Hakai Magazine

Coastal science and societies

Photosynthesizing algae produce bubbles, which make a sound that could one day be used to acoustically assess the health of marine ecosystems. Photo by Lee Rentz/Alamy Stock Photo

Photosynthesis Makes a Sound

The ping of algae turning sunlight into energy adds to the soundscape of marine ecosystems.

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by Sarah Keartes

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Teasing sounds apart in an underwater habitat is like trying to listen for the crackle of a single street light in the middle of Times Square. So when scientists thought they heard the sound of algae on a coral reef, their hunch was met with some skepticism. How could something seemingly so sedentary ring out over some of the ocean’s chattiest (and most flatulent) inhabitants, sloshing waves, and noisy humans? With bubbles, it turns out.

Like the plants that help us breathe, algae also photosynthesize. Underwater, that process of converting sunlight and carbon dioxide into energy and oxygen sends tiny bubbles spiraling toward the surface. And according to new research, when each bubble detaches from the seaweed, it goes ping. The scientists behind the discovery suggest that, like a heartbeat heard through a stethoscope, measuring that unique sound could be a new way to monitor the health of a coral reef.

Spouses Lauren and Simon Freeman, oceanographers with the US Naval Undersea Warfare Center in Rhode Island, first noticed the strange pings in the Hawaiian Islands. They perceived that the soundscape of healthy, protected reefs was dominated by low-frequency sounds (the kind typically made by fish and other large animals), while degraded reefs were noticeably higher pitched.

Though slightly buried under static, the distinctive click or ping sound caused by a photosynthesis-produced bubble can be heard 1.5 seconds into this clip. Audio courtesy of Freeman et al.

“We were told the sound was from snapping shrimp, end of story,” says Simon. “[But] there seemed to be a correlation between the sound and the proportion of algae covering the seafloor.”

Determined to dig deeper, the Freemans and their colleagues housed red algae in tanks devoid of clamorous crustaceans or other animals. The sounds they picked up matched the high-frequency sounds of struggling reefs.

The discovery is an interesting addition to the field of acoustic ecology, a discipline that has typically focused on larger animals such as whales and dolphins, says Erica Staaterman, an acoustic ecologist who was the first to identify sounds made by baby fish and was not involved in the new research.

“When I put a recorder in the water, I’m usually surprised by some cool new fish sound that I’ve never heard before,” says Staaterman. “There’s so much to discover.”

Staaterman is intrigued by the potential applications of monitoring the algae sounds. On coral reefs, algae help produce oxygen. But their prevalence isn’t always positive: if cover grows too thick, the algae can smother sensitive species like corals and even cause a spike in harmful microbes. With some fine-tuning, the researchers’ hope is that eavesdropping on algae could alert scientists to problematic changes in reef composition.

Oceanographer Simon Freeman says photosynthesizing algae in aggregate sound like fizzing champagne. Video courtesy of Freeman et al.

“Making these kind of links between bioacoustics and biodiversity is an exciting field with a lot of promise,” Staaterman says. Acoustic monitoring is already being used in Australia to monitor vital seagrass habitat.

“Right now, reefs are evaluated visually by divers,” Simon says. The process is time-consuming and expensive, which limits the scope of each survey. “In the future, it might be possible to quickly listen to a coral reef soundscape, perhaps by using an autonomous vehicle, and evaluate how it may have changed from the previous year.”

In the ocean’s orchestra, each instrument plays a role. Like the humble triangle, the score just wouldn’t be the same without the subtle ping of the algae.