Can Tiny Bubbles Help Save the Planet?
Seagrasses store a lot of carbon in their tissues, making them a potential counterweight to rising levels of atmospheric CO2.
Seagrasses are more efficient at storing carbon in the soil or sediment, acre for acre, than a tropical rainforest. That could make them a powerful tool for slowing the rapid rise of atmospheric carbon dioxide. The ability to quantify how much carbon a specific seagrass bed stores over time could help governments, businesses and environmental groups better manage these natural carbon sinks. With funding from federal agency APRA-E, Ken Dunton, a marine biology professor and Preston Wilson, an engineering professor may have found one weird trick to measuring carbon storage in seagrass beds: listening to the sound of tiny bubbles.
With current technologies, being able to accurately measure how much carbon a seagrass bed stores from year to year takes a lot of time, people and money. It requires going out and physically digging up plants and sediments and bringing them back to the lab and spending days analyzing them—and doing this repeatedly over time.
Ken Dunton (left) discusses a device for measuring water chemistry with team members at East Flats, a seagrass meadow near Port Aransas, Texas. Photo credit: Marc Airhart.
The new method Dunton and Wilson are developing relies on a simple idea: As seagrasses turn sunlight into energy, they absorb carbon dioxide from the water, store the carbon in their roots and other tissues and release the oxygen back into the water, some in the form of bubbles. The more bubbles a seagrass plant emits during the day, the more carbon it stores. By continuously measuring the sound intensity, they can infer how much carbon is stored over time.
Researchers use a vibrating motor and plastic tube to drill a sediment core at East Flats, a seagrass meadow near Port Aransas, Texas.. Photo credit: Marc Airhart.
Preston Wilson at East Flats, a seagrass meadow near Port Aransas, Texas. Photo credit: Marc Airhart.
Kevin Lee (left) holds a sediment core and discusses acoustic measurements with Megan Ballard at the UT Marine Science Institute in Port Aransas, Texas. Photo credit: Marc Airhart.
TRANSCRIPT
Marc Airhart: In the island town of Port Aransas, near the University of Texas Marine Science Institute, you see visible signs of an energy industry that the Lone Star State is known for. Oil rigs and windmills both loom large. But here, and in coastal places like this, resting just out of sight, there’s a powerful tool for addressing some of the biggest environmental problems linked to the energy sector. That tool? Seagrasses.
Ken Dunton: Most of the biomass of a seagrass plant is below ground. That’s where all the tissues store carbon.
MA: That’s Ken Dunton, a UT Austin marine biologist who has studied the ecology of seagrasses for decades. He says that, unlike land plants, about 80% of seagrass biomass is below the sediment, in roots and rhizomes. This matters for our planetary problem with carbon dioxide, which is accumulating in the atmosphere 100 times faster than back at the end of the last Ice Age—largely due to our burning of fossil fuels like gasoline, coal and natural gas. It turns out, seagrasses are incredibly good at taking carbon from the air and storing it in the sediment. Acre for acre, in fact, they are more efficient at storing carbon than even tropical rainforests. I’m Marc Airhart, and this is Point of Discovery, the podcast that takes you behind the scenes of science and technology.
MA: As seagrass grows in the spring and summer, Dunton told me, it’s “breathing in” a lot of carbon dioxide.
KD: … and all that carbon stays in the sediments. It doesn’t go anywhere. It’s trapped.
MA: Then comes the fall and winter. And just like leaves falling in a forest ...
KD: …in a grass bed, all those leaves die during the fall period, during the winter, and all that ends up in the sediments. And you know, in a productive grass bed, all that sediment now becomes enriched in carbon and eventually buried for anywhere from years to decades or more. And this is what we’re investigating: how much carbon is in the sediments?
MA: With current technologies, being able to accurately measure how much carbon a seagrass bed stores from year to year takes a lot of time, people and money. You might ask, why bother? It requires going out and physically digging up plants and sediments and bringing them back to the lab and spending days analyzing them.... doing this over and over, because it cannot be done just once. Well, the big idea – the real game-changer—would give businesses and whole countries, even, a new excuse to start investing a whole lot more in seagrasses. They would need a way to gauge, easily and precisely, the impact seagrasses have on mitigating and storing carbon. Picture a running meter on underwater carbon: where otherwise-polluters could secure credits as they tally up hundreds of millions of tons of carbon emissions they’re removing from the atmosphere and storing away for good, with the help of seagrasses. We’re not there yet, but Dunton and his team are hard at work on a new tool to measure carbon storage in seagrasses in a much easier way: by listening to the sounds of tiny bubbles.
KD: … and if we can do this using acoustics, by just measuring the sound propagation in water through the grass bed, and obtain those types of indices or metrics remotely with much simpler instrumentation, that would be a big plus.
MA: If it works as planned, their new method would be simpler, cheaper, non-destructive and provide information autonomously, 24/7. That could pave the way for the carbon meter, showing just how much carbon seagrasses are storing.
MA: First, let’s examine the current state of play for these measurements: that is, the hard way to measure carbon stored by seagrasses. I’m with members of Dunton’s field research team, as they pull up long, thin samples of sediment to bring back to the lab. This part of the project takes about five people. They’re holding this clear plastic tube that’s about 4 feet long.
Preston Wilson: It’s called a vibracore. It helps penetrate into the sediment.
MA: That’s Preston Wilson, a UT engineering professor who’s co-leading the acoustics project. We’re standing on squishy mud, in waist-high water, wearing booties to protect our feet from sting rays, crabs and clamshells. Long, slender blades of green seagrass wave like fingers from below. On top of the tube, held by an engineer named Andrew McNeese, there’s an orange box that makes the whole thing vibrate.
Andrew McNeese: It’s essentially like when you take a straw and put it in a cup of water and put your thumb on top and pull it up and create the suction …
MA: Now it’s time.
Megan Ballard: Ready?
MA: Three people are pushing down on the vibrating tube. They’re putting their full weight on it, but it’s going down pretty slow.
PW: Want another set of hands in there? [
Female voice: Come on!
MA: Now there’s four people pushing down.
MB: There it goes!
MA: It’s pretty physical. A couple of minutes later …
Male voice: Alright, stop.
MA: Now they’re pulling the core up and hauling it onto the boat.
MA: So how did that go? Was that typical?
AM: For this site, yeah, it took a bit to get it down. But like we said, from site to site, some of them are soft mud that it goes in easy. And then you get some that have shell hash and other things [that make it] more difficult …
MA: The team collects several more cores and brings them back to a wet lab at the Marine Science Institute, where they’ll get something akin to a very thorough workup at the doctor’s office—these samples are like the patient being sent for ultrasounds, blood work, the whole nine yards. First, the researchers create an acoustic profile of the core while it’s still in its plastic tube—basically measuring how sound travels through each 2-centimeter-thick layer in the core from top to bottom. Kevin Lee, from UT’s Applied Research Laboratories, explains how it works.
Kevin Lee: There’s a core that’s kind of standing upright in this framework, and then there’s a pair of transducers that look like headphones that are scanning down the length of the core.
MA: The transducer on one side is basically a little loudspeaker transmitting sounds across the width of the core and the one on the other side is a receiver. They crawl from the top of the core down to the bottom, stopping every 2 centimeters to take acoustic measurements.
KL: And then, from the time delay going from one side to the other, we can get an estimate of the sound speed. And then, from how much the amplitude decreases as the wave travels across the core, we get an estimate of how much energy is lost in the acoustic wave.
MA: Finding out the correlation between how much carbon is present in the sediment and how sound spreads through it is key. So the team will compare the acoustic properties Kevin is measuring with measurements taken later of the actual organic carbon composition.
KL: What we want to learn out of these cores is: “What are the relationships between organic carbon in the sediment and the acoustic properties of the sediment?”
MA: Knowing that will help in building a new system that uses sound instead of core samples to measure carbon storage. Next, it’s time to get the sediment out of its plastic tube and chop it up into little pieces to find out exactly what each layer is made of. So other team members load the tube up on another rack. They squeeze the sediment out by whacking on a metal cross bar at the top with a mallet …
MA: That forces sediment out through a hole at the top, like squeezing a tube of toothpaste. Then they slice off hockey puck-sized pieces.
MA: Megan Ballard — an acoustics expert, also with the University’s Applied Research Lab— is holding a little plastic tray with a dark, gritty disk.
MB: So this is the puck. …
MA: It’s mud and sand and shell and roots and decayed plant and animal matter. Each puck gets weighed and then dried in a little tabletop oven. Through the background noise, she describes the process.
MB: … and then after they’re dry, they take a little sub-sample and they grind that up and we do chemical measurements on them. And we find out how much carbon is in them.
MA: One part of each puck goes into an incinerator to burn off the carbon. By measuring what’s left, they can infer how much carbon used to be there. Another part of the puck is run through a mass spectrometer to determine the ratios of different isotopes. Just like a big, involved medical workup, all of these tests take hours. When they’re done, the researchers will have a good handle on how much carbon is being stored in the seagrass bed. But the whole point of this project is to find the elusive shortcut to replace all this tedious work with listening for the sounds of tiny bubbles. So up next, we’ll talk about their “one weird trick” for measuring carbon storage.
MA: As seagrasses turn sunlight into energy, they absorb carbon dioxide from the water— store the carbon in their roots and rhizomes—and release the oxygen back into the water, some in the form of bubbles. The more bubbles a seagrass plant emits during the day, the more carbon it stores. This inspired Dunton and Wilson to imagine a new method for measuring carbon storage that doesn’t involve digging up seagrasses and sediments. Again, Preston Wilson:
PW: So just like us in air, if I breathe out, you know enough [hahhhh] you can hear me. Well when the seagrass breathes out under water, it makes bubbles, and the bubbles make noise. And so you can prove this to yourself, just go get a drinking straw and a drinking glass and go [blurb blurb blurb] blow through the straw. I mean, that’s literally what we’re doing. We’re listening to the seagrass make bubbles as part of photosynthesis. And the more photosynthesis, the more bubbles. And so the more bubbles, the more sound. And so basically, through that process, we can correlate the amount of sound of bubbles to the biomass of the plants.
MA: The UT researchers have a “high-risk, high-reward” grant from the U.S. Advanced Research Projects Agency’s Energy division—or ARPA-E. The grant is to develop an automated system that continuously records sound intensity using underwater microphones called hydrophones. Sound intensity increases as more bubbles are released into the water.
MA: Here’s what it sounds like at night when seagrasses are not very active …
MA: And here’s how it sounds during the day …
MA: It’s like if you lived near a high school football stadium.
MA: Just from the sounds, you can infer whether a football game is happening, without actually being there in the stadium, and something about the size of the crowd. With seagrass, it’s like respiration is the football game. When you hear the crowd cheering, the sound of each fan is like a bubble of oxygen in the water: you can infer that a corresponding amount of carbon made its way down into the roots and rhizomes in the sediment. The louder the sound, the more carbon stored. Again, Megan Ballard:
MB: The whole goal of the project is to develop a low-cost network of sensors that someone could just put out and then get the data later. And in future versions of the system, we imagine something that can telemeter data back to shore, so that you don’t have to send people out to go download data, data will come to your computer and you’ll know how much carbon is being stored in the seabed.
MA: By recording the sounds of bubbles and making direct measurements of carbon in the sediments and by correlating the sound intensity collected automatically from hydrophones with the labor-intensive “ground truth” data, the scientists can calibrate a mathematical model that says: “Okay, a sound level of X translates to Y amount of carbon stored.” Ultimately, a commercial tool based on this technology could one day help make seagrasses a viable sequestration tool in a future carbon market.
PW: And to make a market, you need a meter, and then the market sets the price, and the meter ticks off how much you pay. So we would be the meter in a future global carbon market that does not yet exist. But if the technology doesn’t exist, it will never exist, right? So ARPA is wanting to develop the technology that could enable this.
MA: That’s our show. Point of Discovery is a production of The University of Texas at Austin’s College of Natural Sciences.
MA: To see photos from this field project, head on over to our website at pointofdiscovery.org. If you like our show, be sure and tell your friends. We’re available wherever you get your podcasts.
MA: Special thanks today to our guests Ken Dunton, Preston Wilson, Megan Ballard, Kevin Lee and Andrew McNeese. Our theme music was composed by Charlie Harper. Our senior producer is Christine Sinatra. I’m Marc Airhart. Thanks for listening!
Andrew McNeese scrapes a grey, hockey puck-sized disk from a sediment core at the UT Marine Science Institute. Photo credit: Marc Airhart.
An acoustic instrument measures how sound travels through a sediment core at the UT Marine Science Institute. Photo credit: Marc Airhart.
Episode credits
Our theme music was composed by Charlie Harper
Other music for today’s show was produced by: Podington Bear
Cover image: Concept for a new way to infer carbon storage in seagrass beds using sound intensity recorded with hydrophones (black). Illustration credit: 5W Infographics.
About Point of Discovery
Point of Discovery is a production of the University of Texas at Austin's College of Natural Sciences and is a part of the Texas Podcast Network. The opinions expressed in this podcast represent the views of the hosts and guests, and not of The University of Texas at Austin. You can listen via Apple Podcasts, Spotify, RSS, Amazon Podcasts, and more. Questions or comments about this episode or our series in general? Email Marc Airhart.
