Million-Year-Old Dormant Microbes Beneath Ocean Floor Push Life To Its Absolute Limits

Every living creature requires energy in order to subsist, multiply, and pass on its genes. How much energy an animal requires depends on their habitat and size, among many other things. But some cells require so little energy, it just boggles the mind.

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Recently, researchers have identified microbial cells that live in sediments kilometers beneath the ocean floor that require a tiny fraction of a calorie to survive. In fact, many of these cells may be up to 100 million years old, something that is owed to their suspended animation state.

Speaking to Quanta Magazine, James Bradly, a geobiologist at Queen Mary University of London and the lead author of a new study that modeled the suboceanic biosphere, said that “This entire biosphere of cells, equivalent in size to the world’s soils, hardly has enough energy to survive.”

Bradly, along with colleagues from universities across the world, employed existing data from previous drilling operations and lab experiments, which they modeled to extrapolate a detailed profile of sub-seafloor sediments.

Researchers projected values like the age of the sediments, the density of cells living inside them, which nutrients are available to these cells, and the rate at which the cells metabolize the nutrients. The findings were quite staggering.

When the researchers calculated the power consumption of the dormant cells living inside the sediments, they found that they were close to the absolute theoretical limit for energy requirements to sustain life.

These sub-seafloor microbes use only 0.1% of the power consumed by creatures living in the upper 200 meters of the ocean. The buried microbes survive at power levels orders of magnitude lower than any organism ever measured in a laboratory, the authors reported in the journal Science Advances.

Previously, in 2015, Douglas LaRowe and Jan Amend, both at the University of Southern California in Los Angeles, estimated the lowest amount of power required to sustain life. Even life that is dormant for millions of years in a zombified state waiting for the right conditions for reanimation needs at least some energy for fundamental biological processes like the repair of DNA damage.

Power per cell (watts) calculated on a global scale and depth-integrated for the (A) oxic, (B) sulfate-reducing, and (C) methanogenic sedimentary layers. White areas denote absence of the corresponding catabolic zone.​

Even if an individual cell doesn’t divide, it would still need at least a zeptowatt, or 10−21 watts, in order to survive. The sub-seafloor microbes are just slightly above this threshold.

Some of these microbes may be up to 100 million years old, researchers report. Given their phenomenally low energy requirements, this all might change how biologists see cellular evolution.

The findings also open the possibility that life may exist in places that scientists had previously discarded as impossible habitats — and this includes other planets, as well.

The sediment samples that were used for the new theoretical model are around 2.6 million years old. However, deeper sediments might house even more starving cells, pushing energy requirements further to the brink.

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