Breakdown of microbial networks links nutrient stress and reef coral disease.

Coral diseases are increasing in prevalence, accelerating the global decline of tropical reefs, which threatens over 25% of marine biodiversity and vital ecosystem services for human societies. While outbreaks are frequently linked to environmental change, including heat stress, sedimentation, and reduced water quality, the mechanisms by which such factors promote disease remain poorly understood. Here we show that nutrient stress, caused by skewed seawater nitrogen-to-phosphorus (N:P) stoichiometry, promotes the onset of Black Band Disease (BBD), a common and easily recognisable syndrome that affects corals around the globe. Using Turbinaria reniformis as a model system, controlled laboratory experiments demonstrate that skewed N:P ratios disrupt the functional integrity of coral-associated microbial networks while favouring opportunists that exploit dysfunctional host-symbiont interactions. Disease lesion-associated microbial mats are dominated by cyanobacteria and include sulphur-metabolising bacteria, hallmarks of natural BBD communities. Strikingly, similar cyanobacterial taxa are also detected in the visually healthy coral tissue ahead of the expanding lesions, suggesting an opportunistic recruitment of disease-associated members from the resident microbiome. Global analyses of BBD outbreaks reveal that over 88% occurred in regions with skewed N:P ratios, compared with only 16% that were linked to prior heat stress. Together, our findings identify nutrient-driven microbiome destabilisation as a key pathway to coral disease, reinforcing nutrient management as a major lever for reef conservation and restoration practice.