Author: Carrie Arnold / Source: WIRED
Just past the intersection of Centre and Locust in Centralia, Pennsylvania, the microbiologist Tammy Tobin turned the wheel of her aging Prius sharply to the right. As the windshield wipers whipped furiously back and forth to fend off the driving sleet—a reminder that winter had yet to bid farewell—Tobin announced, “We’re here.” We were at the base of a grassy slope nestled behind the SS. Peter and Paul Cemetery. It looked like any of the other countless knolls tucked in the anthracite hills of eastern Pennsylvania. But almost 50 meters beneath our feet lurked a hidden menace. Centralia was burning.
Or rather, the coal seam under what used to be the town of Centralia was burning. The coal has burned for more than 50 years and will likely burn for centuries to come. As we climbed a low rise that hugged the back of the Catholic cemetery, no flames were visible, only puffs of steam where the dirt exhaled excess heat and the grass stubbornly refused to don its icy cap. All but a handful of the town folk had fled when the government revoked Centralia’s postal code in 2002. But Tobin, from Susquehanna University 30 miles west of Centralia, wasn’t here to comb through the wreckage of a once-thriving town.
Instead, she and a group of collaborators had set their sights on something much smaller. The heat and pollution from the underground fire wasn’t just stressful for Centralia’s flora and fauna; it also created a crisis for the area’s microbes. The trillions upon trillions of microscopic single-celled organisms at home in Centralia’s soil suddenly found themselves living in a veritable sauna. It was adapt—or die. Or so scientists thought.
“Centralia is a beautiful sandbox for asking about what happens during an environmental disturbance,” said Ashley Shade, Tobin’s former student, now a microbiologist at Michigan State University and a collaborator on the project. “Even when that disturbance is like a sledgehammer.”
The coal-seam fire at Centralia provides researchers with the perfect opportunity to test a new idea known as a microbial seed bank: that commonly overlooked dormant individuals make up a vast reservoir of biodiversity, ready to spring to life when environmental conditions change. Although scientists had found hints from laboratory and environmental experiments that such a seed bank exists, Centralia represents a rare opportunity to see whether and how a microbial seed bank functions in the real world.
900 Degrees Fahrenheit on the Ground
No one knows exactly how the fire under Centralia started; local legend holds that somebody accidentally ignited the seam while burning trash just outside one of the mine shafts. What is known for certain is that, shortly before Memorial Day in 1962, residents of Centralia reported that a fire had started in the town’s coal mine just east of the Odd Fellows Cemetery. It soon became obvious that even the most aggressive methods wouldn’t stop the spread of the flames. Residents would simply have to wait for the fire to burn itself out. But in an area that dubbed itself “coal country,” there was no lack of underground material to burn through, so the fire outlasted the people. Although residents initially hoped that, because the fire was entirely below ground, they would be able to continue living in Centralia, the release of toxic gases and the opening of sinkholes made it too dangerous.
Most families left by choice or were bought out by the government. A few families, brave or foolhardy (take your pick), continue to live in Centralia, fire be damned. Although Centralia may have had the sharpest reversal of fortune, the economy of the entire region has gone from bad to worse over the past several decades.
Ashley Shade is all too familiar with the travails of central Pennsylvania. She grew up a stone’s throw from Centralia, and although she knew about the fire—you couldn’t really live near Centralia and not know about it, she says—she never really gave it much thought. It wasn’t until her first genetics class as an undergraduate at Susquehanna University in 2002 that she began to think of Centralia as something more than a nearby oddity. The year before, a team of geologists and soil scientists at Susquehanna had approached Tobin, who was Shade’s professor at the time, about setting up a formal study of how the fire was changing Centralia. They asked Tobin if she would help study the soil microbes in Centralia. Although she knew nothing about microbiology, she found the topic quirky and interesting, and so she agreed. She asked her students in 2002 whether any wanted to join her new project in Centralia or an existing study on bovine genetics.
Both Shade and Tobin immediately fell in love with Centralia. The team staked out a range of sites spanning three contrasting areas: one above a spot where the fire had never been, one above where the fire was currently burning, and one where the subsurface flames had already come and gone. This would give the researchers an idea of how the soil microbes changed over time. Some never-burned sites were especially important because the fire was moving in that direction. Tobin and her fellow scientists could track what happened to the soil in real time.
Seventeen years ago, when sequencing the genomes of large numbers of environmental microbes was prohibitively expensive, studying the genetics of soil microbes meant scientists would chop the DNA into small pieces. Each different species of microbe yielded a collection of genetic fragments that could be sorted by size. Using a probe to highlight ribosomal DNA sequences unique to each species, scientists could derive a genetic fingerprint for a microbe and identify its species by comparing their results to a large database of known prokaryotes. Although this “ribotyping” was more time-consuming and less precise than current molecular methods, it nonetheless provided Tobin and Shade with the first clues about what, if anything, may have survived Centralia’s below-ground inferno.
“A place could go from being cool to being very hot pretty quickly, and it fluctuates with all kinds of climatic and geological factors,” Tobin said. “Could things adapt quickly enough?”
Depending on how much oxygen could reach the fire, the flames under Centralia could burn as hot as 1,350 degrees Fahrenheit, and ground temperatures sometimes exceeded 900 F. In 2007, a German documentary film crew bought a single egg from a local cafeteria so they could fry it by a steam vent and eat it for breakfast as an on-camera gimmick. However, the egg didn’t fry. Instead, the soil was so hot that, with a quick sizzle and hiss, the egg charred beyond recognition before the crew could frame their shot, leaving nothing for their toast or their viewers. Under such extreme conditions, Tobin told me as we wound our way along the Pennsylvania byways from her lab at Susquehanna to Centralia, it was entirely possible that nothing had survived. To her delight, she was wrong.
In a 2005 study in Soil Science, Tobin and colleagues showed not only that microbes survive in the soil above actively burning areas, but that some species thrive there. The overall level of diversity was the same in hot areas (with temperatures ranging between roughly 90 F and 170 F) as in areas that the fire had yet to reach. When the researchers looked more closely, they found that although the overall bacterial diversity decreased with higher temperatures, even the hottest samples apparently still held thriving microbial communities. Shade and Tobin also identified heat-loving bacteria (thermophiles) that resembled microbes living near geothermal hot springs in Iceland, though their data wasn’t detailed enough to say just how closely the organisms were related.
What their data couldn’t tell them, however, was whether the microbes living over the fire had lurked there all along in very low numbers or whether they had been blown in or had otherwise arrived from afar, perhaps from other geothermal areas around the globe. It…
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