Does that reef smell good?

Gassy corals make a happy reef, it seems, but these gases may be lost if the water gets too hot.

Animals release gases as part of their normal function – which contributes to how they smell – and the molecules in these gases are a good indicator of health. Until now, however, the “smell” of corals in tropical reefs has been largely overlooked.

In a new study, a team led by Caitlin Lawson from Australia’s University of Technology Sydney (UTS) identified a range of gases produced by reef-building corals off Heron Island on the Great Barrier Reef and highlighted a suite of compounds that may play roles in maintaining healthy reef function.

The findings are published in a paper in the journal Global Change Biology.

“Our results also reveal that heat stress dramatically decreases the chemical diversity, quantity and functional potential of these important compounds,” Lawson says, “which could further impact the capacity of corals to cope with increasing temperatures.”

That’s important because coral loses algae and turns white when stressed – a phenomenon known as bleaching. This in turn affects the entire ecosystem of the reef, as many other plants and animals rely on coral and the gases they release to survive.

“Coral reef diversity is sustained through sights and sounds, and our work shows that smells also play many critical roles,” says co-author David Suggett, also from UTS.

But Is there a certain smell that indicates corals are more vulnerable?

To answer this question, the team identified the volatile organic compounds released by two common corals: Acropora intermedia and Pocillopara damicornis. They detected 87 unique compounds, revealing that the smell of coral is actually rich in chemicals.

They then assessed how gases changed when the corals experienced high temperatures, exposing them to six days at 32 degrees Celsius rather than the 27.5 degrees they usually live in. They found that in the hotter water A. intermedia gases decreased by 41% and P. damicornus gases by 62%.

The stressed corals didn’t produce all of the 87 unique compounds they usually did, so the team was able to identify which compounds were important for the coral to function normally.

This in turn meant that decreased functions due to stress directly corelated to less gases released, providing a potentially simple, non-invasive way of assessing whether a coral is stressed or not by testing which compounds were absent from their smell.

“The discovery of a loss of these smells under heat stress driven by ocean warming is yet more evidence reefs will change as we know them unless we urgently tackle climate change,” says Suggett.

This is, however, one of the first studies of its kind, so more research is needed to explore how this strange mechanism might affect the entire ecosystem of the reef as ocean temperatures rise.