Termites are upstanding citizens of the world. They are efficient architects, maintain nutrient-rich soils and follow orderly social behaviour. A new finding only entrenches this enviable reputation.
According to Australian researchers, these insects build homes that cut their methane emissions.
Carbon dioxide hogs the limelight for its role in global warming but methane is a bigger and badder greenhouse gas. It traps 34-times more heat than the same volume of carbon dioxide over a century, and drives global temperature rise to a greater extent.
The Intergovernmental Panel on Climate Change recently recommended we restrict global average surface temperature rise to less than 1.5º C above pre-industrial levels. According to Joe von Fischer, a climate change expert and researcher at Colorado State University, Fort Collins,, this won’t be possible if we don’t deal with methane.
“Any reductions in [methane] emissions that we can do now will have an immediate benefit on the rate of warming,” he said.
A major chunk of humanity’s methane footprint comes from mining, transport of fossil fuels, the production of natural gas and livestock-rearing.
Also read: A Method in the Madness: How Termites Build and Repair Their Mounds
Like livestock, termites also emit methane when they break down the the leaves and wood they’ve eaten with their gut bacteria.
While still a topic of debate, termites are thought to account for 1-3% of annual global methane emissions.
This is where the story takes a turn. Termite mounds are each populated with millions of these insects. However, scientists have noticed that the mounds release much less methane than they ought to.
Could they have had help? The idea isn’t far-fetched. Scientists have suspected before that termite mounds could also play host to methanotrophic bacteria, which use methane as a source of energy.
Methanotrophs typically live in the soil. However, it wasn’t clear if the mound-associated methanotrophs lived in the walls, under the mound or inside the termites’ guts.
Termite mounds are porous structures. They’re filled with crevices and tunnels, so it’s difficult to accurately measure how much methane they’re emitting. Most termite studies in the past have used invasive methods: the mound is partly or fully destroyed. This won’t work when you’re trying to track down a gas; it’s just going to escape.
Recently, a team of Australian researchers found a way around this problem. They’d pump equal quantities of two gases – methane and argon – into the mound, and then pump them back out.
“When you inject [the mixture] and then extract it at the same point, you can calculate how much methane disappears on the way by comparing it with the tracer gas,” Philipp Nauer, an ecologist at the University of Melbourne, Parkville, and an author of the study, told The Wire.
This way, Nauer and his team studied 29 termite mounds near Darwin, a city in Australia’s Northern Territory.The mounds were host to three native termite species.
The gas-exchange procedure did reveal that more methane was going into the mounds than was coming out. Ergo, something inside was ‘eating’ the gas.
Next, the team repeated the procedure with a third gas in the mix: difluoromethane (DFM). DFM has been known to disrupt the ability of methanotrophic bacteria to break methane down. This time, when the gases were pumped out, the methane and argon concentrations were comparable. This was a giveaway: methanotrophic bacteria were at work inside the mounds.
The team also ruled out the termites’ gut bacteria were chipping in. They performed the gas-exchange procedure on groups of termites removed from their mounds and found no differences between concentrations of the two gases. “This really shows clearly that methane oxidation doesn’t appear to happen in the termites’ bodies to any significant degree,” von Fischer, who wasn’t involved in the study, said.
The final step was to find out how much methane the methanotrophs were eliminating.
Methane is the molecule CH4. Scientists can use two types of methane and carefully track their respective concentrations to deduce how much of the gas the bacteria were digesting. Each type of methane is made of a different type of carbon atoms, or isotope: carbon-12, the common natural form, and carbon-13, a heavier form that can be manufactured. Importantly, scientists can easily tell them apart.
First, an intact termite mound was dug up and isolated in a gas chamber. Second, carbon-13 methane was pumped into the mound. Third, the mound was left to be for a few hours. In this time, the level of carbon-13 methane fell because it was being broken down. The level of carbon-12 methane increased because the termites were producing it.
From the data so obtained, the researchers established that almost half the methane produced by termites was being consumed inside the mounds.
“It was a very clear demonstration that the mound itself is basically a [methane] filter,” Nauer said.
Sanjay Sane, who works on termites at the National Centre for Biological Sciences, Bengaluru, thought the study’s rigour was “really impressive”.
Also read: Wood Beetles Are Nature’s Recyclers – With a Little Help From Fungi
This study focused on termites that live in above-ground mounds. There are other species that build nests that are completely underground or perched atop trees. They’re likely to be the next subjects of study.
Termites are found mostly in the world’s tropical and subtropical regions. These include India, Northern Australia, Africa and South America. Only a handful of termite species live in colder regions.
They are important parts of many ecosystems. They enrich soils with nutrients, reduce their erosion and increase their porosity, allowing it to absorb more rainwater.
There are over 2,500 termite species around the world and fewer than 10% of them are harmful to humans. And this – in Sane’s words – is the “big tragedy of termite research”, because most termite research “serves the purpose of their elimination”.
One can only hope we think differently of them now that we know the mounds three of their kind – maybe more – provide such an important service.
The study was published in the Proceedings of the National Academy of Sciences on November 26, 2018.