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Coral bleaching releases bacteria that put further strain on reefs

A study by the University of Hawai‘i (UH) at Mānoa and the Royal Netherlands Institute for Sea Research (NIOZ) has shed new light on the detrimental effects of coral bleaching on reef ecosystems. 

The collaborative research, which also involved contributions from the Scripps Institution of Oceanography and the University of California, Santa Barbara, has unveiled that during bleaching events, corals release specific organic compounds into their environment. These compounds not only foster an increase in bacterial populations but also encourage the growth of opportunistic bacteria, potentially exacerbating the stress on coral reefs.

Extensive reach of coral bleaching

“Our results demonstrate how the impacts of both short-term thermal stress and long-term bleaching may extend beyond coral and into the water column,” said co-lead author Wesley Sparagon, a postdoctoral researcher at UH Mānoa. 

This insight underscores the extensive reach of coral bleaching, affecting not just the corals but the surrounding aquatic environment as well.

Reef water column microbiology

The research, initiated in response to a bleaching event in French Polynesia in 2019, aimed to explore the lesser-known consequences of coral bleaching on reef water column microbiology and biogeochemistry. 

“Although coral bleaching is a well-documented and increasingly widespread phenomenon in reefs across the globe, there has been relatively little research on the implications for reef water column microbiology and biogeochemistry,” explained senior author Craig Nelson, a professor of Earth Sciences at UH.

Potential pathogens around stressed corals

Through meticulous heating experiments with both bleached and unbleached corals, the team discovered that the composition of organic matter released by corals under thermal stress differs significantly from that of healthy corals. These unique compounds serve as nourishment for microbial communities in the water, leading to a notable increase in microbial abundance.

“Interestingly, the microbes responding to bleaching coral exudates were distinct from those grown on healthy coral exudates,” said Sparagon. This shift towards fast-growing opportunists and potential pathogens surrounding stressed corals poses additional threats, such as suffocation or disease introduction, which could further compromise coral health.

Universal compound shift in stressed corals

A particularly striking finding of the study was the universal occurrence of this compound shift in corals subjected to any form of stress during the experiment. According to co-lead author Milou Arts, an expert in microbiology and biogeochemistry at NIOZ, this suggests that the process occurs throughout the period of coral bleaching, from onset of thermal stress all the way through recovery.

The implications of these findings are profound, indicating that the initial stages of thermal stress are critical and could lead corals towards more severe bleaching and, ultimately, mortality.

Early warning system

The researchers are now focusing on pinpointing specific compounds and microbes in the water column that could signal impending stress on coral reefs. Such an early-warning system holds promise for enhancing coral reef conservation strategies, allowing for timely intervention before irreparable damage occurs. 

Therefore, this study not only contributes significantly to our understanding of coral reef ecosystems but also highlights the urgent need for integrated conservation efforts to safeguard these vital marine habitats against the escalating threats of climate change.

More about coral bleaching

Coral bleaching is a phenomenon that occurs when corals lose their vibrant colors and turn white. This happens because corals have a symbiotic relationship with tiny algae called zooxanthellae, which live inside their tissues. 

Stressful conditions 

The algae provide corals with food through photosynthesis and are responsible for their colorful appearance. However, when corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the algae, leading to a loss of color. This stress is most often associated with increased sea temperatures due to global warming.


Coral bleaching does not mean the coral is dead, but it does indicate a coral under stress, which can lead to decreased growth, reduced reproductive capacity, increased susceptibility to diseases, and, if the stressful conditions persist, death. Bleaching events are becoming more frequent and severe with climate change, posing a significant threat to coral reefs worldwide. 

Valuable ecosystems 

These ecosystems are crucial for marine life, providing habitat, food, and protection for a diverse range of species. Coral reefs also offer important benefits to humans, including coastal protection, sources of food, and income from tourism and fishing. 

The increasing occurrence of coral bleaching highlights the need for global efforts to reduce greenhouse gas emissions and protect marine biodiversity.

The study is published in the journal Communications Biology.

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