Coral-Symbiodiniaceae symbiotic associations under antibiotic stress: Accumulation patterns and potential physiological effects in a natural reef

Antibiotics threaten scleractinian corals, but their accumulation patterns and physiological effects on corals in natural reefs remain unclear. This study investigated antibiotic occurrence in seawater and two coral species, Galaxea fascicularis and Pocillopora damicornis, and explored the physiological effects of bioaccumulated antibiotics in a fringing reef of the South China Sea. Nineteen antibiotic components were detected in seawater, with total antibiotic concentrations (ΣABs) ranging from 17.69 to 44.22 ng L⁻¹ . Eleven antibiotic components were accumulated in the coral hosts, and five components were observed in their algal symbionts. Higher ΣABs were significantly associated with increased total antioxidant capacity in the coral hosts of P. damicornis, while G. fascicularis exhibited a significant increase in algal symbiont density. Furthermore, ofloxacin was linked to increased algal symbiont density of G. fascicularis, while several antibiotic components, including tilmicosin, sulfapyridine, ofloxacin, and lincomycin hydrochloride, were observed to reduce antioxidant levels in the algal symbionts of G. fascicularis. No significant correlations between antibiotic components and physiological activities were detected in P. damicornis. These results highlight species-specific bioaccumulation patterns and physiological responses to antibiotics, suggesting that prolonged contaminations could destabilize coral-Symbiodiniaceae symbiosis. The findings improve understanding of the ecological risks of antibiotic pollution in reefs.