Coral reefs are facing no shortage of threats right now, from bleaching and the impact of warming seas to pollution and the decimation of the biodiversity that lives among them. But what if there was one threat in particular that had been hiding in plain sight?

Research led by the University of Southampton has found that an imbalance of nutrients in seawater – rather than heat stress alone – may be a primary driver of coral diseases, with implications that could reshape how conservationists approach reef management worldwide.

The findings – due for publication in Nature Communications – centre on Black Band Disease (BBD), one of the most destructive coral diseases known to science. A dark, advancing mat of microorganisms, BBD moves steadily across coral surfaces, consuming live tissue and leaving bare skeleton in its wake. The new study found that 88% of recorded BBD outbreaks between 2000 and 2023 occurred in regions with highly imbalanced seawater nutrient ratios – compared to just 16% in reefs recently exposed to heat stress.

The research was conducted at Southampton’s Coral Reef Laboratory, in collaboration with colleagues at the University of Derby. Using controlled experiments, the team demonstrated that skewed ratios of nitrate and phosphate destabilise the coral microbiome – the complex community of bacteria, fungi and other microorganisms that live in and on coral tissue, and which play a critical role in keeping corals healthy.

As that community fragments under nutrient stress, opportunistic microbes move in to fill the void.

Cyanobacteria – dark-coloured, photosynthetic microorganisms – were among the first to capitalise on the disruption, rapidly multiplying to form dense microbial mats across the coral surface. Secondary pathogens follow, intensifying tissue damage and driving the characteristic spread of the disease.

“Strikingly, many of the microbes responsible for the disease were already present in healthy coral tissue before symptoms appeared – highlighting that this disease can emerge from within the organism itself, rather than from external infection,” said Dr Raphaela Gracie, a postdoctoral researcher at the University of Southampton and first author on the paper. 

“Therefore, our research reframes a key coral disease as a micro-ecological imbalance as opposed to a simple pathogenic invasion. This follows similar principles as opportunistic diseases in humans, for instance fungal infections that follow on from the disturbance of the natural human microbiome by antibiotic treatments.”

Just as disrupting the human gut microbiome can leave the body vulnerable to opportunistic infections, so too can a chemically stressed coral become susceptible to disease from within its own microbial population.

Agricultural runoff, wastewater discharge and coastal development all alter the ratio of nitrogen to phosphorus in nearshore waters, placing reefs in precisely the conditions the study identifies as dangerous. Crucially, warming ocean temperatures may themselves shift nutrient balances in ways that further promote BBD – meaning climate change and water quality degradation could act to accelerate reef decline.

The research was led by Associate Professor Cecilia D’Angelo, whose work is supported by a Research Leadership Award from the Leverhulme Trust. 

“Our results show that it’s not just how much nutrients are in the water, but that the balance between nitrogen and phosphorus needs to be considered as well,” says D’Angelo. “Restoring this balance in areas affected by human activities has the potential to reduce disease risk at the local scale.”

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