Coral Reefs are Changing Their Smells in a Warmer World

The aromas of a beach strewn with seaweed or a garden full of blooming flowers are more than just momentary sensory experiences. They also act as entryways into the world of ecosystem health and interspecies communication. Plants, for example, emit gases known as “biogenic volatile organic compounds” to adapt to heat stress, attract pollinators, defend against pathogens, deter predators, and more. 

Scientists have been cataloguing these gases on land for decades, but relatively little work has been done for marine ecosystems. To address this knowledge gap, a group of researchers led by marine biologist Caitlin Lawson at the University of Technology Sydney set to out to measure the full spectrum of gases that two common coral reef-building species emit. Like plants, the coral animals in their experiments emitted a wide range of gases, but as temperatures rose, the corals’ gas production declined. The results were published online in late November in the journal Global Change Biology.

The team collected samples of the coral species Acropora intermedia and Pocillopora damicornis from off the coast of Queensland, Australia and separated them into two aquarium tanks. In one tank they maintained normal water temperatures of roughly 82 degrees Fahrenheit, and in the other, they increased the temperature by 1.8 degrees Fahrenheit (1 degree Celsius) per day for 5 days, then let the samples sit at roughly 90 degrees Fahrenheit for 6 days. In the normal temperature tanks, they detected 87 different gases, each likely serving specific physiological roles for the corals. 

“I expected there would be a lot. But just how many there were was really exciting,” said Lawson, a postdoctoral research associate at the University of Technology Sydney, who helped conduct the study.

In the hotter tank where the corals were exposed to heat stress, gas production dropped significantly. In A. intermedia, the number of gas types dropped by 42%, and in P. damicornis, it dropped by 62%. Additionally, the individual abundances for more than 86% of the gases dropped significantly in the hotter tank.

In recent years researchers have detected gases such as dimethyl sulfide and isoprene coming from corals and their associated microorganisms. Both are thought to play a role in protecting corals from environmental stressors such as hotter temperatures and exposure to air. 

During low tides, for example, corals emit dimethyl sulfide from a mucus-like coating on their surfaces to scavenge harmful “reactive oxygen species” that arise during stressful heat events. These gases then ascend into the lower atmosphere, where they help form low clouds that alter a reef’s local climate. Isoprene, one of the world’s most common biogenic volatile organic compounds, might also help defend coral reefs from high temperatures, as it is known to preserve the photosynthetic process in plants during acute heat stress. 

Lawson and her colleagues detected both of these gases during their experiment, but added a laundry list of others, including toluene, benzene, hexanal, and bromoform, all of which might serve important roles for corals undergoing heat stress. Previous research has found that bromoform, for example, can remove ozone from the atmosphere and serve as an antioxidant. More work needs to be done to pin down the physiological function of these compounds, though, since their role in plant communities doesn’t necessarily translate directly to coral reefs.

Erin McParland, a marine biogeochemist at the Woods Hole Oceanographic Institution in Massachusetts, who was not involved in the project, praised the researchers for identifying a wide range of compounds rather than targeting a few better-known ones like dimethyl sulfide and isoprene. The lesser-known compounds identified in the study “are likely just as important, if not more, as the [gases] previously identified in corals,” wrote McParland in an email.

The sum total of an organism’s emitted gases is called its volatilome, and the status of a coral species’ volatilome is a marker of its health.

“At present, our ability to accurately diagnose a decline in coral reef health often involves time-consuming and destructive methods,” wrote Jennifer Matthews, coral biologist at the University of Technology Sydney who was not involved with the research, in an email. “But similar to how dogs can sniff out explosives, this research could allow us to develop tools to sniff out coral reefs in trouble.”