The world’s glaciers and Greenland’s ice sheet are melting, and fast. The situation in Antarctica is less clear, but scientists generally agree that the continent is losing ice. And as the planet continues to warm, these vast and long-frozen regions will continue to thaw and spill water into our rising seas. Researchers are rushing to answer crucial questions, such as figuring out the dynamics involved in the melt of these giant mantles of ice. Now, a team of scientists has developed a new method to study the process—by using synthetic DNA to track thawing ice.
Hydrologists and glaciologists have traditionally used fluorescent dyes, or salts such as sodium chloride, to track water systems including melting ice. But such tracers have limitations, especially in a place like the Greenland ice sheet. When researchers drop them in a location, some of the dyes and salts are swept downstream as intended. But portions of them often get stuck at some point in the environment. That makes research harder and more time-intensive, because the dyes and salts do not have unique signatures; they can get mixed up with previous or future doses of the tracers. “We don’t have many tracers that are unique, that you can use in the same location,” says Helen Dahlke, an associate professor in integrated hydrologic sciences at the University of California, Davis. “And if you have traces [left over] from a previous experiment, that really complicates the story.”
But Dahlke and her team from U.C. Davis and Cornell University realized that they could make distinctive tracers—with DNA. “Using DNA, you have so many different combinations of the base pairs that you can really make millions of different tracers, and each one has a unique identifier,” Dahlke explains. “You can apply hundreds at the same time, and you can still distinguish them in time and space.” For the tracers, her group developed synthetic single-stranded DNA molecules, about 80 to 100 “letters” long and with random sequences. These DNA strings go into capsules made of a biodegradable plastic, the same type used to contain medicine swallowed by humans—but the capsules used here are microscopic, no bigger than the size of an E. coli bacterium. To prevent bacteria or other organisms from incorporating the tracers into their own DNA, the researchers keep the sequences relatively short and test databases to ensure they are not using a known sequence from other microbes. “It’s not a genetically functional code,” Dahlke told an audience of scientists this month at the American Geophysical Union conference in New Orleans, where she presented the new technique.
Using this method, scientists would drop a huge batch of tracers—trillions of capsules with an identical DNA sequence—to ensure they are concentrated enough to be found downstream. They might drop a single batch of one unique tracer in a single location, or inject several batches of different unique DNA tracers into various melt spots on a glacier or ice sheet. The protective capsules do not dissolve quickly, so researchers could sample water emerging from a glacier, take it to a lab and melt the plastic capsules away, then sequence the DNA present in the sample. With this information they can figure out where the water originated, and when it flowed away from the melt spot and arrived at the glacier’s exit. Such data could help scientists estimate other important factors, such as the flow pathway and how much meltwater a glacier holds.
This kind of information is very valuable, Dahlke and other experts say. “We still have fundamental questions about how meltwater flow in glaciers [and ice sheets] is working,” Dahlke says. “For example, surface meltwater lakes are forming and spontaneously draining, and no one has come up with a good explanation why … or where that water is going.” She notes that while researchers sometimes climb down into glaciers via vertical shafts called “moulins” to see where surface water flows, they can only go so far. And they risk getting crushed.
DNA tracers could transcend this limitation, and also make scientists’ work safer. Plus, since the tracers contain unique codes, researchers would not have to wait to inject them as they have to do with traditional tracers. This could help cut down on time and resources needed for an experiment, and could give glaciologists more accurate information. “Glaciers evolve over a melt season,” Dahlke says, adding that if researchers can drop their tracers over a shorter period of time, they will get more consistent data about how a glacial system works.
So far Dahlke and her team have tested their DNA tracers on a northern Sweden glacier named Storglaciaren, on Wolverine glacier in Alaska and in small streams in New York state. They have also tried the technique at a hill at the Sierra Foothill Research and Extension Center in California, and in lab experiments. Dahlke says scientists could also potentially use this method to better understand questions relevant to human health—such as how pathogens and pollutants are transported through water and soil. A single batch of DNA tracers currently costs about $23, according to Dahlke.
Regine Hock, a professor of geophysics at the University of Alaska, Fairbanks, who was not involved in the new work, says such a tool could be useful for researchers like herself. “The tracer seems extremely promising. It overcomes many of the issues other tracers [such as the dyes and salts] have. It is ‘clean,’ not visible and obviously cheap to produce,” Hock wrote in an e-mail responding to questions from Scientific American. “Therefore it offers new opportunities to investigate the largely inaccessible hydrological system of glaciers and ice sheets.” Asa Rennermalm, an associate professor of geography at Rutgers University, agrees. Rennermalm, who was also not involved in the research, studies hydrology of the Greenland ice sheet—specifically, streams on the surface of the ice sheet, as well as ones that drain out of it. “For those of us who study ice sheets and glaciers, a key question we’re concerned with is how water is flowing in and underneath the ice,” Rennermalm says. “This technique could be really important in understanding that question.”