Effect studies of real life mixtures of persistant organic pollutants (POPs) in a zebrafish model

Abstract

The release of persistent organic pollutants (POPs) into the environment for many decades has led to a worldwide contamination. The impact of pollutants on ecosystem and human health is an urgent and international issue since there are an increasing number of examples of environmental disturbances as well as documented detrimental effects in humans and wildlife. The potential of POPs to cause toxicity in the reproductive and endocrine systems, as well as other adverse effects, has been of growing concern in the Western world. Despite the fact that individuals are more likely to be exposed to combinations of environmental pollutants, many experimental studies have focused on individual compounds or commercial mixtures of POPs. The present study focus on effects of real life mixtures of POPs harvested from aquatic ecosystems in Norway in an in vivo zebrafish model. POPs were extracted from burbot liver from Lake Losna and Lake Mjøsa and were used to expose the zebrafish through their diet in a two-generation study. Genome-wide effects on gene expression, biomarker detection, together with whole-organism responses, such as survival, growth, sex ratio and onset of puberty were investigated. The concentration range of POPs measured in zebrafish was lower than in burbot originating from Lake Mjøsa, but comparable to concentrations previously reported in humans and wildlife. The results showed that exposure of zebrafish with real life mixture of POPs had a negative effect on survival of fish in both generations. The marked drop in survival during 9-20 dpf suggested that this period may be a critical window of development in the zebrafish. In the parent (F0) generation there was a higher proportion of males and an earlier onset of puberty in Losna and Mjøsa exposed groups. Exposure with Losna and Mjøsa mixtures increased body mass and length in the parent generation (F0), while in the next generation (F1) the same exposures were associated with a decrease in body mass and length. Additionally, an indirect transgenerational effect of onset of puberty was indicated by the fact that exposure in F0 (Losna and Mjøsa High) combined with similar exposure in F1 resulted in an earlier onset of puberty in F1 (Losna-Losna and Mjøsa High-Mjøsa High). In terms of sex ratio, a possible transgenerational effect of Mjøsa and Losna exposure was also suggested by the fact that the groups receiving these exposures in F0 maintained a sex ratio skewed towards male dominance, whereas groups exposed to the same mixtures in F1 did not. Subtle differences were detected in mRNA levels, and altered gene expression was only found for the Mjøsa High exposure group. Genes associated with a cellular stress response, apoptosis, AhR signalling, and endocrine regulation was found to react to the toxic challenge. The biomarkers used in the present study (7-ethoxyresorufin-O-deethylase, EROD; Vitellogenin, Vtg) did not reveal strong dioxin-like effects or oestrogenic effects, respectively, that could be associated with exposure with real life mixtures of POPs. It was concluded that long term and low level exposure to real life mixtures of POPs was associated with effects linked to survival, development and reproduction.