Bioaccumulation of mercury in fish exposed to experimentally contaminated water and sediment

Matrix in River Water, Sediments, and Biofilms Mitigates Mercury Toxicity to Medaka (Oryzias Latipes)

Impacts of an environmental matrix on mercury (Hg) bioavailability and toxicity to medaka (Oryzias latipes) were investigated in matrix-free controls and treatments with a stepwise increased environmental matrix of river water, sediments, and biofilms. Generally, river water enhanced but the presence of sediments and biofilms reduced Hg bioavailability to medaka up to 105 times, so that Hgtotal concentrations/amounts among different environmental media cannot mirror Hg availability and toxicity to medaka. On average, 12.9 and 12.4% of Hg in medaka was, respectively, methylated to methylmercury (MeHg) in matrix-free and -containing treatments, indicating no influence of the environmental matrix on Hg methylation in medaka. All oxidative stress, inflammatory injury, and malformation parameters correlated strongly and significantly with Hgtotal and MeHg concentrations in medaka, notably with steeper slopes in matrix-free controls than in matrix-containing treatments, highlighting that the environmental matrix mitigated Hg and MeHg toxicity to medaka. Moreover, oxidative stress was more strongly mitigated than inflammatory injury according to the stronger decreases of the regression line slopes from matrix-free to -containing treatments. Here, we have newly identified that the potential of the environmental matrix to decrease Hg bioavailability and mitigate Hg toxicity to fish together could buffer Hg ecotoxicity in the aquatic environment.

Sediment toxicity assessment using zebrafish (Danio rerio) as a model system: Historical review, research gaps and trends

Sediment is an important compartment in aquatic environments and acts as a sink for environmental pollutants. Sediment toxicity tests have been suggested as critical components in environmental risk assessment. Since the zebrafish (Danio rerio) has been indicated as an emerging model system in ecotoxicological tests, a scientometric and systematic review was performed to evaluate the use of zebrafish as an experimental model system in sediment toxicity assessment. A total of 97 papers were systematically analyzed and summarized. The historical and geographical distributions were evaluated and the data concerning the experimental design, type of sediment toxicity tests and approach (predictive or retrospective), pollutants and stressors, zebrafish developmental stages and biomarkers responses were summarized and discussed. The use of zebrafish to assess the sediment toxicity started in 1996, using mainly a retrospective approach. After this, research showed an increasing trend, especially after 2014-2015. Zebrafish exposed to pollutant-bound sediments showed bioaccumulation and several toxic effects, such as molecular, biochemical, morphological, physiological and behavioral changes. Zebrafish is a suitable model system to assess the toxicity of freshwater, estuarine and marine sediments, and sediment spiked in the laboratory. The pollutant-bound sediment toxicity in zebrafish seems to be overall dependent on physical and chemical properties of pollutants, experimental design, environmental factor, developmental stages and presence of organic natural matter. Overall, results showed that the zebrafish embryos and larvae are suitable model systems to assess the sediment-associated pollutant toxicity.

Detection of mercury in aquatic environments using EPRE reporter zebrafish

It has been proposed that transgenic zebrafish could be designed to detect low levels of chemical contaminants that cause oxidative stress in aquatic environments, such as heavy metals or pesticides. In this paper, we describe such a transgenic zebrafish that produces a luciferase-green fluorescent protein (LUC-GFP) fusion protein under conditions of oxidative stress. The reporter gene expression is under the regulation of the electrophile responsive element (EPRE), which activates gene expression in response to oxidative stressors. The GFP component of this fusion protein allows us to visually detect reporter gene activity in live animals to determine if activity is localized to a particular tissue. The luciferase component is capable of returning a quantitative assessment of reporter gene activity that allows us to determine if reporter gene activity is directly correlated to the concentration of the chemical inducer. We have tested this reporter construct in both transient and stable transgenic fish after exposure to a range of HgCl(2) concentrations. GFP expression from the EPRE-LUC-GFP construct was inducible in transient assays but was below the limit of detection in stable lines. In contrast, we observed inducible luciferase activity in both transient assays and stable lines treated with HgCl(2). We conclude that the EPRE is capable of driving reporter gene expression in a whole animal assay under conditions of oxidative stress. Furthermore, expression was induced at HgCl(2) concentrations that do not result in obvious morphological defects, making this approach useful for the detection of low levels of oxidative contaminants in aquatic environments.