Toxicity pathway-oriented screening of mixtures of organic pollutants in marine wildlife

Abstract

The role of persistent organic pollutants (POPs) in marine wildlife health is an issue of continuous interest worldwide. Dugongs and marine turtles, two iconic species contributing to the unique diversity of the Australian coastal environment, are both experiencing multiple threats of natural and anthropogenic origins. These animals typically feed in coastal areas where release of POPs occurs from urban, agricultural and industrial activities. Given the high fat content of dugongs and a long life span for both species, these animals can accumulate relatively high amounts of POPs. While certain POP groups (such as polychlorinated dibenzo-p-dioxins/furans, PCDD/Fs and polychlorinated biphenyls, PCBs) in these animals can be present at elevated levels where adverse effects may occur, dugongs and turtles are in reality exposed to an even broader spectrum of chemicals, and the combined effect of such complex, unresolved mixtures remains largely unknown. Investigations on mixtures in wildlife are often hindered by conventional exhaustive solvent extraction methods, which require select solvent use and extensive clean-up to remove the complex matrices and therefore can only cover a limited spectrum of chemicals. Another challenge lies in the choice of representative biological endpoints to explore the toxicological relevance of accumulated chemical burdens and quantitatively link chemical exposure and mixture effect. In light of these limitations, this thesis aimed to deliver a methodological framework to link chemical exposure to mixture effects for typical biological sample types. More specifically, the goal was to validate and apply polymer-based passive sampling strategies suitable for dosing into in vitro bioassays with endpoints of relevance to both chemical exposure and wildlife health. Lipid-rich tissue (blubber) of dugongs and blood of green turtles were used for the method development but the methods should be readily transferable to other species. A simple passive sampling technique using polydimethylsiloxane (PDMS) polymer was validated for quantitative extraction of an entire range of neutral organic chemicals from lipid-rich tissue or blood based on equilibrium partitioning between PDMS and major sorptive phases therein (e.g., lipid and proteins). For lipid-rich tissue, chemicals reached equilibrium between PDMS and lipid within 24 h. Governed by a generally constant lipid-PDMS partition coefficient (Klip-PDMS) of 30 gPDMS glip-1 (95% CI: 18-57 Lblood kgPDMS-1), neutral organic chemicals across a wide hydrophobicity range diffuse proportionally from lipid into PDMS. For blood, a quantitative PDMS-blood partitioning system was designed to enrich neutral organic chemicals into PDMS from blood. The sampling kinetics of PCDDs into PDMS was reasonably fast, with an estimated equilibration time of <96 h. The measured PDMS-blood partition coefficient (KPDMS-blood) for a range of PCDDs, PCBs and PBDEs was generally constant with a geometric mean of 14 Lblood kgPDMS-1 (95% CI: 8.4-29 Lblood kgPDMS-1). The constant Klip-PDMS or KPDMS-blood allows the conversion of effect-based chemical burden in PDMS detected by bioassays into lipid-normalized chemical burden in blubber or blood samples. The validated passive sampling techniques were applied to 34 blubber samples of dugongs stranded along the Queensland coast over the last decade and 15 blood samples of green turtles collected from Gladstone, Queensland. Passive sampling extracts of dugong blubber were applied to a range of reporter gene assays indicative of cellular pathways mediated by transcriptional regulators, ranging from induction of metabolism to adaptive stress response. Induction of arylhydrocarbon receptor (AhR) and Nrf2-mediated oxidative stress response were the dominant modes of action, while p53-mediated DNA damage response and NFκB-mediated response to inflammation were of low relevance in both individual chemical profiling and dugong blubber extract screening. A quantitative link was found between the potency of AhR active chemicals, such as PCDDs, PCBs and polyaromatic hydrocarbons (PAHs), and their respective potency to induce the oxidative stress response pathway. While analytically quantified PCDDs could account for the entire AhR-mediated activity, less than 5% of the total oxidative stress response could be explained by PCDDs. An increase or decrease in oxidative stress response was observed with individual chemicals and blubber extracts subject to metabolic activation by rat liver S9 fraction, suggesting the utility of incorporating metabolic enzymes into in vitro bioassays. Similar to what was observed with dugongs, target chemical analysis of PCDD/Fs, dioxin-like PCBs, polybrominated diphenyl ethers (PBDEs) and organochlorine pesticides showed that identified chemicals contributed to the majority of the AhR-mediated activity and less than 0.4% of Nrf2-mediated response. In conclusion, this thesis combined efficient passive sampling techniques to extract chemicals in biological samples with in vitro bioanalytical tools for quantitative screening of total toxicologically relevant chemical burdens in marine wildlife. These innovative tools will enable high throughput screening of biota samples, thus facilitating our future efforts in understanding toxicological implications of wildlife exposure to complex mixtures of POPs. The data generated will inform risk assessment of chemical mixtures and contribute to mitigation strategies for wildlife conservation.