(CN) – Coral reefs in the western Pacific Ocean region of Micronesia are under threat from the emergence of what scientists are calling grey-patch disease, according to a study published Thursday.
Coral reefs are among the most sensitive ecosystems in the world when it comes to changes in their environment. Even the slightest change in oxygen levels, pH, or temperature can leave a coral in distress, which in turn puts the delicate balance of reef life in jeopardy.
In the midst of climate change, coral reefs have experienced an increase in so-called bleaching events that lead to coral disease and subsequent infections. As a result, once abundant coral reefs are beginning to disappear, causing devastating effects to the underwater ecosystem.
Corals exist in a symbiotic relationship with a species of algae called zooxanthellae, in which the reef provides housing for the algae and the algae in turn provides nutrients and protection. When corals face stress from changes in the environment, a bleaching event will occur that expels algal symbiotes, resulting in the lack of the usual rainbow reef color and an increased vulnerability to diseases.
Reefs are also home to numerous types of microbes collectively known as the coral microbiome. In the same way humans have healthy bacteria living inside and outside of their bodies, corals have cohabitating bacteria that function in similar ways. Previous research suggests that the microbiome has a strong influence on a coral’s survival and various diseases have been shown to alter its composition, including a newly discovered coral illness called grey-patch disease.
In a study published Thursday in the journal Microbiome, researchers from the University of Derby and University of Guam report on the new coral disease found in the Micronesian reefs.
Not only does grey-patch disease disease alter the community of microbes living on the host coral, but it could also be a promising tool for monitoring coral health across reefs worldwide.
From 2011 to 2018, researchers travelled sea to sea gathering data on coral reefs. They surveyed reefs all across the Indian and Pacific Oceans for signs of this new disease, and discovered it is currently exclusive to Micronesia, a region made up of many small islands in the western Pacific Ocean.
The disease begins with the growth of a thin grey layer of cyanobacteria, a collection of microorganisms. It then forms what is known as a biofilm or mat that overgrows and smothers the live coral tissue. The disease has been found in 18 different species of coral and seen in 21.7% of surveyed colonies in Micronesia.
To monitor its progression, the team tagged 12 Porites, a large species of coral, at the Luminao Reef in Guam. The results showed that although the disease was able to spread across multiple species of coral, it was slow-moving and in some cases the coral tissue was able to grow back over the biofilm. This could be due to a number of factors including a return of the coral’s optimal environment, the slow spread of the disease in regards to coral’s natural healing time, and so on.
Michael Sweet, lead author of the study, said in a statement that coral samples show “multiple microorganisms appeared to be involved in degrading the corals' defenses causing the onset of the disease.”
“This supports recent findings that suggest we should shift from the one-pathogen-one-disease model, to exploring the importance of multiple pathogens in any given disease,” Sweet said.
Further analysis of the infected coral samples revealed an interesting effect of this infection. When comparing microbiomes across different corals, those with the disease were found to be more similar to each other compared to their healthy counterpart communities. This contradicts a previous hypothesis that suggested a coral’s microbiome becomes hectic and disorganized after infection, and implies that the host coral’s organization of its microbiome is altered in a controlled manner after infection.
Striking a hopeful tone, Sweet said identifying clusters of specific microbes “may be a useful tool for assessing coral health at any given time.”
“We found that in healthy coral tissue, 'infection clusters' were seen in advance of physical signs of the disease. We were also able to identify 'survival clusters' in diseased tissue. These appeared to be essential to the health of the coral and these good bacteria appeared to compete against the disease-causing microbes, allowing certain corals to recover and survive,” he said. “We are now interested in exploring if these microbes may be cultured and used as probiotics to assist corals in overcoming the impacts of climate change."
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