Physiological impairment and metabolic perturbations in Porites cylindrica induced by Hypnea pannosa contact.

Coral reef ecosystems are undergoing accelerated degradation, characterized by phase shifts marked by declining live coral cover and increasing macroalgal proliferation. However, the long-term metabolic adjustments of corals under chronic macroalgal competition remain poorly understood, particularly regarding the integration of physiological changes with metabolic responses resilience mechanisms. This study conducted a 21-day experiment to investigate the physiological and metabolomic response between coral Porites cylindrica and macroalgae Hypnea pannosa. Key findings revealed that direct physical contact with H. pannosa induced severe oxidative stress in P. cylindrica, manifested by a 19.06 % increase in malondialdehyde (MDA) levels, a 12.26 % reduction in Fv/Fm, an 8.65 % decrease in endosymbiotic microalgal density, a 66.73 % diminishment in chlorophyll a content, and depletion of lipid, carbohydrate, and protein reserves by 44.21 %, 7.91 %, and 12.03 %, respectively. Metabolomic profiling identified 81 upregulated and 114 downregulated metabolites. KEGG pathway enrichment analysis demonstrated that these differentially expressed metabolites were predominantly associated with lipid-mediated defense mechanisms, including arachidonic acid metabolism and unsaturated fatty acid biosynthesis pathways. These mechanistic insights advance our understanding of coral-macroalgal competitive dynamics and provide critical theoretical foundations for formulating targeted reef restoration strategies.