Impairment of reproduction of adult zebrafish (Danio rerio) by binary mixtures of environmentally relevant concentrations of triclocarban and inorganic mercury

• TCC aggravated liver lesions caused by Hg2+ by altering concentrations of hormones.
• Mixture of Hg2+ and TCC under environmentally relevant concentrations induced severe gonad histological lesions.
• The candidate mechanism of binary mixture for reproductive toxicity was to affect steroidogenesis genes.

Effects of chemical mixtures at environmentally relevant concentrations on endocrine systems of aquatic organisms are of concern. Triclocarban (TCC) and inorganic mercury (Hg2+) are ubiquitous in aquatic environments, and are known to interfere with endocrine pathways via different mechanisms of toxic action. However, effects of mixtures of the two pollutants on aquatic organisms and associated molecular mechanisms were unknown. This study examined effects of binary mixtures of TCC and Hg2+ on histopathological and biochemical alteration of reproductive organs in zebrafish (Danio rerio) after 21 d exposure. The results showed that: 1) At concentrations studied, TCC alone caused little effect on hepatic tissues, but it aggravated lesions in liver caused by Hg2+ via indirect mechanisms of disturbing homeostasis and altering concentrations of hormones; 2) Histological lesions were more severe in gonads of individuals, especially males, exposed to the binary mixture. Exposure to TCC alone (2.5 or 5 μg/L) (measured concentration 140 or 310 ng/L) or Hg2+ alone (5 μg/L or 10 μg/L (measured concentration 367 or 557 ng/L) slightly retarded development of oocytes, whereas co-exposure to nominal concentrations of 5 μg/L TCC and 10 μg /L Hg2+ promoted maturation of oocytes. In males, maturation of sperm was slightly delayed by exposure to either TCC or Hg2+, while their combinations caused testes to be smaller and sperm to be fewer compared with fish exposed to either of the contaminants individually; 3) Lesions observed in fish exposed to binary mixtures might be due to altered transcription of genes involved in steroidogenesis, such as cyp19a, 3beta-HSD, cyp17, 17beta-HSD and modulated concentrations of testosterone and estradiol in blood plasma. The observed results further support the complexity of toxic responses of fish exposed to lesser concentrations of binary chemical mixtures. Since it is impossible to collect empirical information in controlled studies of all possible combinations of toxicants, the application of omics methods might improve the predictive capabilities of results of single classes of chemicals.