Researchers, including those from the University of Derby in the UK, surveyed coral reefs across the Indian and Pacific Oceans between 2011 and 2018 for signs of this new grey-patch disease, and found that it is currently restricted to Micronesia, a sub region in the western Pacific Ocean consisting of thousands of small islands.
The study, published in the journal Microbiome, noted that the disease is characterised by the growth of a thin grey layer of a type of bacteria called cyanobacteria. The bacteria form a mat, or a biofilm, that over grows the live coral tissue, the study said.
The researchers tagged 12 Porites a large species of coral at Luminao reef in Guam a US island territory in Micronesia. They found that the disease had affected 18 different coral species, and was seen in nearly 22 per cent of the surveyed colonies.
According to the researchers, the disease was slow to progress, and some coral tissue was able to grow back over the bacterial mat. “Upon analysis of the coral samples, multiple microorganisms appeared to be involved in degrading the corals’ defences causing the onset of the disease,” said study lead author Michael Sweet from the University of Derby. “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 added.
The researchers said the community of microbes, or the microbiome, living on the corals was significantly altered after the cyanobacterial infection. When the scientists compared microbiomes across different corals, they found that those with the disease were more similar in terms of their new bacterial community profiles. Based on this finding, the researchers suspect that cyanobacterial infection causes alteration of the coral’s organisation of its own microbiome in a controlled way.
“Our analysis highlighted the clustering of specific microbes. Identifying these clusters may be a useful tool for assessing coral health at any given time,” Sweet said. The researchers said in healthy coral tissue these ‘infection clusters’ were observed in advance of physical signs of the disease.
They also found clusters of bacteria that indicated better survival of the corals from the disease. “These appeared to be essential to the health of the corals and these good bacteria appeared to compete against the disease-causing microbes, allowing certain corals to recover and survive,” Sweet said.
The researchers are currently planning to explore if these microbes may be cultured and used as probiotics that can assist corals in overcoming the impacts of climate change.