Key points:

• Abnormal ratios of seawater nutrients reduce a coral’s resistance to infection.
• Opportunistic microbes take over its microbiome and cause disease.
• Diseases can emerge from within the coral, not just externally.
• Heat stress remains important, but scientists think skewed nutrient ratios are key to the development of the so-called ‘Black Band Disease’.

Scientists led by the University of Southampton have revealed that an imbalance of nutrients in seawater can cause coral disease – possibly to a greater extent than that from heat stress of warming oceans.

New research conducted at Southampton’s Coral Reef Laboratory, and with colleagues at the University of Derby, shows disruption of the delicate nutrient balance of the sea can destabilise microbial communities that live in harmony with corals, triggering disease.

Devastating outbreaks include the common and highly destructive Black Band Disease (BBD) which moves across the coral surface, killing coral tissue, and leaving behind just bare skeleton.

Although not the only cause of BBD, the team found 88 percent of recorded incidents of the disease occurred in regions with highly imbalanced seawater nutrient ratios.

Findings of the new study are due for publication in the journal Nature Communications.

Corals are a group of unique animals, which live in symbiosis with photosynthetic microalgae. Together they form three-dimensional structures known as coral reefs, which about 25 percent of all marine biodiversity depends on.

Coral reefs in tropical seas are under mounting threat from climate change. Along with rising sea temperatures, coastal developments, overfishing and pollution from agriculture and industry also remain consistent threats to their future.

The hidden disease trigger

Led by Associate Professor Cecilia D’Angelo of the University of Southampton, the team demonstrated that skewed ratios of seawater nutrients, such as an imbalance of nitrogen and phosphorus, disrupt the delicate microbial communities living with corals. These microbes, both in and on corals, are collectively called the ‘coral microbiome’.

“When nutrients are out of balance, the interactions between members of the coral microbiome begin to break down,” says Dr D’Angelo. “This creates spaces for opportunistic microbes to take over and cause disease.”

Using controlled laboratory experiments in the Coral Reef Laboratory at the University of Southampton, the researchers demonstrated that imbalances of nitrate and phosphate can promote the formation of disease lesions similar to those of Black Band Disease. Importantly, the microbial communities that caused disease in the lab, not only visually resemble their real-life counterparts on reefs, they also contain related microbe species with an equally destructive effect on corals.

Microbial networks collapse under stress

Healthy corals rely on their complex microbial networks to help maintain stability and resist infection. The study found that nutrient imbalance in the seawater fragments these networks in the laboratory corals, reducing their connectivity and resilience.

As networks break down, opportunistic microbes, in particular dark-coloured, photosynthetic ones called ‘cyanobacteria’, rapidly increase in abundance. These form disease-causing ‘microbial mats’ – dense, web-like structures that cover the coral tissue. Secondary pathogens benefit from this and join the disease communities to intensify tissue damage and cause the mats to spread across the coral surface.

Dr Raphaela Gracie from the University of Southampton, a postdoctoral researcher in the team and first author on the paper, explains: “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.”

“Therefore, our research reframes a key coral disease as a micro-ecological imbalance as opposed to a simple pathogenic invasion,” adds Dr D’Angelo. “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.”

Implications for coral reef conservation

To understand the broader relevance of their findings, the researchers analysed global records of BBD outbreaks between 2000 and 2023. They found that over 88 percent occurred in regions with highly imbalanced nutrient ratios, whereas only 16 percent were found in reefs that were recently exposed to heat stress.

“Our results show that a vast majority of BBD outbreaks occur in reefs exposed to chronic nutrient imbalance, indicating that water quality management could be a crucial tool for mitigating reef coral diseases in the future,” says Dr Raphaela Gracie.

However, warming oceans still remain a most severe concern for coral reef survival, as anomalously high temperatures can cause fatal coral bleaching, a major driver of coral reef decline. Furthermore, rising seawater temperatures may shift the nutrient balance in a way that promotes BBD.

Nutrient imbalances can also be caused by human activities including agricultural runoff and wastewater discharge. Reducing such disturbances and managing nutrient levels at a local scale could help to stabilise coral microbiomes and prevent disease.

Cecilia D’Angelo, whose work is supported by a Research Leadership Award from the Leverhulme Trust, concludes: “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,” she continues, “Restoring this balance in areas affected by human activities has the potential to reduce disease risk at the local scale.”

Ends