Wherever an animal goes, it leaves behind little pieces of its DNA. Sloughed off skin cells, hair follicles, bits of saliva, and droppings of feces settle into the ground or float around in bodies of water, giving a clue to researchers that a specific animal had been there long after it leaves. Now, scientists have found that such DNA is also floating around in the air—enough for them to detect those tiny traces of wafting genetic material and use them to identify which animals live in an area.
Any DNA that wasn’t taken directly from an organism—like skin cells left on a chair as opposed to ones taken via a cheek swab—is known as environmental DNA, or eDNA. Researchers have been collecting this kind of DNA for about 20 years, from things like permafrost, soil, water, snow, and even honey, says Elizabeth Clare, an assistant professor of biology at York University. While it’s been speculated that environmental DNA could be collected from air, too, the concept hasn’t been tested until recently.
In the Hamerton Zoo Park in England (Clare was previously a senior lecturer at Queen Mary University of London, where she was the lead researcher for this work), Clare and her team used a vacuum pump to pull air through an extremely fine filter, from which they then extracted DNA. By literally pulling DNA from thin air this way, the researchers were able to identify 25 species of animals, including 17 known zoo species, like tigers and lemurs, as well as food for zoo animals, like chicken and fish, and even species that exist naturally in areas around the zoo, like the Eurasian hedgehog.
“The zoo is this marvelous scenario for testing a technique like this,” Clare says. If she had gone to a farm, for example, and detected cow DNA from the air, it wouldn’t be clear where exactly that DNA came from—a cow nearby, manure on the ground, a cow in a field down the road? “But zoos are amazing because they’re these collections of non native species. And if I detect tiger DNA, there’s really only one place that DNA could come from. It’s the tiger. There’s no other thing I’m going to mix it up with in the British countryside. So the zoo is this marvelous place to to test whether or not you can really detect the things that are there.”
Zoos have the added bonus of having confined pens and enclosures the animals stay within. If tiger DNA were detected somewhere other than the tiger enclosure, that shows how far it traveled through the air. Researchers confirmed that environmental DNA could travel at least a few hundred meters.
This work, which was published in the journal Current Biology, happened to coincide with another study, published in the same journal, by researchers based in Denmark who took samples from air at the Copenhagen Zoo. Both teams conducted their research independently, and then learned of the other’s work when their studies were complete (Clare and the other team head, Kristine Bohmann, have worked together before), and then worked to get them published simultaneously.
Along with being a surprising coincidence, it was also a boon for the research—both studies proved this idea of capturing DNA from the air and using it to identify animals works, confirming the science. The Danish study took a different experimental approach, using both a water-based vacuum and blower fans with filters attached, to collect air samples; that another method also worked suggests doing this kind of monitoring might not be as hard as everyone thought it would be, Clare adds. (Another study from December suggests airborne DNA can be used to detect insects, but that work hasn’t yet been peer reviewed.)
Though this work is early, it has promising implications for monitoring biodiversity in the wild. Researchers have already been using environmental DNA found in water to monitor aquatic species; now, with DNA taken from the air, it opens up the possibility of collecting information on terrestrial animals that might be hard to monitor with other methods, like cameras.
“Almost every other method we have of biodiversity monitoring requires the animals to be physically present when you are watching,” Clare says. “So if you have a camera trap and you’re trying to take pictures, the animal has to walk in front of the camera. If it walks behind, you’ll never know it was there. …Environmental DNA is more like a footprint. The animal may have left days ago, but it leaves behind the signature that you can collect.”
This method could potentially be used as an early warning system for invasive species, before that animal is so abundant that it’s physically seen in an area. It could also be used to detect endangered species that are rarely seen, or where it would be unethical for a researcher to capture such an animal to collect its DNA or monitor it directly. Before that happens, though, more work needs to be done to learn how factors like wind, solar radiation, temperature, and weather affect the distribution or accumulation of environmental DNA in the air.
There might also need to be some getting used to this idea that all this DNA is in the air around us. In earlier studies, Clare and her team collected DNA from the air around a mole rat enclosure in a lab. In those experiments, dog DNA appeared in the samples, and the researchers couldn’t work out where it was coming from—until they realized one of the people who takes care of the mole rats also takes care of his mother’s dog, and he must have tracked that DNA in. “It’s quite strange to think that we basically exist in this soup of material,” Clare says, “and some of that material is DNA from all the other animals and people and things that are around us.”