Proteomic insights into the combined effects of near-threshold bleaching temperature and prometryn exposure on the coral Galaxea fascicularis.

Ocean warming events increasingly co-occur with coastal agrochemical pollution, yet their joint effects on coral holobiont molecular responses remain poorly resolved. We exposed the reef-building coral Galaxea fascicularis to near-threshold warming (30 °C), prometryn (1 μg L-1), and their combination for 4 days, and quantified coral host and Symbiodiniaceae protein abundance using TMT-based proteomics to capture early-stage molecular responses. Heat was the primary driver of proteome shifts, consistent with suppressed biosynthesis and reallocation toward maintenance, whereas prometryn imposed additional changes on specific pathways under heat. Under combined stress, symbionts showed a mismatch in which light harvesting increased without a proportional rise in reaction-center/electron-transport capacity, which may elevate excitation pressure and oxidative stress risk. Consistent with this interpretation, photophysiological measurements showed that light-adapted PSII parameters were more responsive than dark-adapted maximum photosystem II quantum yields (Fv/Fm) during acute exposure, with the combined treatment showing modest but the clearest decline in photochemical performance. In parallel, host stress-associated signaling pathways were enriched, while antioxidant/detoxification functions were reduced in both partners. Together, these proteomic patterns suggest that prometryn exposure may compromise glutathione-based redox buffering capacity, thereby weakening the molecular response of a relatively heat-tolerant coral to near-threshold warming. The present study provides valuable insights into the critical need to prioritize the reduction of PSII herbicide pollution as part of coral reef protection strategies. Our findings support management efforts to reduce coastal PSII herbicide inputs as a local lever that could mitigate bleaching susceptibility under warming.