Phylosymbiosis on the Great Barrier Reef: A complex ecosystem with complex microbiomes

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Australia’s Great Barrier Reef is listed as one of the seven natural wonders of the world. A living structure so large it can be seen from outer space, yet it is tiny coral polyps, millimetres in diameter, that are largely responsible for its formation. Smaller yet, are the microbes that form a symbiosis with reef building corals and underpin the health of the entire ecosystem. The importance of microbes is unquestionable, and it is critical we understand how these communities are structured along with the functions they provide.

Perhaps the only thing rivalling the size of the Great Barrier Reef is its extraordinary complexity. Marine invertebrates alone make up several thousand different species, each with their own unique microbiome, and each microbiome is not without its own complexity. They may shift with the seasons, transform with latitude and respond to the surrounding environment. However, through the layers of complexity, at the foundations of a host-microbe symbiosis, we expect that there would be an element of host control. To begin exploring this, we first asked the question – do closely related marine invertebrates share more similar microbiomes than distantly related invertebrates? As it turns out, this pattern is widespread in nature and already had a name – phylosymbiosis.

Within a month, I was spontaneously thrown into three back-to-back field trips with the goal of collecting as many species of marine invertebrates as possible. Our focus was on scleractinian corals, however given that they are notoriously difficult to work with on a microbial level, we expanded our search to octocorals, sponges and ascidians. Field trips are often the most exciting aspect when you work on marine animals. They can take you to remote locations with pristine reefs and leave you with experiences not many get to share. However, they can also be the most stressful part of the work. Solutions for makeshift laboratories seem to be finished the night before, volunteers eager to see the reef drop out last minute, and bad weather causes unexpected delays. Despite the challenges, we came back with a species list we were satisfied with and began to explore the patterns of phylosymbiosis.

I mentioned that corals are notoriously difficult to work with. Extracting high quality DNA is tricky and inhibitors tend to plague PCRs. Aside from that, the coral microbiome tends to be highly dynamic, particularly in comparison to sponges. That we still saw significant patterns of phylosymbiosis in our corals, and that this pattern extended to our other groups of coral reef invertebrates, really emphasised to us that a close relationship between host phylogeny and the microbiome exists. Whether or not this is due to host control/cophylogeny/coevolution or more passive mechanisms is our current research focus.

If you’d like to know more, the full article can be found at https://www.nature.com/articles/s41396-020-0671-x

Paul O'Brien

PhD Candidate, James Cook University

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