Polycyclic aromatic hydrocarbons (PAHs), consisting of two or more fused benzene rings, are ubiquitous pollutants in aquatic systems. Crude oils typically produce PAH mixtures with high concentrations of 2-, 3-, and 4-ringed PAHs families and correspondingly low levels of ≥5-ring compounds, while combustion of fossil fuels and other carbon sources generate mixtures that may be dominated by larger 4–6 ring compounds. Although industrial PAH emissions have declined in the developed world, loadings to aquatic habitats have increased with human population growth and increased motor vehicle use1,2. Chronic aquatic PAH loading occurs by multiple pathways, including airborne deposition of soot and exhaust particles1,3, runoff from impervious surfaces4, and direct absorption of vapor-phase compounds from the air5. By contrast, catastrophic oil spills such as the 1989 Exxon Valdez tanker grounding6 and the 2010 Deepwater Horizon wellhead blowout released very large quantities of PAHs directly into the marine environment7. Certain heterocyclics contain nitrogen, sulphur, or oxygen, and are more technically termed polycyclic aromatic compounds (PACs). However, following general convention, we refer to heterocyclics here as PAHs.

As expected for a large and complex family of structurally related compounds, the biological activities of PAHs are both overlapping and multifaceted. A major challenge has been determining which individual compounds contribute to the overall toxicity of complex mixtures with different chemical compositions. The carcinogenicity of 4–6-ringed PAHs such as benzo(a)pyrene was recognized more than half a century ago8 and occurs in both terrestrial and aquatic organisms. However, the cardiotoxicity of 3-ringed PAHs was not suspected until the aftermath of the 1989 Exxon Valdez disaster when studies documented developmental defects and mortality in fish embryos exposed to Alaskan North Slope crude oil6. Embryos were found to accumulate waterborne PAHs dissolved from oil, with toxicity increasing as a mixture enriched with 2-ringed compounds (i.e., naphthalenes) shifted with weathering, to one dominated by 3-ringed (tricyclic) subfamilies (fluorenes, dibenzothiophenes, and phenanthrenes)9,10. Subsequent research in aquatic systems in particular has revealed distinct activities for virtually all PAH subfamilies, and even individual compounds within subfamilies. However, mechanistic studies of single compounds often use exposure concentrations that are much higher than those observed in contaminated habitats. More work is needed to understand environmentally relevant complex mixtures.

Studies in zebrafish embryos have demonstrated cardiotoxic effects of many single PAH compounds and PAH mixtures, but also revealed a minimal binary division of mechanisms. PAHs that are strong carcinogens are potent agonists of the aryl hydrocarbon receptor (AHR), inducing their own metabolism and carcinogenic activation by cytochrome P4501A8. In general, these PAHs are also cardiotoxic to developing fish embryos at relatively high concentrations. Similar to dioxins and polychlorinated biphenyls11,12, these PAHs inappropriately activate the AHR in developing cardiomyocytes, leading to primary defects in cardiac morphogenesis (poor chamber looping and reduced cardiomyocyte proliferation) followed by secondary functional defects. This form of toxicity is entirely dependent on the AHR and is prevented by AHR gene knockdown. This has been demonstrated, for example, for 4- and 6-ring compounds such as benz(a)anthracene and benzo(a)pyrene13,14,15,16, and a C4-alkylated 3-ring compound, retene17. In contrast, exposure to either complex PAH mixtures derived from crude oil or single tricyclic compounds that dominate in these mixtures leads to cardiac function defects, followed by secondary morphological defects. Indeed, embryos of many fish species exposed to crude oil-derived PAHs show functional abnormalities that include bradycardia and arrhythmias characteristic of atrioventricular conduction block as well as reduced ventricular contractility18,19,20,21,22. Importantly, the cardiotoxicity of both crude oil and single non-alkylated tricyclic PAHs occurs without activation of the AHR in cardiomyocytes18,19,20,21,22, and is not prevented by AHR gene knockdown11. Moreover, the cardiotoxicity of crude oil is exacerbated by knockdown of cyp1a23, indicating that CYP1A-mediated metabolism of crude oil-derived PAHs is protective. Fractionation-based toxicity assays also linked the developmental cardiotoxicity of petroleum products to tricyclic PAH families15, as well as a limited number of studies in adult zebrafish [e.g., ref. 24]. Collectively, this work supports the existence of an AHR-independent mechanism by which crude oil-derived PAHs induce cardiac arrhythmia and reduce cardiomyocyte contractility.

We have recently demonstrated that complex PAH mixtures from crude oil affect excitation-contraction (EC) coupling in fish hearts25. EC coupling is the physiological process that links electrical excitation with contraction in a cardiomyocyte26,27. In fishes of the family Scombridae (e.g. mackerels, tunas28,29), as in mammals26, EC coupling begins when an action potential (AP) depolarizes the surface membrane of the cardiomyocyte, and opens voltage-gated ion channels, allowing calcium (Ca2+) entry into the cell through L-type Ca2+ channels (I CaL ). This extracellular Ca2+ influx triggers the release of additional Ca2+ from the internal Ca2+ stores of the sarcoplasmic reticulum (SR) via a process termed ‘Ca2+-induced Ca2+-release’ (CICR). The consequent systolic Ca2+ transient, which activates the contractile machinery within the heart muscle cells, is the spatial and temporal sum of such local Ca2+ releases30,31. Relaxation occurs when Ca2+ is returned to resting levels by reuptake into the SR via the SR Ca2+ ATPase (SERCA) and extrusion from the cell via the sodium calcium exchanger (NCX). Critical for AP repolarization are the opening and closing of voltage-gated sodium (Na+), Ca2+ and potassium (K+) channels, which renew the EC coupling process at every heartbeat. Our earlier work25 revealed crude oils disrupt these EC coupling pathways in scombrid fish cardiomyocytes, which explains the bradycardia and arrhythmia previously observed in the whole-heart.

Despite evidence that the tricyclic PAH fraction causes the crude oil heart failure syndrome in developing fish20, a direct link between an individual tricyclic PAH and the disruption of EC coupling has not been established. The present work set out to define the molecular moiety(s) of cardiotoxic PAHs from crude oil in three scombrid fishes: the Pacific mackerel (Scomber japonicas), the yellowfin tuna (Thunnus albacares) and the Pacific bluefin tuna (Thunnis orientalis). We then reveal the mechanism for this disruption in atrial and ventricular myocytes from these pelagic predators.