|Other Abstract||Dense blooms of cyanobacterial (blue–green) algae are one of the consequences of the increasing eutrophication in many waters worldwide. With frequent occurrence of cyanobacterial blooms in recent years, cyanotoxins have become a great threat to aquatic animals, livestock, and human health. Among these toxins, microcystins (MC) are the most common and dangerous hepatotoxins. Fish, one of the main inhabitants in aquatic systems, is usually subjected to MC directly or passively, which consequently causes fish kill or fish poisoning episodes. Till now, most experimental studies documenting the toxicity of MC exposure to fishes have been prone to acute toxicity experiments. Few investigations have been conducted on post-event of wild animal poisonings in which toxic cyanobacteria are suspected, especially when long-term and/or frequent exposure occurs. The main aim of this study was to examine physiological and biochemical responses of fishes with different trophic levels to toxic cyanobacterial blooms in Meiliang Bay, Lake Taihu, with discussion on the possible mechanisms.
Deaths from microcystin toxication have widely been attributed to hypovolemic shock due to hepatic interstitial hemorrhage, while some recent studies suggest that cardiogenic complication is also involved. So far, information on cardiotoxic effects of MC has been rare and the underlying mechanism is still puzzling. We conducted intravenous injection of extracted MC in rats，in order to evaluate the roles of oxidative stress and mitochondrial dysfunction in cardiotoxic effects by MC.
The main results and conclusions were summarized as follows:
1. Physiological and biochemical responses of four fishes with different trophic levels to toxic cyanobacterial blooms were studied in a large net cage in Meiliang Bay, a hypereutrophic region of Lake Taihu. We sampled four fishes: the phytoplanktivorous Hypophthalmichthys molitrix and Aristichthys nobilis, the omnivorous Carassius auratus, and the carnivorous Culter ilishaeformis. Alterations of the antioxidant (GSH) and the major antioxidant enzymes (CAT, SOD, GPx, GST) in livers were monitored monthly, and the ultrastructures of livers were compared between the bloom and the post-bloom periods. During the cyanobacterial blooms, the phytoplanktivorous fishes displayed only slight ultrastructural changes in liver, while both carnivorous and omnivorous fishes presented serious injuries such as swollen endomembrane system and morphologically altered nuclei in hepatocytes. Biochemically, the phytoplanktivorous fishes possessed higher basal GSH concentrations and better correlations between the major antioxidant enzymes in liver, which might be responsible for their powerful resistance to MC.
2. Physiological and biochemical responses in kidneys of fishes with different trophic levels (phytoplanktivorous, omnivorous and carnivorous) to toxic cyanobacterial blooms were studied in a large net cage in Meiliang Bay. Catalase and glutathione S-transferase were significantly higher during blooms than before and after blooms. All fishes showed ultrastructural alterations in kidneys during blooms, which mainly are inosculation of foot processes in epithelial cell of glomeruli. The results suggested that kidney impairment from chronic exposure of toxic cyanobacterial blooms might be the first step, and then followed by hepatic failure. Compared with livers in terms of physiological status, the weaker antioxidant ability of kidney made it more susceptible to chronic MC exposure, besides the effective accumulation of MC metabolites in kidney.
3. To evaluate the impact of toxic cyanobacterial blooms on the planktivorous fish, 12 serum chemistry variables were investigated in silver carp and bighead carp, in a large net cage in Meiliang Bay, a hypereutrophic region of Lake Taihu. The results confirmed adverse effects of cyanobacterial blooms on the two phytoplanktivorous fishes, mainly characterized with potential toxicogenomic effects and metabolism disorders in liver, and kidney dysfunction. In addition, cholestasis was intensively implied by distinct elevation of all four related biomarkers (ALP, GGT, DBIL, TBIL) in bighead carp. Based on a multivariate discriminant analysis, the combination of LDH and AST activities and DBIL and URIC contents for silver carp, and the combination of ALT and ALP activities and TBIL, DBIL and URIC concentrations for bighead carps strongly indicate toxic effects from cyanobacterial blooms.
4. A laboratory experiment was conducted to examine toxic effects of microcystins on heart muscle of rats intravenously injected with extracted MC at two doses, 0.16LD50 (14µg MC-LReq kg-1 body weight) and 1LD50 (87 µg MC-LReq kg-1 body weight). In the dead rats, both TTC staining and maximum elevations of troponin I levels confirmed myocardial infarction after MC exposure, besides a serious interstitial hemorrhage in liver. In the 1LD50 dose group, the coincident falls in heart rate and blood pressure were related to mitochondria dysfunction in heart, while increases in creatine kinase and troponin I levels indicated cardiac cell injury. The corresponding pathological alterations were mainly characterized as loss of adherence between cardiac myocytes and swollen or ruptured mitochondria at the ultrastructural level. MC administration at a dose of 1LD50 not only enhanced activities and up-regulated mRNA transcription levels of antioxidant enzymes, but also increased GSH content. At both doses, level of lipid peroxides increased obviously, suggesting serious oxidative stress in mitochondria. Simultaneously, complex I and III were significantly inhibited, indicating blocks in electron flow along the mitochondrial respiratory chain in heart. In conclusion, the findings of this study implicate a role for MC induced cardiotoxicity as a potential factor that should be considered when evaluating the mechanisms of death associated with microcystin intoxication in Brazil.|