A two-year expedition across the Pacific Ocean has revealed that microbes on coral reefs can be as varied as the microbiome of the rest of the planet’s ecosystems combined. This could mean that we have grossly underestimated the total microbial diversity on Earth.
Coral reefs make up less than 1% of the ocean, but are home to almost a third of marine animal and plant species. An expedition launched in 2016 and led by Serge Planes at the University of Perpignan in France visited 99 reefs across the Pacific Ocean. On each reef, Planes and his colleagues collected seawater and sampled three species of coral and two species of fish.
They sequenced a key section of DNA from bacteria and archaea present in the samples. Of the approximately 3 billion sequences that resulted, they found more than half a million unique sequences, indicating vast microbial diversity. Samples from different parts of the ocean also had distinct microbiomes.
“The diversity of corals is reflected in the diversity of the microbiome,” explains Pierre Galand at the Sorbonne University in France.
Extrapolating these results to the microbiomes of hundreds of other coral species and thousands of fish species in Pacific reefs, the researchers claim that the total microbial diversity present on all these reefs would be almost six times higher. They say this could be higher than some estimates of microbial biodiversity for the entire planet.
This microbial diversity plays an essential role in coral reef ecosystems. Some corals have symbiotic relationships with bacterial species, for example. Other microbes speed up nutrient cycling or protect against pathogens.
“Microbes really run these ecosystems because they’re involved in so many different kinds of processes,” says Deron Berkepile at the University of California, Santa Barbara.
However, Planes says it’s unclear how the microbiome is affected by factors driving the loss of macrodiversity in reefs, such as rising temperatures due to climate change, ocean acidification and overfishing.
“There is no longer a direct link between the decline of coral reefs and changes in the microbiome,” he says. “It’s mainly because we know so little about the microbiome.”