Researchers from the Marine Biological Laboratory (MBL) and colleagues from other institutions have discovered that ancient groundwaters harbor not only diverse and active microbial communities, but also unexpectedly large numbers of microbial cells.

Some of these microbes seem to produce “dark oxygen” (in the absence of sunlight) in such abundance that the oxygen may nourish not only those microbes, but also may leak into the environment and support other oxygen-reliant microbes that can’t produce it themselves, according to a study titled, “Hydrogen and dark oxygen drive microbial productivity in diverse groundwater ecosystems” which was published in Nature Communications.

“Around 50% of humankind relies on groundwater as a source of drinking water. Here we investigate the age, geochemistry, and microbiology of 138 groundwater samples from 95 monitoring wells (<250 m depth) located in 14 aquifers in Canada. The geochemistry and microbiology show consistent trends suggesting large-scale aerobic and anaerobic hydrogen, methane, nitrogen, and sulfur cycling carried out by diverse microbial communities,” the investigators wrote.

groundwater sampling
Apparatus for sampling groundwater from aquifer wells. [Muhe Diao]

“Older groundwaters, especially in aquifers with organic carbon-rich strata, contain on average more cells (up to 1.4 × 107 mL−1) than younger groundwaters, challenging current estimates of subsurface cell abundances. We observe substantial concentrations of dissolved oxygen (0.52 ± 0.12 mg L−1 [mean ± SE]; n = 57) in older groundwaters that seem to support aerobic metabolisms in subsurface ecosystems at an unprecedented scale. Metagenomics, oxygen isotope analyses, and mixing models indicate that dark oxygen is produced in situ via microbial dismutation.

“We show that ancient groundwaters sustain productive communities and highlight an overlooked oxygen source in present and past subsurface ecosystems of Earth.

Unexpected finding

“When doing research, it is common to find surprising results; we just still know very little about the cosmos,” said Emil Ruff, PhD, a microbial ecologist and team leader. “But it is a highlight to find something so utterly unexpected as oxygen that seems to be produced deep beneath our feet in permanent darkness. At first, we thought that we had contaminated all our samples, but additional analyses supported a source of the gas within the aquifers.” 

The study investigated 138 groundwater samples drawn from 14 aquifers that lie beneath more than 80,000 square miles of prairie in the province of Alberta, Canada, an area three times the size of Ireland. The aim was to investigate the biogeochemistry and microbial ecology of a broad range of aquifer environments.

“The leakage of dark oxygen into the groundwaters could have important consequences relevant for climate change research,” noted co-author Mark Strous, PhD, of the University of Calgary. “We have indications that the microbes use the groundwater oxygen to consume methane, a greenhouse gas. Especially in the province of Alberta, methane is common in groundwaters and may leak out of the ground into the atmosphere. We will now seek to understand if and how much methane these microbes prevent from being emitted.”

The team found significantly more microbial cells in older groundwater samples than in younger groundwaters, suggesting these ancient groundwaters have evolved over time to provide energy for microbes to grow. This discovery runs counter to prior studies in subsurface ocean and land ecosystems that found microbial cell density commonly decreases as depth increases, presumably due to energy limitations.

“Counting bacterial cells under a microscope requires enormous patience and skill and is rarely done for large numbers of samples,” explained Strous. “Yet, when Isabella Hrabe de Angelis joined Dr. Ruff as a project student, she did just that, and spent hundreds of hours behind a microscope. It was thanks to this effort that we could show that these groundwaters are actually productive ecosystems, where everybody had expected them to be subsurface ‘deserts,’ generally devoid of nutrients and energy.”

The study included collaborators from the University of Calgary, Canada; Max Planck Institute for Chemistry, Mainz, Germany; Woods Hole Oceanographic Institution, and Alberta Environment and Protected Areas, Calgary, Canada.


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