Uptake and effects of microcystin-LR on detoxication enzymes of early lifestages of the zebra fish (Danio rerio)

The effects of cyanotoxins on fish have been studied mainly in adults, rath er than in early life stages which could be more sensitive or, in view of t heir immobility, more readily affected. The uptake of microcystin-LR by dif ferent early life stages of the zebra fish (Danio rerio) was investigated u sing C-14-labelled microcystin-LR. The effects on the activity of the detox ication enzymes, microsomal and soluble glutathione S-transferases (GST), a nd glutathione peroxidase (GP-X) were examined. There was a detectable upta ke of microcystin from the first day of embryonic development up to 5 day o ld larvae. On average, an absorption of 0.5 ng microcystin for eggs and ele uthero-embryos was calculated over the entire exposure time, Because of the differences in volume of the eggs and eleuthero-embryos, there was an incr ease in the microcystin-LR concentration between these stages. In the eggs, approximately 25% of the medium concentration was found, and in eleuthero- embryos an equilibrium between fish and medium was reached. The activity of the detoxication enzymes differed during ontogenesis, but t he effects of activation and suppression of these enzymes were similar at a ll stages. Minor activation of the soluble GST was found and a marked activ ation of GP-X was evident. The reaction of the microsomal GST was not so ob vious. These results showed that there was an uptake of microcystin-LR by early li fe stages of the zebra fish and that the detoxication system reacted to thi s toxin, possibly indicating the ability of the organism to metabolize micr ocystin-LR to a less harmful compound. Chronic toxic effects, such as reduc tion in growth, in such early life stages when organogenesis is not finishe d and hence the microcystin-LR affects not one single target organ but the whole organism, might be due to the increased energy demand of these detoxi cation processes. (C) 1999 by John Wiley & Sons, Inc.