The Underground Biosphere: A New Understanding of Life in the Dark

The Earth beneath our feet holds a secret: a vast biosphere with a global volume nearly twice that of all the world’s oceans. This underground world, teeming with microbial life, may even surpass the diversity found in surface-dwelling ecosystems. Yet, until recently, these subsurface organisms were believed to inhabit oxygen-deficient dead zones, scraping by on the barest traces of nutrients. However, new research published in Nature Communications challenges these assumptions and propels us into a deeper understanding of life in the dark.

Life thrives in ways we never thought possible in the darkness beneath our feet.

A Landmark Study

The study in Alberta, Canada, discovered that these underground microbes produce large amounts of oxygen, even without light. The volume of this "dark oxygen" is astonishing, equivalent to the oxygen produced by photosynthesis in the Amazon rainforest. This revelation is revolutionary, as it appears to create conditions favorable for oxygen-dependent life in the surrounding groundwater and strata.

“It is a landmark study,” commented Barbara Sherwood Lollar, a geochemist at the University of Toronto. Until now, the generation of oxygen-containing molecules in the subsurface environment was overlooked mainly due to their rapid consumption. This study is the first to pull together all the pieces of this microbial puzzle.

The Crowded Depths

While surveying Alberta's groundwater microbiology, Emil Ruff made a puzzling discovery. Contrary to expectations, older, deeper groundwaters held more cells than the fresher waters. The team identified a variety of microbes, including methanogenic archaea and several types of bacteria, many of which required oxygen to digest methane and other compounds.

The presence of aerobes in an environment devoid of photosynthetic oxygen production was perplexing, and yet chemical analyses revealed high levels of dissolved oxygen in the deep groundwater samples. Initially doubting the results, Ruff spent considerable time trying to disprove his findings before accepting the consistency across hundreds of models.

Making Oxygen for Everyone

To unravel the mystery of the dissolved oxygen, the research team turned to mass spectrometry. The lightness of the oxygen atoms suggested a biological, rather than geological, origin. Further investigations identified microbes as the most plausible source of this unexpected oxygen production.

The researchers discovered that a type of methane-feeding bacteria creates its oxygen by breaking down nitrites, known as dismutation. This finding challenges the belief that dismutation is a rare method for generating oxygen. The team theorizes that this oxygen leakage from dismutation could enable entire communities of aerobic microbes to thrive in the groundwater and the surrounding soils.

Implications for Life on Other Planets

Beyond altering our understanding of Earth's subsurface, these findings could also translate to other planets. The soil of Mars contains perchlorate compounds that Earth microbes could convert into chloride and oxygen, and Jupiter's moon Europa’s deep, frozen ocean could potentially harbor oxygen produced by microbial dismutation. Even Enceladus, a moon of Saturn, could host life using dismutation pathways in its subsurface ocean of liquid water.

While the implications for extraterrestrial life are intriguing, the discovery also challenges our preconceived notions about the needs of life. It highlights our lack of understanding about one of the planet’s biggest biospheres. As Karen Lloyd, a subsurface microbiologist, puts it, “It’s as if we have had egg on our face all along.”