New research has supplied unique insights at a genetic level into how corals collaborate with their microscopic partners – including algae, bacteria and archaea – to grow and thrive.
It’s well established that the symbiotic relationship between corals and algae is vital for the survival of reefs, according to lead author Steven Robbins from the University of Queensland, Australia.
“The most striking example of this is coral bleaching,” he explains, “where corals expel their algal symbiotic partners at high water temperatures. As algae make the bulk of the coral’s food through photosynthesis, the coral will die if temperatures don’t cool down fast enough to allow the symbiosis to re-establish.”
The team used genomic analysis to see if coral had equally important relationships with other microorganisms, in collaboration with researchers from James Cook University.
The research is published in the journal Nature Microbiology.
“This is the first study of its kind to tease apart all of the genetic material of a healthy reef building coral and each of the microscopic organisms that live in association with it,” says co-author Lauren Messer, a notoriously complex task, according to Robbins.
The multi-disciplinary team collected carefully selected samples of Porites lutea from Orpheus island, just north of Townsville in Queensland, Australia, a coral species that grows abundantly throughout the Indo-Pacific region.
These corals and their associated algae are several hundred years old and more resilient to warming and extreme weather than most, so they could yield insights into reef survival mechanisms.
To separate the coral and all the microorganisms, the researchers used a filtration procedure normally used to sequence marine sponge-associated microbiomes and with state-of-the-art computer algorithms managed to identify 52 bacterial and archaeal genomes.
“What the team was able to do is an exciting first for the field”, says Robbins. “It’s truly ground-breaking – this is the blueprint for corals and their symbiotic communities.”
Checking the entire library of genes that each organism has to work with, they hypothesised which nutrients the coral needs but could not make by itself, then investigated which of the microorganisms produce those nutrients.
Their key finding was that the bacteria and archaea appear to recycle nutrients like carbon and nitrogen within the coral, providing essential vitamins and amino acids that the reef relies on to grow.
Messer says this represents a significant shift in current understanding of coral symbiosis, as many of these functions were previously attributed to algae, and importantly pinpoints how the microbiome is important for its survival.
These organisms are “much more integral to coral health than we’ve historically given them credit for”, adds Robbins.
This is critical, given that nearly half of all corals on the Great Barrier Reef – the world’s largest living organism and one of its richest ecosystems – died in 2016 and 2017, a scenario that is unlikely to reverse if carbon emissions remain at current levels.
And it’s not just the corals that are at risk, says Robbins.
“That figure doesn’t take into account that coral reefs support a massive number of marine species that may disappear with the reef – fish (Nemo and Dory), anemones, crabs, sponges, sea stars,” and more, he explains. “As we lose reefs, we lose far more than corals.”
Being a bioinformatician, Robbins was amazed to visit them himself. “I think if you aren’t concerned with saving the reefs you must not have seen them,” he says. “They’re stunningly beautiful and enough fascinating animals live in them to support decades of Attenborough documentaries.”
The next steps include investigating other coral species. “Corals can be about as different from each other phylogenetically and genetically as a human is to a kangaroo,” Robbins explains, “so we need to do more studies like this in different species and across other important reef invertebrates, like sponges.”
They do note that many of the microbial species they identified are found in other coral species, so lessons learned will likely have broader applications.
Robbins suggests it may even be possible to culture some of the species to inform attempts to make a marine probiotic.
“As scientists, we are incredibly saddened by the recent mass bleaching and large-scale deaths of corals on the Great Barrier Reef,” says Messer. “We hope that this data set will provide a foundation for future research that will help corals survive global climate change.”