Researchers investigating the coral microbiome in Australia’s Great Barrier Reef have found two clusters of co-existing bacteria. And, for the first time, they’ve discovered that one of them is a close relative of the bacteria that causes chlamydia in humans. The discovery provides more information about coral reef health and may help tackle the problem of coral bleaching.
The rich biodiversity of coral reefs makes them one of Earth’s most crucial ecosystems. Corals’ survival relies on multiple microorganisms, including bacteria, fungi, and viruses. The Great Barrier Reef, on the northeast coast of Australia, is the world’s largest and best known coral reef system, composed of over 2,900 individual reefs and 900 islands covering an area of about 132,973 square miles (344,400 sq km).
Bacteria play a critical role in protecting corals against pathogens, cycling nutrients, and producing vitamins and essential amino acids. They commonly colonize the corals’ mucus and skeleton and are less frequently seen in the tissues. When bacteria are seen in the tissues, they are known to form large, dense clusters called cell-associated microbial aggregates (CAMAs).
Researchers from the University of Melbourne, the Australian Institute of Marine Science, Townsville, and the University of Vienna have discovered two types of CAMAs within the tissues of Pocillopora acuta corals in the Great Barrier Reef.
The first, belonging to the Endozoicomonas genus, is known to be widespread in corals. It’s generally thought that this bacterium is beneficial to corals because of its ability to produce B vitamins and antimicrobial compounds.
The second kind of bacteria was unexpected. It was found to be from the genus Chlamydiales, which includes the bacteria that causes chlamydia, a sexually transmitted infection, in humans. It’s the first time that Chlamydiales has been found in corals.
“We worked with Chlamydiales specialists Dr Astrid Collingro and Professor Matthias Horn from the University of Vienna, and found that these bacteria steal nutrients and energy from their hosts to survive,” said Justin Maire, lead author of the study.
Using a combination of imaging techniques, microdissection and genome sequencing, the researchers found that the CAMAs were in the tentacle tips of P. acuta corals and that Endozoicomonas and Chlamydiales occurred in distinct but adjacent CAMAs.
The discovery of the novel Chlamydiales and the fact that it co-exists with Endozoicomonas advances our understanding of the complex coral microbiome and coral reef health.
“There is a possibility that this bacterium gets nutrients and energy from other coral-associated bacteria, and for those of us working to understand coral biology, the possibility that the bacteria living inside coral tissues are interacting with each other is quite thrilling,” Maire said.
Global warming has triggered marine heat waves, leading to coral bleaching on the Great Barrier Reef and other coral reefs around the world. When the reef’s waters stay too hot for too long, the corals become stressed and expel the colorful marine algae (zooxanthellae) living inside their tissues, leaving behind a white skeleton. Coral reefs can take decades to recover from bleaching, if they recover at all.
The researchers hope their findings will help tackle the problem of coral bleaching on the Great Barrier Reef using probiotics.
“One of the focus areas in my lab is the development of bacterial probiotics for corals, helping to improve their resistance to thermal stress and survival rates caused by climate warming,” said Madeleine van Oppen, a co-author of the study. “We still know very little about the functions of coral-associated bacteria, and this new study will help us to figure out whether probiotics are a feasible solution and if bacteria such as Endozoicomonas are best placed to do the job.”
The study was published in the journal Science Advances.
Source: University of Melbourne