The discovery of clusters of two species of bacteria in coral tentacles gives light on their involvement in coral reef health.

Each organism, from a tiny bacterium to a giant clam, contributes significantly to the health of coral reefs, which are intricate ecosystems with intricate species relationships.

Our new review uncovers one more layer of intricacy in coral reefs.

Strangely, a close relative of the chlamydia-causing bacteria was found in clusters of two different kinds of bacteria in coral tissue.

These brand-new findings, which were recently published in Science Advances, suggest that these bacteria might interact not only with their coral host but also with one another. To determine whether these interactions are advantageous to the coral or detrimental, additional research is required.

The bacterial microbiome of corals Like humans, corals have a diverse bacterial microbiome that is closely related to their health. Therefore, in order to comprehend how corals function, it is essential to comprehend the intricate relationships between bacteria and coral.

Credit: University of Melbourne

Bacteria can assist corals in a number of biological processes, such as the movement and processing of nitrogen or sulfur or the production of antibacterial compounds that shield corals from pathogens. While the majority of these bacteria are found in the mucus layer on the coral’s surface, some are found within the coral’s tissues.

Tissue-associated bacteria are probably some of the most important members of the coral microbiome, despite the fact that there is a lack of information on them.

Samples were taken from a long-term experiment on the Great Barrier Reef coral species Pocillopora acuta at the Australian Institute of Marine Science in Townsville to learn more. These examples were delivered to our lab at the College of Melbourne to research the tricky tissue-related microorganisms.

Image of a coral polyp using fluorescence microscopy (Pocillopora acuta). The bacteria are shown in red, and the coral tissues are shown in green. Tentacles surround the polyp’s center, forming its mouth. Credit: Justin Maire

A glowing needle in a haystack

The first obstacle was recognizing the tiny bacteria.

We used a method known as “fluorescence in situ hybridization” to accomplish this. Basically, we add fluorescent tests to the coral that explicitly tie it to microbes. These probes, and consequently the bacteria, light up when excited by a laser beam.

Our corals’ tentacles contained large clusters of bacteria, as we discovered. Why are tentacles there? Tentacles are involved in both catching prey and repelling predators, so we think this could be related to nutrition or defense.

We wanted to know what kind of bacteria were in these bacterial clusters after we discovered them. Regularly, to distinguish a coral’s bacterial microbiome, coral examples are crunched up, and the DNA of the multitude of microorganisms is sequenced and contrasted with that of other known microscopic organisms.

This method, however, would not work because it would not distinguish between bacteria in the tissue, mucus, gut, skeleton, etc. because we were only interested in the bacteria in the tentacular clusters.

Instead, we utilized a method known as “laser capture microdissection” to precisely remove extremely small portions of tissues, similar to the clusters that the bacteria formed. With this strategy, we could only isolate tissue-related microorganisms and sequence their DNA to distinguish them and grasp their capabilities.

Before and after laser capture microdissection, a method that enables very precise sampling from tissue sections, a bacterial aggregate (in red) Credit: Justin Maire.

Does Chlamydia infect corals?
The coral’s tentacle aggregates contained two types of bacteria.

One of them belongs to the bacterial order known as the Chlamydiales, which includes the pathogens that cause Chlamydia infections in mammals. Since chlamydia infections in corals have never been documented before, this is an unexpected finding.

It is known that chlamydiales steal energy from their host in the form of adenosine triphosphate (ATP, which is the cell’s primary energy transfer source). The disease with which these bacteria infect mammals like humans and koalas is based on this energy parasitism.

We demonstrated, in collaboration with Dr. Astrid Collingro and Professor Matthias Horn, experts in Chlamydiales at the University of Vienna in Austria, that this bacterium thrives on coral-provided nutrients and energy.

Likewise, it’s additionally conceivable that this species gets supplements and energy from other coral-related microorganisms — something that we have never seen.

The possibility that the bacteria that live in coral tissues are interacting with one another is very exciting for those of us who are attempting to learn as much as we can about coral biology.

During Dr. Maire’s visit to the University of Vienna later this year, it will be determined whether these novel Chlamydiales are harmful to coral or beneficial to it, despite the fact that they share many similarities with pathogens that affect mammals.

Approaches to conservation based on the microbiome

The Endozoicomonas genus includes the other bacteria found in coral tissues. Corals are known to have a lot of these bacteria, which are generally thought to be good.

In our review, we found that Endozoicomonas can create a few B nutrients and antimicrobial mixtures, affirming its capability to give advantages to its coral host, given that corals themselves can’t deliver specific B nutrients.

There are a variety of threats to corals, including climate change. As a matter of fact, it is assessed that without significant mitigation, most coral reefs will vanish by 2035. New approaches to coral conservation and restoration will be needed to save these reefs.

Probiotics are one possibility for a solution. Similarly, just as we eat yogurt loaded up with probiotics to further develop our stomach wellbeing, immunizing corals with valuable microscopic organisms might work on their protection from raised temperatures.

However, before these solutions based on the microbiome can be implemented, we need to know exactly how coral-associated bacteria work.

Coral probiotics may be an option for the preservation of these vital and beautiful ecosystems as studies like ours begin to unravel the interactions between coral and bacteria.

More information: Justin Maire et al, Colocalization and potential interactions of Endozoicomonas and chlamydiae in microbial aggregates of the coral Pocillopora acuta, Science Advances (2023). DOI: 10.1126/sciadv.adg0773

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