Microbes Deep Under Sweden Survived Hundreds of Millions of Years of Mountain-Building

Microbial life deep within Earth has persisted and repeatedly flourished through hundreds of millions of years of mountain-building and erosion, according to a new study that analyzed minerals, fluids and gases from a 2.3-kilometer-deep borehole in central Sweden. The researchers, led by scientists at Linnaeus University, found episodic bursts of methane-producing life linked to major geological events that shaped Scandinavia.

The deep continental subsurface is one of Earth's least-explored ecosystems, yet it hosts microbial communities capable of ancient metabolisms such as methanogenesis, the production of methane by microorganisms. The team analyzed fracture-filling calcite minerals from the COSC-2 drill core, drilled west of Ostersund in central Sweden, which extends more than 2,300 meters into bedrock and passes through geological layers formed at different times in Earth's history.

The calcite showed exceptionally high carbon isotope values, a hallmark of microbial methanogenesis, because microbes preferentially use lighter carbon isotopes and leave behind a strongly enriched carbon reservoir. "These are some of the clearest mineral-scale fingerprints of deep microbial methane production that we've seen," said Henrik Drake, a professor at Linnaeus University and senior researcher on the study. "They show that microbial communities have left a chemical archive of their activity deep in the crust."

By dating the minerals with high-precision uranium-lead dating, the team found not a single event but multiple distinct episodes of microbial activity, spanning from about 378 million years ago to as recently as 1.8 million years ago. "This tells us that the deep biosphere is not static. Instead, it responds to geological processes over vast timescales, turning on and off as conditions shift," said Femke van Dam, a researcher at Linnaeus University and the study's lead author.

The borehole intersects rocks once part of the Caledonian mountain range, an ancient belt that stretched across Scandinavia and has worn down over hundreds of millions of years. "These microorganisms have survived tectonic upheaval, burial, uplift and changing temperatures," Drake said, adding that the findings could help refine where scientists look for life elsewhere, including on other planets, and how subsurface ecosystems might respond to future changes driven by energy use and climate.