The Kidd Creek Mine, located in Ontario in Canada, is a copper, silver, and zinc mine reaching three kilometers deep, and miners are still digging. This rich vein of metals formed about 2.7 billion years ago along the ocean floor, through extensive hydrothermal vent activity. Tectonic forces tipped the floor vertically, but otherwise the site has remained fairly stable, neither roasted by volcanic activity, nor further deformed. This stability, combined with unprecedented access provided by mining tunnels, make Kidd Creek Mine the ideal place to study groundwaters within a network of fractures that have been isolated from the surface for millions or even billions of years.
A new paper  in Geomicrobiology Journal provides the first evidence that these isolated groundwaters support a small, but persistent population of bacteria. A team of DCO Deep Energy Community members including Oliver Warr, Barbara Sherwood Lollar (both at University of Toronto, Canada), and Magdalena Osburn (Northwestern University, USA) performed careful microbiological, geochemical, and isotopic analyses of the ancient groundwater. The study establishes the context necessary for ongoing investigations into the identities and origins of microbes living at the site, and provides a roadmap for future comprehensive investigations into Earth’s deep life.
“To me, the most exciting part isn’t about finding yet another environment where we previously thought life couldn’t exist. It’s about changing the way we think about investigating novel environments to focus on using techniques across many disciplines in concert with each other,” said first author Garnet Lollar (University of Toronto, Canada).
The researchers collected water at the Kidd Creek Observatory, located 2.4 kilometers below the surface within the mine. The Observatory includes two boreholes drilled in 2007 that have bubbled up continuously with deep water. They also sampled the service water used for mining activities, which comes from the surface, so that they could compare the different water sources and identify any surface contamination. An earlier study  established that these boreholes tap into the oldest known groundwater on Earth. In this paper, new analyses of the dissolved ions, gases, and the hydrogen and oxygen isotopes, which are atoms of the same element with a different number of neutrons in the nucleus, confirm that the underground fracture water remains uncontaminated.
Attempts to culture different types of microbes from the ancient groundwater consistently yielded only organisms called sulfate-reducers, which use sulfate to gain energy. “It’s a very low biomass, relatively low biodiversity microbial ecosystem that is eking out an existence in these deep, isolated systems based on the miracle of chemistry – water-rock reactions,” said Sherwood Lollar.
The findings confirm the results of a previous study  by this group where they used sulfur isotopes to provide indirect evidence that sulfate-reducing microbes have been active in these fluids over geologic timescales. Kidd Creek has remained at a temperature that can support life for the last two billion years, which presents the possibility that the community may have persisted throughout that time.
This study lays the groundwork for ongoing experiments to identify the microbes surviving in these ancient waters. “The DCO was very catalytic in this and supported us in this idea of bringing together all kinds of people so that multiple teams could take samples and compare and contrast their final results,” said Sherwood Lollar. Currently, DCO colleagues are sequencing the genomes of individual microbes collected from the site, as well as the metagenome, which is all the genomes of organisms in the water.
The low numbers of microbes living in waters from the Kidd Creek Observatory mean that researchers also can use the site as a test bed to investigate abiotic compounds, created in the absence of life, from deep water-rock interactions. “This system is a powerful abiotic chemistry factory, and we don’t have many of those on Earth,” said Sherwood Lollar. At most accessible sites, life has obscured the abiotic production of methane and other organic compounds.
This investigation borrows the “follow the water” mantra that guides the search for life on other planetary bodies and urges future studies of deep life to adopt this approach. By integrating data from the water geochemistry, mineralogy, and isotopic signatures at a site, scientists can provide important context for interpreting how the microbial community gains food and energy, and how long the organisms may have lived there.
Furthermore, Kidd Creek Observatory and other boreholes into ancient bedrock can serve as analogs for planetary bodies, like Mars or Titan. These sites can show what types of geology and chemistry have the potential to support life to guide future searches for extraterrestrial life.
But Sherwood Lollar emphasizes that despite the allure of extraterrestrial life, there is still much to explore here on Earth. “There are fundamental principles on which the planet operates that we’re still just beginning to figure out,” she said. “There are still amazing discoveries to be made.”
Main image: The researchers collect ancient water from the boreholes at the Kidd Creek Observatory at 2.4 kilometers below surface. Credit: Stable Isotope Laboratory, University of Toronto