What is coral? If you ask the general public this question, you might get a range of guesses. Those more marine or biologically inclined will know that they are an animal, within the phylum Cnidaria. But corals are so much more than just an animal or a single organism; they can be best described as a holobiont, which means 'whole unit of life'. A holobiont, or a metaorganism, is an assemblage of microbes living in symbiosis with a multicellular animal host. The coral holobiont is made up of a large diversity of algae, fungi, viruses, and prokaryotes (bacteria and archaea), and it is, to say the least, fascinating. The image below (extracted from our online Coral Biology course) provides a simplified depiction of this holobiont metaorganism as a diagram of a coral tentacle.
I first became aware of the coral holobiont in my undergraduate marine biology program. Whilst I do not specifically remember having any classes on coral, I did have a marine microbiology class and my lecturer focussed his research on coral microbiology. I was never too interested in microbes, and had always wanted my career to focus more on marine megafauna (real unique, I know). However, for some reason I decided to conduct my undergraduate honours research project on coral microbiology, and ended up studying the interactions between a consortium of probiotic bacteria and a virulent disease-causing bacteria, all extracted from a single coral species. From that time on, I was hooked on the coral holobiont and the network of complex interactions between the coral and its microbes that allow the holobiont to maintain homeostasis and survive, thrive and evolve within its marine environment.
In fact, you cannot understand corals without understanding the mechanisms of this microbial world. This community of microbes are the means by which corals flourish in the nutrient-poor environment that is tropical seas. So, understanding the mechanisms involved in the interactions between themselves and with the coral host is key to understanding how corals might respond to environmental changes such as warming temperatures and ocean acidification.
Perhaps the more well-known coral-microbe relationship is that of the symbiotic zooxanthellae, which are microscopic dinoflagellates (microalgae) within the family Symbiodiniaceae. Numerous species of zooxanthellae live within the gastrodermis of the coral and photosynthetically fix carbon in the forms of glycerol, glucose, and organic acids to support respiration, growth, and reproduction of the coral. In return, the coral provides a safe, high light environment, for the zooxanthellae as well as CO2 (a product of its respiration) which the zooxanthellae can use in photosynthesis. The phenomenon of coral bleaching is the loss of this colourful zooxanthellae from the coral tissue as a result of environmental stress, most notably elevated temperature, which causes a disfunction in the coral-zooxanthellae symbiotic relationship.
However, the bacteria, viruses, fungi and archaea play equally important roles within the coral, such as fixing nitrogen for both the coral and zooxanthellae to use, providing a probiotic health benefit, recycling nutrients, and exchanging essential metabolites. After zooxanthellae, bacteria are the most understood component of the holobiont, whilst the other groups of microorganisms remain more cryptic in their function. A new (March 2023) paper in the journal FEMS Microbiology Reviews by Mohamed et al. titled 'The coral microbiome: towards an understanding of the molecular mechanisms of coral - microbiota interactions' reviews the current understanding in functional diversity of microbes within the coral microbiome, with a focus on their role in coral health and disease and how these specific interactions could be studied to further understand the ecology of the coral holobiont in the reality of a warming ocean. There are some really interesting nuggets of information throughout the review. What really grasped my attention was how they highlight the recent discovery of an apicocomplexan called Corallicolids. Apicocomplexans are well known parasitic agents of humans, which suggests that their relationship with corals is likely not a beneficial one, but no evidence of harmful interactions has so far been found. This is just one example of how much we still have to learn about the coral holobiont and all of its microbial components and mechanisms.
And guess what? The microbiome doesn't end at the coral either... if you zoom out a little (or a lot), you can discuss the microbiome of a whole coral reef! If there are 1-100 million microbial cells per ml of surface mucus layer of an individual coral polyp, then imagine how many microbes there are in a whole coral polyp, and an entire coral reef... and not just individual cells, but biodiversity as well. It's pretty mind boggling to think about it. A recent publication (June 2023) in Nature Communications titled 'Diversity of the Pacific Ocean coral reef microbiome' details some results from the incredibly cool Tara Pacific expedition; a science expedition from the platform of a 118 ft schooner sailing yacht called Tara. As the authors state, the Tara Pacific expedition "provides new insight into the global microbial diversity, the factors driving their distribution, and the biocomplexity of reef ecosystems". They not only sampled coral, but also 2 fish species and planktonic communities in 99 reefs of 32 islands across the entire Pacific Ocean. Their aim was to assess the composition and biogeography of reef microbiomes. What they find is incredible; when they extrapolate the amount of microbes found within their sampled coral and fish to all the fishes and corals of the Pacific Ocean, the number of microbes they get is within range of the number of prokaryotes (remember, that's bacteria and archaea) currently estimated for the entire Earth. The authors therefore suggest that the global microbial biodiversity is largely underestimated and go on to state that these microbes represent undiscovered and potentially important taxonomic and metabolic diversity that of course warrants further study, but also conservation effort.
There is SO much still to discover about the diversity and complexity of coral and whole reef microbiomes. Not only is it a biologically fascinating subject, but being able to identify and untangle some of these complex microbial interactions could prove timely in helping corals through assisted evolution to survive the elevated sea temperatures that continue to threaten them with bleaching and disease outbreaks (see our previous Corals News article about assisted evolution in corals). If this article has sparked within you a bit of interest in the coral holobiont, then I encourage you to check out our interactive online Coral Biology course which has a whole chapter dedicated to the coral holobiont.