During anaerobic degradation of Microcystis spp. blooms, the cell walls burst, releasing the toxins into the water, such as ammonia and microcystin. The released toxins could then come into contact with a wide range of aquatic organisms and can be responsible for the toxic effects observed in many organisms. To assess the responses of oxidative stress to a combination of ammonia and microcystin in larval fish, 11-day-old larvae of grass carp Ctenopharyngodon idella were exposed to solutions with different concentrations of ammonia (NH(3)-N) and microcystin-LR (MC-LR). Results showed that NH(3)-N and MC-LR had significant effects on catalase (CAT), superoxide dismutase (SOD), glutathione (GSH), and malondialdehyde (MDA), and significant interactions were detected between NH(3)-N and MC-LR, suggesting that oxidative stress and lipid peroxidation occurred in grass carp larvae exposed to a combination of ammonia and microcystin. Furthermore, CAT, SOD, GSH, and MDA fluctuated significantly with time, suggesting that the effects were time-dependent. Above results indicated that grass carp larvae may encounter toxic effects from ammonia and microcystin during the degradation of Microcystis spp. blooms. As CAT, SOD, GSH, and MDA of grass carp larvae are highly sensitive to ammonia and microcystin, they can be used as biomarkers for assessing oxidative stress responses of grass carp larvae to multiple toxins released from senescence and lysis of cyanobacterial blooms.
