Fracking has mostly been studied because of what it can potentially do to the surface environment. The chemicals used in fracking fluid, as well as the gas and brines that can come back up the wells, all pose environmental risks that have to be managed. What's often not considered is that the well is a bit like a two-way street. The fracking fluid, which is anything but sterile when injected, also contaminates the environment deep under the Earth's crust.

A new study, released this week by Nature Microbiology reveals that fracking creates an entire ecosystem 2.5km below the Earth's surface, one that can persist for at least a year after the frack. And the microbes that thrive there may actually have implications for the production and durability of the fracking wells.

The people behind the work (14 of them at three different institutions) took a relatively simple approach: sample the fracking fluid at a couple of wells before it's sent underground, then sample the fluid that comes back up the wells at various time points, including over 300 days after the fracking. The sampling included a look at the organic chemicals in the fluids, and DNA sequencing that's sufficient to reconstruct entire genomes from anything present.

The chemical analysis revealed that the fracking fluid that's injected actually contains chemicals that can act as energy sources for microbes, including sucrose and ethylene glycol. After the fracking process is complete, some of this fluid comes back up the wells, but it's typically quickly displaced by the salty brines that are common deep below the surface. But those brines were accompanied by molecules that are typically produced by salt tolerant bacteria and archaea (more on that below).

That's where the DNA analysis came in. After fracking, the authors were able to detect the presence of at least 29 different species of microbes in the fluid coming back up the wells. Over time, the populations changed; by 80 days after fracking, the population largely consisted of just six salt-tolerate bacterial and archaeal species. At least one of these is genetically distinct from its previously characterized relatives; the authors propose the name "Frackibacter" for its genus.

At least one of these species was clearly present in the input fluid, despite the fact that a biocide was added to it. Two other species that the researchers looked for were not present in the input fluid, although the authors suggest that this is because they were rare enough to be below the detection limit. It was only after they reached the deep brine that they could thrive.

The species that thrived are all salt tolerant. In fact, some of the biological-signature chemicals found in the fluid that came back out of these wells are used by microbes to adapt to high-salt environments. Meanwhile the nutrients in the fracking fluid seemed to be metabolized quickly and were gone by 80 days. The authors suspect that, by this point, the community was surviving by feeding on the chemicals used for salt tolerance.

There was just one hitch there: those chemicals are normally produced and kept inside cells. How'd they get out into the environment to provide food? The answer, it seems, is viruses. There was evidence of over 300 different viruses in the genomic sequence, with the prevalence of different species changing over time. And there was evidence in the genomes of the microbes that they were getting exposed to viruses as the ecosystem developed. The viruses spread by bursting open cells, which the authors suggest could ensure that enough nutrients got released back into the ecosystem to allow the surviving microbes to feed.

Overall, there's probably not a lot going on chemically in these brines, so the resulting ecosystem may not survive in the long term. But that doesn't mean there can't be short-term consequences. The bacteria could produce organic acids and sulfides that could react with the steel in the well hardware. And the species present had the potential to form biofilms, which could potentially interfere with the movement of materials within the wells or in their vicinity. Either of these could pose problems for the production of these wells.

Still, the whole thing is an impressive testament to life's resilience. The fracking fluid contained chemicals that should kill of any microbes, and it's injected into an area with crushing pressures and salt concentrations that would kill off the majority of species on our planet's surface. Yet somehow species that are so rare as to not be detectable end up forming an ecosystem that persists for at least a year.

Nature Microbiology, 2016. DOI: 10.1038/NMICROBIOL.2016.146 (About DOIs).