Coral recruits are highly sensitive to heavy fuel oil exposure both in the presence and absence of UV light

Oil pollution remains a prominent local hazard to coral reefs, but the sensitivity of some coral life stages to oil exposure remains unstudied. Exposure to ultraviolet radiation (UVR), ubiquitous on coral reefs, may significantly increase oil toxicity towards these critical habitat-forming taxa. Here we present the first data on the sensitivity of two distinct post-settlement life stages of the model coral species Acropora millepora to a heavy fuel oil (HFO) water accommodated fraction (WAF) in the absence and presence of UVR. Assessment of lethal and sublethal endpoints indicates that both 1-week-old and 2-month-old recruits (1-wo and 2-mo) were negatively affected by chronic exposures to HFO (7 and 14 days, respectively). Relative growth (1-wo and 2-mo recruits) and survival (1-wo recruits) at end of exposure were the most sensitive endpoints in the absence of UVR, with no effect concentrations (NEC) of 34.3, 5.7 and 29.3 μg L-1 total aromatic hydrocarbons (TAH; ∑39 monocyclic- and polycyclic aromatic hydrocarbons), respectively. On average, UVR increased the negative effects by 10% for affected endpoints, and latent effects of exposure were evident for relative growth and symbiont uptake of recruits. Other sublethal endpoints, including maximum quantum yield and tissue colour score, were unaffected by chronic HFO exposure. A comparison of putative species-specific sensitivity constants for these ecologically relevant endpoints, indicates A. millepora recruits may be as sensitive as the most sensitive species currently included in oil toxicity databases. While the low intensity UVR only significantly increased the negative effects of the oil for one endpoint, the majority of endpoints showed trends towards increased toxicity in the presence of UVR. Therefore, the data presented here further support the standard incorporation of UVR in oil toxicity testing for tropical corals

The effects of ultraviolet radiation and climate on oil toxicity to coral reef organisms – a review

Oil pollution remains a significant local threat to shallow tropical coral reef environments, but the environmental conditions typical of coral reefs are rarely considered in oil toxicity testing and risk assessments. Here we review the effects of three environmental co-factors on petroleum oil toxicity towards coral reef organisms, and show that the impacts of oil pollution on coral reef taxa can be exacerbated by environmental conditions commonly encountered in tropical reef environments. Shallow reefs are routinely exposed to high levels of ultraviolet radiation (UVR), which can substantially increase the toxicity of some oil components through phototoxicity. Exposure to UVR represents the most likely and harmful environmental co-factor reviewed here, leading to an average toxicity increase of 7.2-fold across all tests reviewed. The clear relevance of UVR co-exposure and its strong influence on tropical reef oil toxicity highlights the need to account for UVR as a standard practice in future oil toxicity studies. Indeed, quantifying the influence of UVR on toxic thresholds of oil to coral reef species is essential to develop credible oil spill risk models required for oil extraction developments, shipping management and spill responses in the tropics. The few studies available indicate that co-exposure to elevated temperature and low pH, both within the range of current daily and seasonal fluctuations and/or projected under continued climate change, can increase oil toxicity on average by 3.0- and 1.3-fold, respectively. While all three of the reviewed environmental co-factors have the potential to substantially increase the impacts of oil pollution in shallow reef environments, their simultaneous effects have not been investigated. Assessments of the combined effects of oil pollution, UVR, temperature and low pH will become increasingly important to identify realistic hazard thresholds suitable for future risk assessments over the coming century

Comparative sensitivity of the early life stages of a coral to heavy fuel oil and UV radiation

During an oil spill, shallow, tropical coral reefs are likely to be simultaneously exposed to high intensities of ultraviolet radiation (UVR), which can exacerbate the toxicity of petroleum oils. While successful recruitment of corals is critical for reef recovery following disturbances, the sensitivity of several early life stages of coral to petroleum hydrocarbons has not been investigated, particularly for UVR co-exposure. Here we present the first dataset on the relative sensitivity of three early life stages (gametes, embryos and planula larvae) in a model broadcast spawning coral species, Acropora millepora, to the dissolved fraction of a heavy fuel oil (HFO), both in the absence and presence of UVR. All early life stages were negatively impacted by HFO exposure but exhibited distinct sensitivities. Larval metamorphosis was the most sensitive endpoint assessed with a 10% effect concentration of 34 μg L−1 total aromatic hydrocarbons (TAH) in the absence of UVR. The impact on fertilisation success was highly dependent on sperm density, while the fragmentation of embryos masked embryo mortality. Larval metamorphosis was conclusively the most reliable endpoint for use in risk assessments of the endpoints investigated. Putative critical target lipid body burdens (CTLBBs) were calculated for each life stages, enabling a comparison of their sensitivities against species in the Target Lipid Model (TLM) database. A. millepora had a putative CTLBB of 4.4 μmol g−1 octanol for larval metamorphosis, indicating it is more sensitive than any species currently included in the TLM database. Coexposure to UVR reduced toxicity thresholds by 1.3-fold on average across the investigated life stages and endpoints. This increase in sensitivity in the presence of UVR highlights the need to incorporate UVR co-exposure (where ecologically relevant) when assessing oil toxicity thresholds, otherwise the risks posed by oil spills to shallow coral reefs are likely to be underestimated

Petroleum hydrocarbon ecotoxicology for coral reef risk assessments

Mikaela Nordborg studied the sensitivity of tropical coral to petroleum oil exposure. She found that all assessed life stages of coral were sensitive to oil exposure, that impacts were exacerbated during ultraviolet light co-exposure and that oil toxicity modelling can be a useful tool in tropical oil spill risk assessments

Phototoxic effects of two common marine fuels on the settlement success of the coral Acropora tenuis

Coral reefs are at risk of exposure to petroleum hydrocarbons from shipping spills and uncontrolled discharges during extraction. The toxicity of petroleum hydrocarbons can substantially increase in the presence of ultraviolet radiation (UVR), therefore spills in shallow coral reef environments may be particularly hazardous to reef species. Here we investigated the sensitivity of coral larvae (Acropora tenuis) to dissolved hydrocarbons from heavy fuel oil (HFO) and diesel in the absence and presence of UVR. Larval settlement success decreased with increasing concentrations of dissolved HFO, and co-exposure to UVR doubled the toxicity: 50% effect concentrations (EC50) decreased from 96 (−UVR) to 51 (+UVR) total petroleum aromatic hydrocarbons (TPAH). Toxic thresholds for HFO were similar to concentrations reported during marine spills: EC10s of 24 (−UVR) and 15 (+UVR) µg l−1. While less toxic, diesel also reduced settlement and exhibited phototoxicity: EC10s of 122 (+UVR) and 302 (−UVR) µg l−1. This study demonstrates that the presence of UVR increases the hazard posed by oil pollution to tropical, shallow-water coral reefs. Further research on the effects of oils in the presence of UVR is needed to improve the environmental relevance of risk assessments and ensure appropriate protection for shallow reef environments against oil pollution

Effects of aromatic hydrocarbons and evaluation of oil toxicity modelling for larvae of a tropical coral

Application of oil toxicity modelling for assessing the risk of spills to coral reefs remains uncertain due to a lack of data for key tropical species and environmental conditions. In this study, larvae of the coral Acropora millepora were exposed to six aromatic hydrocarbons individually to generate critical target lipid body burdens (CTLBBs). Larval metamorphosis was inhibited by all six aromatic hydrocarbons, while larval survival was only affected at concentrations >2000 μg L-1. The derived metamorphosis CTLBB of 9.7 μmol g-1 octanol indicates larvae are more sensitive than adult corals, and places A. millepora larvae among the most sensitive organisms in the target lipid model (TLM) databases. Larvae were also more sensitive to anthracene and pyrene when co-exposed to ecologically relevant levels of ultraviolet radiation. The results suggest that the application of the phototoxic TLM would be protective of A. millepora larvae, provided adequate chemical and light data are available