Abstract
Microcystins (MCs), the most common cyanobacterial toxins in freshwaters, can accumulate in aquatic animals in free and bound forms. MCs therefore pose a risk to human health upon consumption of contaminated fish tissues, as they cause severe liver damage and death with potential carcinogenic effects. While there have been extensive studies on free MC contents in fish tissues, the detection of bound MCs in fish is largely unexplored. In the present study, both free and bound MCs were monthly monitored in tilapia fish from three tropical fishponds in Egypt for one year. Fishpond waters contained high concentrations of MCs (18-44 mu g/l). Free MCs were estimated in tilapia fish organs at levels up to 11.8 ng/g in intestines, 8.3 ng/g in livers and 0.38 ng/g in edible tissues. Concentrations of bound MCs in edible tissues as determined by Lemieux oxidation technique (15,000-19,000 ng/g fresh weight, FW) were several-folds higher than free MCs. The estimated daily intake (EDI) for free MCs (0.0006-0.002 mu g/kg/day MCs) in edible tissues was lower than the WHO limit (0.04 mu g/kg/day), while EDI of bound MCs exceeded this limit by a factor of 77-2396. Therefore, both free and bound MCs in fish tissues should be considered when fish are monitored for human consumption.