Investigation of effects of the toxic cyanobacteria Microcystis aeruginosa on zebrafish reproductive system physiology at molecular, tissue and whole organism levels of biological organization

Abstract

Blooms of toxic cyanobacteria in freshwater bodies are a global ecological and public health concern. Microcystin (MC) toxin concentrations can exceed the recommended levels of the World Health Organization. Among microcystin (MC) toxin types, MC-LR is generally accepted as the most toxic congener and the one most considered in research. MCs have been associated with fish mortality and the sublethal effects of aqueous and dietary exposure on larval and adult zebrafish (Danio rerio) at environmentally relevant concentrations are relatively unknown. The objectives of this research project were novel. First of all, to investigate the toxicity of M. aeruginosa and MC-LR in zebrafish larvae using gene expression profiles and histopathology with dose and time response. A further objective was to investigate the effects of sub-lethal dietary exposure of both the cyanobacterium, Microcystis aeruginosa and the MC-LR toxin on adult zebrafish, through evaluations of target gene expression, changes in the gut microbiota and histopathology. The results of the sub-lethal aqueous exposure of MC-LR and M. aeruginosa for 96 hours on zebrafish larvae indicated oestrogenic effects for MC-LR and M. aeruginosa. Significant changes in gene expression were observed according to treatment concentration and exposure duration for oxidative stress and biotransformation related genes. However, treatment-related changes in gene expression did not relate to hepatocellular or intestinal lesions in larval zebrafish. Results after the sub-lethal dietary exposure of adult zebrafish to MC-LR or M. aeruginosa for 14 days increased significantly for the target genes depending on treatment and exposure duration. Additionally, no histopathological changes in the liver or the gut were observed, apart from normal variations. However, the trunk kidney showed indications of karyorrhexis, hypertrophy and hyperplasia. Moreover, changes in gut microbial community structure were recorded. Finally, the results of the present study showed changing gene expression profiles for different target genes at dose and time relationships after exposure to the sub-lethal aqueous and dietary exposures. Such exposures also suggested that oxidative stress could be another toxic route along with the protein phosphatase pathway. Changes in the gut structure imply that effects on gut physiology could in addition be an important toxicological response to M. aeruginosa.