Earth's 'deep biosphere' is home to as many single-celled organisms as its surface. They may hold the key to explaining how life began—and even to how it might travel through space.
Image: Dennis Jarvis / Flickr
Earth's "deep biosphere"—the vast, subterranean world that's home to as many single-celled organisms as our planet's surface—has a rep for being a stark and lonely place. But a new study finds that deep oil reservoirs, miles beneath the ocean floor, are anything but solitary. Here, bacteria are social critters that have been swapping genetic material back and forth for eons.
What's more, rapid DNA swapping between oil-dwelling bacteria could hold clues to how life survived on early Earth—and, perhaps, on extraterrestrial worlds.
Oil reservoirs, formed over millions of years as carbon-rich sediments are compressed and cooked, are scattered like islands across Earth's subsurface. Like other deep biosphere habitats, we know they harbor life, but we aren't really sure how or when life got there.
"There's a hypothesis that these bacteria were buried, then continued to live on in complete isolation," study author Olga Zhaxybayeva told me.
To test that hypothesis, the team of researchers, hailing from Dartmouth College, the University of Alberta, and the University of Oslo, analyzed 11 genomes of the heat-loving bacterium Thermotoga. The bacteria was taken from oil reservoirs in the North Sea and Japan, and marine sites near the Kuril Islands, Italy and the Azores. They compared their results with publicly available Thermotoga genomes from North America and Australia.
Their analysis revealed a complex evolutionary history between the different genomes, suggesting rampant gene swapping across far-flung communities. And since the oil beds themselves are ancient, this genetic exchange has probably been going on for millions of years.
How microbes half a world apart actually exchange genetic material isn't totally clear. Some bacteria are genetic scavengers, sucking up stray DNA willy-nilly. Others use microscopic tubes to pass genes back and forth in a weird bacterial version of sex. And viruses—which cut and paste DNA among surface-dwellers' genomes all the time—might also shape the genetic landscape of the deep biosphere.
"The answer is probably that it happens in a variety of ways," Zhaxybayeva told me. "But it's really surprising to see how much it's happening. It's clear that these organisms are not nearly as isolated as we once thought."
The author's findings may also shed light on the nature of life on early Earth. Zhaxybayeva, who has been mapping Thermotoga's lineage for over a decade, says the organism has deep roots in the tree of life.
"This lineage is perhaps one of the most ancient that exists today," Zhaxybayeva said. "The fact that it's anaerobic, and likes hot environments, fits with our understanding of where life on Earth first evolved."
Thermotoga's penchant for gene swapping may indicate a once-widespread adaptation for life in hydrothermal vents, where high heat and acid have no trouble shredding DNA apart.
"As temperatures rise, organisms accrue more DNA damage. One way to potentially repair their genome is to actually recombine it— to patch their genomes with similar DNA," Zhaxybayeva said.
Top-notch DNA repair machinery may be life's most precious survival tool. Who knows, maybe it's Earth's most ardent gene-swappers that could actually survive the long, dark, radiation-filled trip to another world.