day 7 to 34.48 jk/g on day 28) to compliment energy loss whenever carbohydrate was exhausted. Protein was least affected (from 1.78 kj/g on day 7 to 1.23 kj/g on day 28) with mild reduction in its reserve. Increased energy consumption was recorded amongst the exposed groups, with the highest concentration (0.828 mg/l) offering the most Ec of 58.54 kj/g on day 56. Significant reduction in CEA occurred across the exposed groups as Ea and Ec dwindled.  These results have shown the adverse effects of Cd on the energy status of C. gariepinus and the sensitive effectiveness of CEA technique in assessing the toxic effects of metallic pollutants on freshwater animals. Key words: Clarias gariepinus, cellular energy allocation, energy available, electron transport system, biomarker, energy metabolism.