科学美国人60秒:Microbes Deep under Seafloor Reflect Ancient Land Origins

Microbial organisms live in you, on you, in soil, in clouds 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. A gram of rich garden soil can hold a billion bacteria—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, Exploring deep microbial life in coal-bearing sediment down to ~2.5 kilometers below the ocean floor]

In a commentary in the same issue, 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