Coral reefs are complex ecosystems with intricate interactions between species, where every living thing, from microscopic bacteria to enormous clams, is essential to preserving the reef’s health.
Our new study reveals yet another layer of complexity in coral reefs.
In the tissues of corals, we found groups of two different types of bacteria, one of which was strangely a near related of the chlamydia-causing bacteria.
These recent discoveries, which were reported in Science Advances, suggest that these bacteria may communicate with both their coral host and one another. To determine whether these interactions are advantageous or detrimental to the coral, more research is necessary.
The coral microbiome
Corals have a varied bacterial microbiome that is closely linked to their health, just like people do. Therefore, it is essential to comprehend the intricate interactions between bacteria and coral in order to comprehend how corals function.
Numerous biological activities, such as the transfer and digestion of nitrogen or sulfur, or the creation of antibacterial chemicals that guard corals against pathogens, can be assisted by bacteria. Some of these bacteria are found inside the coral tissues, while the majority of them are found in the mucus layer that covers the coral surface.
Very little information is available on tissue-associated bacteria, yet they are likely some of the most important members of the coral microbiome.
To learn more, samples from a lengthy investigation on the Great Barrier Reef coral Pocillopora acuta species were collected by the Australian Institute of Marine Science in Townsville. To look into the illusive tissue-associated bacteria, these samples were sent to our lab at the University of Melbourne.
A glowing needle in a haystack
The first challenge was to see the bacteria they are very small.
For this, we used a technique called “fluorescence in situ hybridization.” Essentially, we add fluorescent probes to the coral that specifically bind to bacteria. When excited by a laser beam, these probes (and hence the bacteria) light up.
We found that bacteria formed large clusters in the tentacles of our corals. Why in the tentacles? We don’t really know, but given that tentacles are used for both capturing prey and fending off predators, we speculate that this may be related to nourishment or defense.
After finding these bacterial clusters, we wanted to know what kind of bacteria they were. Typically, to identify a coral’s bacterial microbiome, coral samples are crunched up, and the DNA of all the bacteria is sequenced and compared to that of other known bacteria.
However, because this method would not distinguish between bacteria in tissue, mucus, the gut, the skeleton, etc., we were only interested in the bacteria in the tentacular clusters, therefore it was ineffective.
Instead, we employed a method known as “laser capture microdissection” to precisely remove very minute chunks of tissues, such as the bacterial clusters. We could only sample tissue-associated bacteria using this method, allowing us to sequence their DNA in order to identify them and comprehend their roles.
Do corals get chlamydia?
We found two types of bacteria in the aggregates in the coral’s tentacles.
One belongs to the bacterial order Chlamydiales, which includes the pathogens that cause chlamydia infections in mammals. This is an unexpected finding because coral infections with chlamydia have never been documented.
Chlamydiales are known to steal energy from their host, in the form of adenosine triphosphate (known as ATP, this is the main source of energy transfer in cells). This energy parasitism is the basis of the disease these bacteria cause in mammals, like humans and koalas.
In collaboration with Chlamydiales specialists at the University of Vienna in Austria, Dr. Astrid Collingro and Professor Matthias Horn, we showed that this bacterium relies on nutrients and energy provided by the coral to survive.
It’s also likely that this species receives nutrients and energy from other coral-associated bacteria, which is something we have never observed previously.
The potential that the bacteria living inside coral tissues are communicating with one another is pretty exciting for those of us trying to learn as much as we can about coral biology.
While these novel Chlamydiales exhibit many similarities with mammalian pathogens, Dr. Maire to the University of Vienna will examine whether they are detrimental or beneficial to coral later this year during a visit.
Towards microbiome-based conservation approaches
The other bacteria present in the coral tissues belong to the Endozoicomonas genus. These bacteria are known to be widespread in corals and are generally considered to be beneficial.
Given that corals cannot naturally manufacture some B vitamins, our investigation revealed that Endozoicomonas can produce a number of B vitamins as well as antimicrobial chemicals, demonstrating its potential to help its coral host.
Corals are under threat from a range of factors, including climate change. In fact, it is estimated that without major interventions, most coral reefs will disappear by 2035. Saving these reefs will require new coral conservation and restoration approaches.
One potential solution involves probiotics. Corals may be better able to withstand high temperatures if they are injected with helpful bacteria, similar to how probiotic-rich yogurt helps to enhance our gut health.
But before these microbiome-based remedies can be put into practice, we must fully comprehend how coral-associated microorganisms work.
Our research sheds light on how coral and bacteria interact and explores the possibility of coral probiotics as a means of preserving these important and stunning ecosystems.
