Algae makes a noise when it photosynthesizes, and it sounds like a ping

This reef is much noisier than it appears.
This reef is much noisier than it appears.
Image: AP Photo/Keoki Stender
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Some underwater snapping sounds are obvious—the jaws of a shark, the movements of a pistol shrimp, maybe even a parrotfish having a snack. Now scientists are adding a more surprising culprit to the list: plants. Incredibly, photosynthesis seems to have a snap all of its own.

Hawaii-based oceanographers with the US Naval Undersea Warfare Center first heard curious snapping noises while listening to the soundscape of different coral reefs. Healthy ones sounded low and rumbly, replete with the flatulent grumblings and mumblings of fish or other large animals, while degraded reefs were higher pitched—pings, snaps, and crackles.

At first, the snaps were chalked up to shrimp, just doing what they do. But oceanographer Simon Freeman, working with his spouse Lauren, had his doubts. “There seemed to be a correlation between the sound and the proportion of algae covering the seafloor,” Freeman told Hakai magazine. To test this correlation, the oceanographers tested red algae on its own in “barren” tanks, without the aural distractions of other organisms. Again, they heard the high-frequency snaps.

But how? Algae doesn’t move, and has nothing to snap with. Moreover, the oceans are so noisy—crashing waves, hooting boats, farting fish—that it’s hard to understand how these underwater wallflowers could possibly make themselves heard. The answer, writes Sarah Keartes in Hakai magazine, is bubbles.

When algae photosynthesizes, much like trees and plants above sea level, it performs an incredible equation: sunlight and carbon dioxide in, oxygen and energy out. That oxygen takes the form of myriad tiny bubbles, cascading up to the surface like fizz in a champagne glass. When each of these little bubbles separates itself from the plant, it makes a distinctive snapping noise: ping.

The researchers have since published their work in the journal PLOS ONE, where they propose using these pings as a way to monitor the health of reefs—an overabundance of algae is usually a bad sign. “Right now, reefs are evaluated visually by divers,” Simon Freeman told Hakai. But the cost of such evaluations often limit their scope. “In the future, it might be possible to quickly listen to a coral reef soundscape,” he said, “perhaps by using an autonomous vehicle, and evaluate how it may have changed from the previous year.”