Scientists have been recording the cacophony generated by creatures living on coral reefs. And it turns out that amongst all the noise is the sound of seaweed.
Oceanographer Simon Freeman, from the United States Navy describes a coral reef soundscape as a “busy and bustling place.”
If you put your head in the water on a tropical coral reef, you’ll hear what he means - an entire orchestra of animals that sounds like heavy rain, or perhaps a pan of frying bacon.
The loudest section in the coral reef orchestra belongs to the snapping shrimps.
Then there’s the seaweed section.
I know what you’re thinking. Noisy seaweed? How does that work? What are they trying to say?
We’ll get to that in a minute, but first up, some context about sounds on a coral reef.
Simon says that most of the sounds are what scientists called ‘transients’ – “pings, pops and clicks. And many thousands of those together combine to form the coral reef soundscape.”
“What’s special on a coral reef is that each transient conveys information about its producer,” says Simon.
“So the frequencies in each sound, the duration, the rise time and fall time – all of those pieces of information give us evidence for what the sound producer may be, what it might be doing and its characteristics.”
Lower frequency sounds are produced by larger animals. Research by Simon and fellow US navy oceanographer – and wife - Lauren Freeman has shown that “when you have a healthy reef with larger animals on it, the soundscape has a larger low frequency component.”
Lauren says that this study also revealed that each reef had its own distinct acoustic signature, which also indicated its overall health.
“That’s when we got our first clue that there might be a sound signature to algae,” says Lauren.
“The most degraded reefs also have the most algae, and they have this curious high frequency acoustic signature that we only noticed during daylight hours.”
Marine algae photosynthesise during the day, and produce oxygen bubbles, which Simon and Lauren speculated might be responsible for the noise.
Enter marine acoustician Giacomo Giorli, now with NIWA in New Zealand.
With his help, they set up an experiment in a lab in Hawaii, using a common invasive seaweed called Gracilaria salicornia. They suspended a very sensitive hydrophone or underwater microphone in a tank of water containing only Gracilaria and began recording.
During the day, as lots of small oxygen bubbles appeared on the algae and floated to the surface, they recorded a high frequency sound. They could even identify very short high clicks that were correlated with individual bubbles.
And at night, when the seaweed stopped photosynthesising? Nothing.
So, how does a bubble make a noise? The answer is physics.
“After the bubbles reach a certain size,” says Simon, “the buoyancy force overcomes the surface tension force and the bubble will separate from the body of the algae. When it does that the bubble becomes momentarily non-spherical, and that creates a perturbation. As the bubble becomes spherical again it oscillates in pressure and volume … and it rings.”
Different sized bubbles ring at slightly different frequencies.
“[It] will ring at a very high frequency if it is a small bubble, and that frequency is proportional to the radius.”
“Lots and lots of bubbles, going off at a very similar time,” says Simon, “will create a cacophony that is audible even above other biological noise.”
Lauren says this is the first time that research into the acoustic properties of bubbles has been applied to marine algae, and the team’s results have created a lot of interest amongst marine biologists.
The collaborators speculate that in future it may be possible to tell the health of coral reefs, temperate marine algae forests or seaweed farms by the sound they are making.
Photosynthesis by marine algae produces sound, contributing to the daytime soundscape on coral reefs by Simon Freeman, Lauren Freeman, Giacomco Giorli and Andreas Haas was published in the journal PLOS One.