The ocean is a noisy place. If it’s not trendy humpback pop music and chatty narwhals, it’s killer whale death screams and ominous communications from the abyss – not to mention the terrifying acoustic war waged on marine life by humans every day.

But according to scientists, there’s another player in this oceanic orchestra – and it’s something so fundamental to life you could easily forget it was there at all: photosynthesis.

“My goodness we were surprised,” lead researcher Simon Freeman told IFLScience. “Pervasive natural phenomena can be so subtle, sometimes.”

Freeman, along with fellow US Navy oceanographer and spouse Lauren Freeman, first heard the high-pitched ‘click’ of the algae’s photosynthesis back in 2015, while listening to coral reefs off the coast of Hawaii.

Now, eavesdropping on these beautiful but not exactly sprightly structures isn’t as strange as it may seem. The noise coming from a coral reef, Freeman explained, can actually tell you how healthy it is – flourishing, protected reefs feature the low-frequency rumblings of animal life, while degraded reefs have a higher soundtrack, filled with snaps, crackles, and, yes, pops. But while the pair could pinpoint this noise, they couldn’t tell what was behind it.

“We wondered, what mechanism was causing the sound?” said Freeman. “There seemed to be a loose correlation between the sounds and the proportion of algae covering the sea floor.”

To test their algal observation, they designed a simple experiment, described in a paper published in the journal PLOS ONE. They kept some red algae in a tank, free from the flora and fauna it would share its natural environment with, and recorded it with a hydrophone. Sure enough, they heard the tell-tale high-pitched clicks of a coral reef struggling to survive.

“We've gone out snorkeling many times and now often notice bubbles on algae,” said Freeman. “I've been shown videos of Mediterranean sea grass beds bubbling vigorously in the Sun... Why did we not notice this phenomenon acoustically before?”

For those of us who haven’t been in a biology classroom for a while, photosynthesis is the process plants use to turn water, carbon dioxide, and sunlight into energy. But the system is not perfect, and for every molecule of glucose the plant makes, it also creates six pretty useless (if you’re a plant) molecules of oxygen. And it’s this oxygen, it turns out, that’s the clicking culprit.

“All the sound is created when the bubble is released from the algae,” Simon explained. “Each bubble 'rings' in a quiet, short 'ping' sound as it detaches from where it was formed … The ringing occurs at the natural resonance frequency of the bubble, which is inversely proportional to the radius. Just by listening, it is possible to accurately estimate the size of each bubble by ear!”

With their discovery, the researchers hope to help fellow oceanographers monitor the health of coral reefs – a key barometer of climate change. Although this monitoring is very important, current technology makes it extremely difficult, Simon explained.

“Right now, reefs are evaluated visually by divers – a time-consuming process that cannot cover large areas due to the enormous expense involved,” he said. “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.”