Polar cod, lipid metabolism and disruption by polycyclic aromatic hydrocarbons

Past experience has shown that releases of crude oil can have important long-term ecosystem effects at the regional level, leading to significant economic consequences through the loss of ecosystem services. In ice-covered environments, oil recovery is considered exceptionally challenging because the remoteness of the Arctic renders detection and access to the impacted area difficult, especially during the polar night. Spilled oil can easily become encapsulated into the ice and released in the following melt seasons when it may be widely distributed over larger areas; this will potentially affect ecosystems during periods of high productivity and at biological hotspots. In addition, our increasing understanding of ecosystem processes during the polar night shows a system with high activity levels and biological interactions across most trophic levels that may be more vulnerable than previously assumed. Weathering processes in the Arctic, and in particular those of crude oil encapsulated in sea-ice, are significantly prolonged, thereby also increasing the time of exposure to marine organisms. Water-soluble hydrocarbons within brine channels in the sea ice can also reach concentrations that are toxic to ice-associated organisms and they can be released over several months, potentially contaminating food webs. Polar cod (boreogadus saida) is the most abundant pan-Arctic fish species, and it sustains the majority of other species that comprise higher trophic levels. Populations of this small gadid are at risk of experiencing significant changes in ecosystem interactions and alterations in their life cycle strategies in regions of enhanced warming. Additional impacts from anthropogenic pollution, and in particular accidental oil spills, may accelerate the decline of this key species and thereby its central role in the Arctic food web. The present project provided novel insights into the sensitivity of both adult polar cod and their early life stages to crude oil exposure. Through a total of 11 experimental studies carried out over the 4-years project period, from short (days) exposures of in vitro systems to medium (weeks) and long-term (months) in vivo exposures of polar cod, we provide new knowledge related to 1) the toxicokinetics of polycyclic aromatic hydrocarbons (PAHs) in dietary exposed polar cod (WP1), 2) some pathways of toxicity (WP2) and 3) the sensitivity of adults and early life stages to exposure to dietary and waterborne crude oil, respectively (WP2/WP3). The project further allowed developing in vitro methodologies to further investigate toxic responses and their pathways (WP2). Finally, our current ecological knowledge of polar cod has been widely advanced in the past years, both within the present project, but also internationally, and 17 new ecological publications on polar cod life cycle strategies, population genetics, habitat preferences and role in the food web are compiled in a special issue, to be published in Polar Biology in June 2016 (WP4). Within POLARISATION, some studies were designed to study effects and toxicity pathways at molecular and cellular levels, using both short- and long-term exposure to the model compound benzo(a)pyrene, known for its toxicity. Other studies were designed to provide environmentally realistic conditions, i.e. dietary exposure of adult polar cod found in the mesopelagic layer, and waterborne exposure to early life stages found in the epipelagic layer. Concentrations of crude oil or related compounds used in these studies were thus representative of levels in water or food that may be expected post-oil spill. Among all results obtained within the project, the key findings include differential body distribution of different PAH compounds but no significant effects of different temperatures (0 and 6°C) on their toxicokinetics (WP1). Also, depending on exposure doses, differential toxicity pathways seemed to be activated in polar cod (WP2, part 1). At environmentally realistic levels of crude oil dietary exposure, lipid homeostasis seemed to be altered, at least to some degree at the physiological level, although clear effects were not observed at transcription levels (WP2, part 2). Mechanisms of action need to be further investigated, but preliminary data suggests that polar cod responds differently than other fish species (WP2, part 2). Long-term exposure to low levels of dietary crude oil altered sperm quality in maturing males (WP3 part 1), but no effects were observed for plasma levels in sex steroid hormones or oocyte maturation. Pelagic embryos and larvae were, on the contrary, highly sensitive to extremely low levels of water-soluble fraction of crude oil, showed significant malformations and reduced grows (WP3 part 2). Additional work should evaluate the consequences of these observed effects on later developmental stages, as well as overall population sensitivity under various oil spill and climate scenarios.

The polar cod is considered a key species in the arctic marine food web. In the highly seasonal Arctic environment, polar cod is relying on the accumulation of lipid reserves during the short productive season for overwintering and reproduction during the polar night. No studies have reported on eventual disruptions of the lipid metabolism by anthropogenic contaminants and their mode of actions. There is, however, evidence that polycyclic aromatic hydrocarbons (PAHs) may disrupt the lipid metabolism and l ead to long-term biological effects such as reduced growth. The exact mechanisms involved are, however, far from understood. Furthermore, there is an obvious gap in knowledge regarding the bioavailability of ingested PAHs in polar cod, including the facto rs influencing the uptake of chemicals and their fate within the organism. Therefore, this project aims at studying the importance of exogenous and endogenous factors affecting the uptake of chemicals and tissue distribution in polar cod by using advanced methods in chemistry and toxicokinetics (WP1). Furthermore, the role of the nuclear receptor PPAR in the disruption of lipid metabolism will be studied (WP2) through in vivo experiments and the use of advanced molecular and toxicological methods. In vitr o cell cultures will be developed to investigate basic mechanisms of actions and the implication of nuclear receptors in the toxic response in polar cod. The long-term biological consequences of a PAH exposure including growth, reproduction and tumorigene sis, will be investigated in polar cod exposed 6 to 8 months to realistic doses of a mixture of PAHs using bioenergetics and histopathology techniques (WP3). In order to give the project a holistic perspective, annual workshops will be organized for shari ng knowledge on polar cod with the project participants and invited speakers. Finally, a special issue will be published that will provide a comprehensive review on all existing knowledge on polar cod (WP4).