Compounds released from reef bleaching promote bacteria, potentially further stressing corals

On healthy reefs around the world, corals, algae, fish and microbes live interconnected and in balance, exchanging nutrients, resources and chemical signals. However, new research led by the University of Hawaii (UH) at Mānoa and the Royal Netherlands Institute for Marine Research (NIOZ) has found that when coral bleaching occurs, corals release unique organic compounds into the surrounding water which not only promotes bacteria. overall growth, but select for opportunistic bacteria that can further stress reefs.

The work appears in Communication biology.

“Our results demonstrate how the impacts of short-term thermal stress and long-term bleaching can extend beyond corals and into the water column,” said co-senior author Wesley Sparagon, a postdoctoral researcher at the UH Mānoa College of Tropical Agriculture and Human Resources and a former doctoral student in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).

The research team, which included scientists from UH Mānoa, NIOZ, Scripps Institution of Oceanography and the University of California, Santa Barbara, conducted experiments on bleached and unbleached corals collected during A bleaching event in Moorea, French Polynesia in 2019.

“Although coral bleaching is a well-documented and increasingly widespread phenomenon in reefs worldwide, there has been relatively little research into the implications for the microbiology and biogeochemistry of the reef water column ” said Craig Nelson, lead author of the study and professor at SOEST. .

In a heating experiment, the team determined that heat-stressed and bleached corals exuded a different composition of organic matter in response to heat stress compared to unbleached coral. These unique compounds nourished the microbial communities in the surrounding waters, causing an increase in their abundance.

“Interestingly, the microbes responding to exudates from bleaching corals were distinct from those grown on exudates from healthy corals,” Sparagon said. “And there was a greater abundance of fast-growing opportunists and potential pathogens. The growth of these microbial communities around stressed corals can harm corals, either through suffocation or by introducing disease.”

The biggest surprise was that this change in the release of coral compounds occurred in corals that experienced stress during the study: corals that had been warmed but not yet bleached, corals that had been both heated and bleached, and corals that had previously bleached in the field. .

“This suggests that this process occurs throughout the coral bleaching period, from the onset of heat stress to recovery,” said NIOZ co-lead author Milou Arts. “Importantly, this is more pronounced in healthy corals under heat stress, suggesting that it is more influential at the onset of heat stress and may push corals toward more severe bleaching, and ultimately a mortality.”

Researchers are now actively working to identify compounds and microbes in the water column that serve as an early warning system for coral reefs under stress. This could strengthen or complement other coral reef conservation efforts, particularly in terms of identifying coral reef stress before catastrophic damage occurs.