Coral die-off in restoration project tied to low levels of beneficial bacteria in microbiome
by University of GuamGaby Clark
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Corals are the architects of reefs throughout tropical seas, providing protection and sustenance for the species that depend on them. Coral reefs protect coastlines from storm surge, put tourist dollars in coffers, and provide cultural richness in the way of food and fishing practices.
The loss of coral ecosystems is a critical problem that has propelled coral scientists into action to save species that may not survive the rapid pace of climate change and environmental degradation.
Coral researchers at the University of Guam Marine Laboratory are attempting to understand the relationship between declining water quality and coral health and how these factors affect coral restoration efforts in Tumon Bay, Guam. A recent publication by Laurie J. Raymundo, Maria D. Andersen, and Héloïse Rouze in the journal iScience looks at the issue and offers protocol suggestions for coral restoration efforts.
In a five-year period (2013 to 2017) Guam lost 53% of key staghorn Acropora species, which triggered restoration efforts and the creation of near-shore coral nurseries.
In February 2022, more than 160 nursery-grown Acropora aspera were outplanted in Tumon Bay, which supports an extensive population of the closely related species Acropora pulchra. The outplants showed a 100% survival rate for the first three months.
During regular monitoring, the restoration team noticed signs of disease and then bleaching three months after outplanting. They sampled tissue to document potential shifts in the microbiome that accompanied the decline in health, until the complete mortality of all outplants in August, six months after planting.
This study implicated poor water quality in the total mortality of the outplants from compounding effects of disease and bleaching. The summer months in Tumon Bay bring extreme low tides, hot windless days, and a decline in water quality.
Without adequate flushing of water in the bay by wind and wave action, a buildup of nutrients and bacteria occurs. There were also two red tide events during the period that the outplants were dying.
Microbiome testing revealed that the corals were very low in beneficial Endozoicomonas bacteria, which is associated with coral health. Also, patterns in the microbiome communities suggested these beneficial bacteria may have not been present when the corals were first outplanted.
"The discovery that nursery-grown coral fragments may be low in beneficial bacteria has inspired us to see how we can encourage these health-promoting bacteria before outplanting," said lead author Raymundo.
"Sampling for microbiome analysis is not commonly applied to assess the performance of coral outplants. We are hoping to learn more about their resilience and recovery process by including this type of analysis in our work."
Innovation in applied research may be part of the lifesaving tools needed to protect the corals and the stunning reefs they build.
More information: Laurie J. Raymundo et al, Coral restoration in a stressful environment: Disease, bleaching, and dysbiosis in Acropora aspera in Guam, Micronesia, iScience (2025). DOI: 10.1016/j.isci.2025.112244
Journal information: iScience
Provided by University of Guam