Microplastic-mediated enrichment of polycyclic aromatic hydrocarbons (PAHs) and their toxic effects on coral symbionts: Evidence from oxidative stress and energy metabolic disturbance

Microplastics (MPs) and polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants prevalent in coral reef ecosystems. MPs may exacerbate PAH bioaccumulation and toxicity via a “Trojan horse effect”, yet their combined impacts on coral holobionts remain poorly understood. This study used the scleractinian coral Pocillopora acuta as the model organism, deployed polyethylene microplastics (PE-MPs) in situ in the waters off Xidao Island to assess their PAH enrichment capacity, and subsequently exposed the coral to the resulting contaminated complex to investigate its toxicological effects on the coral-Symbiodiniaceae symbiosis relationship. The total PAH concentrations in seawater near the dock of Xidao Island, Sanya, was 50.65 ng/L, dominated by low-molecular-weight PAHs. PE-MPs incubated in situ for three months showed significantly elevated absorbed PAH levels, particularly anthracene (ACE), phenanthrene (PHE), and pyrene (PYR). Exposure to these PE-MPs carrying PAHs significantly increased PAH accumulation in both coral hosts and their algal symbionts, with higher levels in the algae. PE + PAHs exposure also triggered oxidative stress, energy metabolism disruption, and enhanced apoptosis. Moreover, Pearson correlation analysis showed that cellular energy allocation in corals and Symbiodiniaceae was strongly negatively correlated with ΣPAHs. Strong correlations between host and symbiont responses indicate a coordinated stress mechanism. These findings demonstrate that PE-MPs enhanced the bioavailability and toxicity of PAH, ultimately destabilizing the coral-Symbiodiniaceae symbiosis and posing ecological risks to the coral reef ecosystem. • PE-MPs can effectively adsorb low- and medium-ring PAHs from seawater. • PE-MPs can serve as carriers of PAHs and enhancing their toxicity to the coral-Symbiodiniaceae. • A strong coordinated stress response mechanism exists between coral and Symbiodiniaceae.