Sunday December 04, 2022

How Life in the ‘Deep Biosphere’ Thrives Despite Temperatures That Would Fry Humans

The Japanese scientific drilling ship used to detect microbes living deep below the seafloor.Photo: JAMSTEC

A science expedition in 2016 revealed a subsurface habitat in which microbes were found living at temperatures approaching 250 degrees Fahrenheit. Now, a follow-up study reveals how this remarkable microbial community manages to beat the heat.

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High metabolic rates make life possible for microorganisms living in sediments buried deep beneath the seafloor, according to new research published in Nature Communications. The study, led by marine geomicrobiologist Tina Treude from the University of California Los Angeles, casts subsurface microbes in a new light, showing some of them to be surprisingly active and capable of thriving in deep and hot conditions.

“We always found that microbes in the deep biosphere are an extremely sluggish community that slowly nibbles on the last remains of million-year-old, buried organic matter. But the deep biosphere is full of surprises,” Bo Barker Jørgensen, a microbiologist at Aarhus University in Denmark, said in a University of California press release. “To find life thriving with high metabolic rates at these high temperatures in the deep seabed nourishes our imagination of how life could evolve or survive in similar environments on planetary bodies beyond Earth.”

In an email, Virginia Edgcomb, a geologist at Woods Hole Oceanographic Institution who wasn’t involved in the new study, said she’s excited by the research because it shows “we cannot assume that microbial activities are insignificant simply because of the depth below seafloor or extreme temperatures,” particularly when “sufficient sources of carbon and energy are available.”

In this case, the required sources of carbon and energy were found in the Nankai Trough subduction zone off Japan. Seven years ago, a scientific expedition led by the same team drilled 3,930 feet (1,200 meters) below the seafloor, pulling up marine sediment samples and evidence of the extremophile microbes. They did so to investigate the temperature limit of the deep subseafloor biosphere and the extent to which life might be resident in this extreme habitat. Incredibly, they found a small community of microbes that appeared to be thriving despite temperatures reaching 250 degrees F (120 degrees C). It wasn’t totally obvious to the researchers how this was possible, prompting further study.

For the new investigation, Treude and her colleagues ran radiotracer experiments to measure the metabolic rates of the microbes, which they did under highly sterile conditions to prevent contamination. This wasn’t easy, given the low population density of the microbes; less than 500 cells were present in each cubic centimeter of sediment. The team also made special provisions to ensure that the observed metabolic rates were the same in the lab as they would be in the microbes’ natural environment.

This work resulted in the discovery of the microorganisms’ rapid metabolism, which the researchers say is what makes it possible for them to survive such extreme conditions. The scientists theorize that the high metabolic rates are a necessity, allowing the microbes to repair cells damaged by heat.

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“The energy required to repair thermal damage to cellular components increases steeply with temperature, and most of this energy is likely necessary to counteract the continuous alteration of amino acids and loss of protein function,” said Treude.

At the same time, the microbes have ample access to nutrients supplied by the heating of organic materials, specifically hydrogen and acetate from water leaking through the marine sediment.

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The new observations “might seem counterintuitive to many, which is that cells living close to the thermal limits of life at this location, and so deep below the seafloor, where we would expect them to be barely eking out an existence, are actually very active,” said Edgcomb. But their high rate of activity is for a very interesting reason: “To be able to provide enough energy to repair thermal cell damage so they can survive,” she added.

In an email, Jennifer Biddle, an associate professor at the University of Delaware who’s not affiliated with the research, said the new work “appears well done” and “nicely compliments” pre-existing work showing changes to microbial communities and increases in cell division as sediment temperatures get hotter. An argument presented in the new paper is that cells only get kick-started once they’re already buried—a finding that agrees with recent research co-authored by Biddle demonstrating that “once cells find their ‘happy place’ in the subsurface, they have plenty of power to grow,” she said.

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One limitation, Biddle said, is that the researchers described microbial activity but didn’t provide any names or identify the microbes in question. She said “it would be great to know who is there, so we could even better estimate how fast they may be going,” adding that it would also be good to “culture some of these subsurface lineages to test their thermal ranges and how they may have adapted to this environment.”

Interestingly, these subseafloor microbes approach the thermal limits of life as we know it, but some scientists think microbes can survive in even hotter environments. Sounds like we need to dig a bit deeper next time, as even more extreme microbes could still be waiting to be found.

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