How do stressed corals smell?

If you ever move To be unnecessarily cruel to someone huddled up to spend the winter in a dull, drafty place, you can send them a photo of Heron Island. A lush, sand-fringed freckle several hours by boat from Gladstone, Australia, looks like a screensaver version of paradise. Adorably serene looking black noddies perch on Pisonia trees Green and loggerhead turtles emerge from the turquoise water each year to bury their eggs in the sand. But the most amazing feature of the island is offshore: a part of the Great Barrier Reef.

Pocillopora damicornis, sometimes known as “cauliflower coral,” is lumpy and brown, like an unfortunate brassica left to rot in remote places in the refrigerator. Colonies of Acropora intermedia they huddle together in steps several meters wide, their creamy branches resembling the graceful and agile hands of a pianist. The reef is struggling with warming waters. In a new report, the International Union for Conservation of Nature categorized the reef’s outlook as “critical,” the group’s most dire forecast, due in part to bleaching, which occurs when the coral expels its symbiotic algae.

Many healthy coral polyps are teeming with zooxanthellae, a single-celled photosynthetic alga that provides the corals’ main food supply in addition to their tone. Algae funnel glucose, amino acids, and other byproducts of photosynthesis into reef-building corals, and corals use them to fuel the production of calcium carbonate, the substrate of the reef. When the water temperature rises (or sometimes falls), the algae shoots up, leaving pale corals in its wake. “Animal tissue is naturally transparent, like a jellyfish, so you look directly at the white skeleton when the chlorophyll pigments in the algae disappear,” says Carly Kenkel, a biologist at the University of Southern California at Dornsife. The Great Barrier Reef has seen three mass bleaching events in the past five years, according to a report from Australia’s Great Barrier Reef Marine Park Authority. Such events are not necessarily death sentences (reefs can recover if algae return), but losing them, even temporarily, can be both a stress response and a powerful stress in itself, leaving corals more vulnerable to disease. .

The reefs have generally grown well in the turquoise waters around Heron Island.
The reefs have generally grown well in the turquoise waters around Heron Island. Ciambue / Wikimedia Commons / CC BY 2.0

We know that stress makes humans sick, slippery and stinky, and stressed corals smell different, too. Of course, heat stress doesn’t make the corals sweat, but it does change the type and composition of the gases known as biogenic volatile organic compounds (BVOC) that rise from the reef. A team led by Caitlin Lawson, a researcher at the Sydney University of Technology, recently chiseled some samples of P. damicornis and A. intermedia from the Great Barrier Reef near Heron Island to measure the effects of heat stress on those gases – the smell of a threatened reef.

Many researchers who work with coral do not smell it. They take samples underwater, with their noses covered by diving masks, and return to shore or a boat without stopping to inhale. After collecting samples of tropical coral, “I immediately immerse the piece of coral in a container of liquid nitrogen to freeze it or in a tube of saturated salt to preserve it,” says Christina Kellogg, a research microbiologist at the US Geological Survey. Petersburg Marine and Coastal Sciences in Florida. “In both cases, speed is critical to stopping the microbial diversity clock, so I have never recorded the odor.”

<em>A. intermedia</em> and <em>P. damicornis</em> corals ready to be sniffed.  “width =” auto “data-kind =” article-image “id =” article-image-79397 “src =” “/><figcaption class=A. intermedia and P. damicornis corals ready to be sniffed. Courtesy of Caitlin Lawson

Kenkel describes the smell of live corals as mild and salty, like fresh oysters. Things get weirder if the coral has bleached or if it sits out of the water and produces mucus. Taking coral out of its natural environment is a stressor in itself, so when someone smells a bump – a fragment no bigger than a couple of polyps – they’re already struggling a bit. “Whenever I can smell them, it’s when they’re out of the water or when I start to stress them out,” Lawson says.

The compound most commonly associated with corals is dimethyl sulfide, which is generated by phytoplankton and released into the atmosphere. The smell is often compared to rotten cabbage. “It’s not the most pleasant smell,” Lawson says. “If you’ve walked on a beach at low tide when there’s algae exposed, it’s pretty similar to that. But the rotten cabbage is probably a little rough. “

To measure the ways in which the gases in the samples changed under heat stress, Lawson and his collaborators used tanks containing freshly collected healthy samples, and then increased temperatures by 4.5 degrees for five days, until the water reached the lowest temperatures. 32 degrees Celsius (89.6 degrees Celsius). Fahrenheit), a temperature at which whitening is known to occur.

In artificially heated waters, P. damicornis and A. intermedia they produced less BVOC than their control sample counterparts, and those that emitted were present in smaller quantities. When temperatures stayed at 32 degrees C, the team noticed a decrease in overall health and even noticed some whitening. As he walked through the lab, Lawson didn’t smell anything different, but that doesn’t mean the olfactory landscape stayed the same. “Yes [the corals are] turning pale, there is a corresponding change in the gases, ”he says. The team published their findings in Biology of global change.

“It’s a fascinating article,” says Bryan Wilson, a marine biologist at the University of Oxford who studies corals. Wilson, who was not involved in this research but advised one of the co-authors on earlier work, appreciates the paper’s focus on “a relatively under-studied component of coral biology” and hopes that future studies will examine key species and species. which are known to be more tolerant of heat.

He’s also excited about the possibility that this research could eventually help scientists diagnose threatened reefs. Lawson hopes to explore if there are specific compounds that are precursors to visible whitening, and that has Wilson really excited. “Biological prediction of bleaching events prior to observed coral stress symptoms is something of a Holy Grail in the field,” says Wilson. Over time, we may not have to wait for corals to turn ghost white to know they are in trouble; With the right tools, we may be able to smell your distress before we see it.