Tropical sunlight induces the photo-enhanced toxicity of polycyclic aromatic hydrocarbons from the Brazilian oil spill to Danio rerio early life stages.

Between 2019 and 2020, the Brazilian coast was impacted by ∼5400 tons of weathered, emulsified oil, causing widespread ecological and socioeconomic damage. Marine oil spills pose severe threats to aquatic ecosystems, and toxicity can be amplified by environmental factors such as ultraviolet (UV) radiation. This study investigated the photo-enhanced toxicity on Danio rerio early life stages (ELS) exposed to a water accommodated fraction (WAF) of this oil with concurrent UV exposure from natural sunlight (WAF + UV) compared to exposure protected from sunlight (WAF-UV). Concentration of total polycyclic aromatic hydrocarbons (ΣPAH33) in the WAF-100 % was 32.95 μg-ΣPAH33 L-1. Exposure to WAF + UV increased toxicity 3.5-fold from a lethal concentration to 20 % (LC20) of exposed larvae after 168 h of exposure of 29.1 ± 5.8 μg-ΣPAH33 L-1 (WAF-UV) to 8.4 ± 0.7 μg-ΣPAH33 L-1 (WAF + UV). Developmental delays based on the General Morphology Score (GMS) and developmental abnormalities based on the General Teratology Score (GTS) increased based on a fourfold reduction in lowest observed effect concentrations (LOEC) for GMS and GTS under WAF + UV exposures. Morphometric LOECs for reduced total length and eye area decreased 8-fold and 16-fold, respectively. Frequency of larvae with absent cardiac looping increased four-fold and bradycardia increased after WAF + UV exposure, indicating physiological disruption. Biochemical analyses revealed oxidative stress induction, with increased GST, SOD, and LPO levels, correlating with increased developmental abnormalities. These results demonstrated that translucent ELS fish exposed to oil from the 2019 Brazilian spill and natural sunlight suffer enhanced toxicity, highlighting the need to include phototoxic effects in tropical oil spill risk assessments.