Marine scientists using underwater microphones have managed to eavesdrop on the sound of photosynthesis.
The process works by measuring the rate at which underwater plants such as seagrass, kelp and other algae release tiny bubbles of oxygen, created as a byproduct of photosynthesis.
At low rates of photosynthesis, marine plants simply release dissolved oxygen into the water. But at higher rates, oxygen collects on the leaves as bubbles. Eventually, these grow large enough to break loose and rise to the surface, where they reach the air without dissolving.
Traditional measurements of dissolved oxygen miss these bubbles and therefore underestimate the rate of photosynthesis in a seagrass bed or kelp forest — an important omission for climate scientists wanting to use oxygen production as a proxy for the rate at which marine plants removes carbon dioxide from the water and, by extension, from the atmosphere.
But that doesn’t have to be the case. It is also possible to use a sonar-like process to monitor the rate at which the bubbles are released, Jean-Pierre Hermand, an acoustical oceanographer at the Free University of Brussels, Belgium, reported this week at a meeting of the Acoustical Society of America in Minneapolis, Minnesota.
“Physical acoustics is very sensitive to the presence of bubbles,” he says, adding that cardiologists use the same effect when they inject microbubbles into the body, then monitor them by ultrasound to determine blood flow through constricted arteries.
Better yet, says Hermand, it’s possible to use this process to monitor oxygen production over the length or breadth of an entire seagrass meadow. “The longest I have done is 1.5 kilometres,” he says.
Seagrass, he adds, is important because it is not only a major player in both marine ecology and the Earth’s climate cycle, but because it has also been declining.
“Over the last century, about 30% has been destroyed,” he says.
In 2010 to 2011, he says, a heat wave destroyed about one-fifth of Western Australia’s seagrass, reducing its ability to remove carbon from the atmosphere by nine million tonnes per year, “which is enormous.”
In addition, carbon previously sequestered by the dead seagrass, accumulated in roots and sediments, will gradually decay and escape.
“The estimation is that in the next 40 years there will be about 21 million tonnes of carbon released into the atmosphere,” Hermand says.
Similar issues apply to kelp, the oxygen production of which can also be monitored by acoustics.
“In Tasmania, the [ocean] current pattern has changed, so there is a significant impact on the ecology of the kelp forests,” explains Hermand.
In another presentation at same conference, Simon Freeman, a US Navy oceanographer, reported preliminary results (now under review for publication) showing that with sensitive-enough microphones, it may be possible to truly listen in on the release of these oxygen bubbles without the need for beaming them with sonar-like signals.
Freeman’s interest is in monitoring the health of coral reefs — for which the appearance of algae is a death knell. “Once a reef has been taken over by algae, the ecosystem is in an ‘alternate stable state’,” he says. “The coral will never come back because the algae is established and makes it almost impossible for coral to return.”
A couple of years ago, he adds, he and his wife (also part of the team on his present paper) spent six months in Hawaii, using acoustics to study coral reefs at 23 different sites. One of the things they found was that healthy reefs were dominated by low-frequency sounds, while unhealthy ones (where the coral was dead and replaced by algae) were dominated by high-frequency sounds.
Intriguingly, the high-frequency sounds occurred disproportionately during the day, not at night.
Eventually, the two Freemans and a team that included colleagues from New Zealand and The Netherlands formed a hypothesis: what they were hearing was a sound created by the release of oxygen bubbles from the algae that had invaded the unhealthy reefs. As these bubbles broke free from the algae, they deformed then vibrated, ringing like bells as they rose toward the surface.
To test the hypothesis, they put algae in a tank, illuminated it with grow lights and carefully listened to the sounds of the bubbles it produced.
The result matched what he’d heard in Hawaii. That suggests that listening for similar sounds could be a simple way of monitoring the health of other reefs, such as Australia’s Great Barrier Reef, where recent “bleaching” crises appear to have killed a great deal of coral, allowing algae to invade.