Revealing the Function of Coral Reefs through the Uncovering of Dual Bacterial Clusters in Coral Tentacles

Coral reefs are complex ecosystems with complex inter-species relationships, where every living organism–from tiny bacteria to giant clams–plays a vital role in keeping coral reefs healthy.

Our new study reveals another layer of complexity in coral reefs.

We detected populations of two types of bacteria within the corals’ tissues, which included, oddly enough, a close relative of the chlamydia-causing bacteria.

These new findings have been published in Science advances, suggesting that these bacteria may interact with their coral host and with each other. More work is required to understand whether these interactions are beneficial or harmful to corals.

Coral microbiome

Just like humans, corals have a diverse bacterial microbiome that is closely linked to their health. So, understanding the complex relationships between corals and bacteria is critical to understanding how corals function.

Bacteria can assist corals in many biological processes, such as the movement and processing of nitrogen or sulfur, or the production of antibacterial compounds that protect corals from pathogens. While most of these bacteria live in the mucous layer that covers the surface of the coral, some bacteria occur within the tissues of the coral.

Very little information is available on tissue-associated bacteria, however they are probably some of the most important members of the coral microbiome.

To find out more, samples were taken from a long-running experiment on the Great Barrier Reef coral species Pocillopora acuta conducted at the Townsville-based Australian Institute of Marine Science. These samples were shipped to our laboratory at the University of Melbourne to examine the elusive tissue-associated bacteria.

Glowing needle in a haystack

The first challenge was seeing the bacteria – they are so small.

For this, we used a technique called ‘fluorescence in situ hybridization’. Essentially, we add fluorescent sensors to corals that bind specifically to bacteria. When excited by a laser beam, these tentacles (and thus the bacteria) light up.

We found that the bacteria formed large clusters in the tentacles of the corals. Why in claws? We don’t really know, but we think this could be related to feeding or defense, as the tentacles are involved in both catching prey and fending off predators.

After finding these bacterial groups, we wanted to know what kind of bacteria they were. Typically, to determine a coral’s bacterial microbiome, coral samples are crushed, and the DNA of all the bacteria is sequenced and compared with that of other known bacteria.

However, we were only interested in bacteria in tentacle clusters, so this technique would not work, as it would not distinguish between bacteria in tissue, mucus, gut, skeleton, etc.

Instead, we used a technique called “Precision laser capture“To precisely cut out very small sections of tissue, such as clusters formed by bacteria. With this technique, we can exclusively sample tissue-associated bacteria and sequence their DNA to identify them and understand their functions.

Does coral get chlamydia?

We found two types of bacteria in the assemblages of the coral’s tentacles.

One is a member of Chlamydiae, a bacterial order that contains the pathogens responsible for chlamydial infections in mammals. This is a surprising finding as no cases of chlamydia have been previously reported in coral reefs.

Chlamydia is known to steal energy from its host, in the form of adenosine triphosphate (otherwise known as ATP, this is the main source of energy transfer in cells). This energy parasitism is the basis for the disease these bacteria cause in mammals, such as humans and koalas.

In collaboration with chlamydia specialists at the University of Vienna in Austria, Dr. Astrid Kollingrow and Professor Matthias Horn, we have shown that these bacteria depend on nutrients and energy provided by coral to survive.

In addition, it is also possible that these species obtain nutrients and energy from bacteria associated with coral reefs – something that has not been seen before.

For those of us working to understand all we can about coral biology, the prospect of bacteria living within coral tissues interacting with each other is very exciting.

While this novel Chlamydiales shows many similarities to mammalian pathogens, both harmful and beneficial to coral reefs, it will be examined later this year during Dr. Meyer’s visit to the University of Vienna.

Towards a microbiome-based conservation approach

Other bacteria found in coral tissues belong to the genus Endozoicomonas. These bacteria are known to be widespread in coral reefs and are generally considered beneficial.

In our study, we found that Endozoicomonas can produce several B vitamins and antimicrobial compounds, confirming their ability to provide benefits to their coral host, since corals themselves are unable to produce specific B vitamins.

Coral reefs are under threat from a range of factors, including climate change. In fact, it is estimated that without major interventions, most coral reefs will be gone by 2035. Saving these reefs will require new approaches to reef conservation and restoration.

One possible solution involves probiotics. In the same way that we eat probiotic-packed yogurt to improve gut health, inoculating corals with beneficial bacteria may improve their resistance to high temperatures.

But before implementing these microbiome-based solutions, we need to understand exactly how coral-associated bacteria work.

Studies like ours are beginning to reveal how coral and bacteria interact with each other and whether coral probiotics are an option for maintaining these vibrant and beautiful ecosystems.