Uptake dynamics and effects of traditional and emerging persistent environmental pollutants to the Antarctic keystone species, Antarctic krill

Abstract

The schooling Euphausiid, Antarctic krill (Euphausia superba) is considered a keystone species in the Southern Ocean due to its abundance, prominent role in nutrient cycling, and dependency of almost all Antarctic predatory species on this single species. It has often been postulated that polar species may be more sensitive to anthropogenic contaminants on account of their evolutionary isolation from exposure. Despite geographical isolation, anthropogenic contaminants have frequently been detected in the Southern Ocean and Antarctica biota, including, heavy metals, petroleum products, persistent organics pollutants (POPs) as well as microplastic (plastics <5 mm diameter) marine debris. This thesis examined the response of Antarctic krill exposed to two common pollutants; the POP compound para, para-Dichlorodiphenyldichloroethylene (p,p’-DDE), a metabolic by-product of the pesticide Dichlorodiphenyltrichloroethane (DDT); and polyethylene (PE) microplastics. To investigate the fate of microplastics ingested by a grazing crustacean of high dietary plasticity, commercial microplastic PE beads were offered to Antarctic krill as a proportion of their diet. Krill were exposed to either a low (20%) or high (80%) dose plastic diet for four days, after which the faecal pellets and internal body burdens were examined. Antarctic krill were found to mechanically alter ingested microplastic beads into irregular fragments and nanoplastics. The capacity for fragmentation was found to be dependent on the concentration ingested. Further, the krill displayed size dependant depuration of the altered beads. This is the first time pristine microbeads have been noted to be physically altered by ingestion. With regards to the fate of commercial PE beads ingested by Antarctic krill, it appears that larger microplastics are fragmented into pieces that are small enough to cross biological barriers after ingestion, or are egested as a mixture of irregular triturated particles. These findings suggest that the current literature, based on observations from laboratory-based feeding studies, may be oversimplifying the way in which zooplankton interact with microplastics. To investigate the uptake and depuration kinetics, bioaccumulation potential and detrimental health effects of irregular triturated microplastics Antarctic krill were exposed to commercial microplastic PE beads in a range of concentrations (10, 20, 40 and 80% plastic mixed diet). Toxicological endpoints of mortality and weight loss were both found to be non-sensitive for acute exposure in Antarctic krill. The depuration of particles large enough to be detected was found to be extremely fast, with krill eliminating 80% of their accumulated body burden in a matter of hours. Effective depuration was proposed to be the primary mechanism for mitigation of bioaccumulation and toxicity in krill, with bioaccumulation over 10 days of exposure found to be negligible. However, as uptake rates were similarly fast, and organisms in the marine environment are unlikely to experience microplastic free conditions to depurate their accumulated burden, chronic exposure over the lifetime of the organism with a continuous, yet variable, uptake and egestion is suggested to be a more likely scenario. Further, particles small enough to cross biological barriers were not quantified in this study, and the bioaccumulation potential of these particles remains to be evaluated. To investigate sublethal p,p’-DDE exposure and identify potential biomarkers of sub cellular damage Antarctic krill were aqueously exposed to five treatments (1, 5, 10, 15 and 20 μg L-1) of p,p’-DDE. The response of enzymes with known roles in exogenous compound metabolism (glutathione S-transferase, GST and cytochrome P450 2B, CYP2B), neurotoxicity (acetylcholinesterase, AChE) and oxidative stress (glutathione peroxidase GPx) were quantified. CYP2B was not detectable in Antarctic krill. None of the enzymes detected induced linear concentration-dependant responses. GST was elevated at all exposure concentrations compared to the control, however no treatments were significantly different. GPx and GST followed similar trends throughout the treatment responses suggesting that an underlying biological factor may be influencing both enzymes. AChE activity was not correlated with p,p’-DDE exposure concentration, but all concentrations displayed lower activity than the solvent control however no treatment was significantly inhibited by p,p’-DDE in Antarctic krill. These findings did not provide evidence for an activated detoxification response to p,p’-DDE via the targeted biochemical pathways in Antarctic krill. These findings provide an important baseline for future work to establish the mechanisms of organochlorine toxicity and further our understanding of Antarctic krill detoxification capabilities. Overall, the results of the work described in this thesis characterise several key interactions between anthropogenic pollutants and Antarctic krill. This work provides a comprehensive foundation for further investigation on the combined effect of p,p’-DDE and polyethylene microplastic stressors, as well as other microplastics and POPs in isolation and as mixtures, which may better reflect environmental conditions.