Reef-building corals are products of co-evolution

Corals and the microbes they host evolved together, researchers have found, providing important clues for reef management practice.

“Many corals have gone extinct during industrialisation and many others are threatened with extinction,” says microbiologist Rebecca Vega Thurber, from Oregon State University in the US, which led the international study.

“If we see patterns of evolution between microbiomes and corals, that gives us an idea of which microbes to target: to learn what they do, how they help corals resist climate change and how they help to buffer against nutrient pollution.

“We can look in more depth at the microbes and understand how they help or hurt their hosts.”

Corals are home to a complex composition of fungi, bacteria, and single-celled organisms called dinoflagellates that together make up the coral microbiome. Shifts in microbiome composition are connected to changes in coral health.

Vega Thurber and her colleagues studied 600 samples of scleractinian, or stony, corals collected from 21 reefs off the coast of Australia, spanning 17 degrees of latitude, working with local partners at James Cook University in Queensland.

For each sample set, they looked at the corals’ tissue, skeleton and mucous to see what microbes were present. They did so by sequencing the 16S rRNA gene, which is present but slightly different in every living organism.

From there, they could look for patterns between different corals’ microbial communities and determine whether co-evolution between residents and host had taken place.

“We found strong support for coral-microbe phylosymbiosis, in which coral microbiome composition and richness is reflected in coral host’s evolutionary history,” Vega Thurber says.

“When speciation for modern reef-building coral families began between roughly 25 million and 65 million years ago, that was accompanied by large changes in microbiome richness. And changes continued to accumulate during more recent speciation events.”

Coral diversity is too great for assessing the various factors that maintain the microbiome of every single coral species, Vega Thurber says, but the findings provide general rules for microbiome assembly “that inform estimates of the effects of microorganisms in understudied portions of the coral tree”.

“Now we have a framework for analysing scleractinian coral microbes that can reveal how the corals’ evolutionary history, host traits and local environment interact to shape microbiomes,” she says.

“In the coral world, there’s been a longstanding hypothesis that microbes and coral co-evolved, but there hadn’t been a sufficient data set to test that before now.”

The findings are reported in a paper published in the journal Nature Communications.