Safer solutions for sea: Montipora capricornis coral responses to naturally-derived versus copper-based antifouling agents.

While conventional synthetic antifouling agents (AFs) threaten coral reef ecosystems, the environmental safety profiles of naturally-derived AFs remain inadequately studied. To this end, a synthetic AF Cu2O, and 7 naturally-derived ones (7-Amino-4-methylcoumarin (AMC), butenolide, ε-polylysine (EPL), camptothecin (CPT), eugenol, gramine, and BAF-1) were chosen for a comparative study to reveal the physiological responses of coral Montipora capricornis. The selected AF concentration was 50 μg/L, referenced from Cu2O bleaching-inducing concentration. After 15 days, Cu2O demonstrated the most substantial adverse effects: tissue loss and bleaching severity, zooxanthellae density reduction, chlorophyll a (Chl a) content decline, and antioxidant enzyme activity alteration; while butenolide had the least significant effects; and the rest AFs showed intermediate stress responses. Further proteomic landscapes focusing on Cu2O and butenolide revealed that Cu2O had intensely downregulated proteins involved in photosynthesis, the Calvin cycle, glycolysis, the tricarboxylic acid (TCA) cycle, and calcification in both symbiont and coral host. Notably, butenolide had no effect on calcification-related proteins. This integrative proteomics-physiological approach represents the first simultaneous assessment of multiple naturally-derived antifoulants on coral holobiont responses, revealing distinct mechanisms of toxicity divergence between metallic and non-metallic compounds. Notably, butenolide preferentially suppresses energy metabolism while sparing calcification-related proteins, suggesting fundamentally different toxicological pathways than Cu-based alternatives.