Microbes Deep Under Seafloor Reflect Ancient Land Origins

Microbes 2,500 meters below the seafloor in Japan are most closely related to bacterial groups that thrive in forest soils on land, suggesting that they might be descendants of ones that survived when their terrestrial habitat was flooded 20 million years ago

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Microbial organisms live , on you, in soil, and below the ocean floor. That last batch of single-celled critters is particularly hard to study. Not only are the samples difficult to obtain, but they can easily be contaminated with all that other microbial life once we dig them up.

So researchers had to take special precautions when they investigated microbial life in a coal bed deep below the seafloor near Japan. This material was once dry land, but got submerged some 20 million years ago.

The research team drilled nearly 2,500 meters below the seafloor and brought up samples. They carefully avoided contamination and evaluated only the inner portions of the samples, which were protected by the outer parts.  

Analysis found life tenaciously holding on well under the ocean. —at 2,500 meters below the seafloor a gram of sediment might be home to just a single microbe. And those deeply buried organisms are quite different from microbes to be found just under the seafloor.

In that deep layer, the microbes are most closely related to bacterial groups that thrive in forest soils on land. The scientists thus suggest that the deeply buried undersea microbes might be descendants of ones that survived when their terrestrial habitat became flooded. Again, that was 20 million years ago. The finding is in the journal Science. [F. Inagaki et al, ]

In a , Julie Huber of the Marine Biological Laboratory at Woods Hole says the fact that there’s “a massive buried biosphere” has global importance “with sub-seafloor microbes playing a crucial role in carbon sequestration…and Earth’s evolution, and likely encompassing staggering metabolic and genetic diversity.” She adds, “We still have a long way to go in uncovering and understanding microbial life deep beneath the seafloor.”

—Cynthia Graber

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