Billions of years of evolution on Earth have produced “endless forms most beautiful,” as Darwin poetically noted. Some have evolved to live in “extreme” environments, such as deep sea hydrothermal vents and around the rims of lava lakes. Now, a new study published in Nature Microbiology reveals that extremely rare lifeforms have even managed to eke out a living inside the Earth’s crust itself.

The microbes in question are called achaea, highly primitive lifeforms that have been found thriving in acidic hot springs and salt likes, environments that would be uninhabitable to most other critters. Although they resemble bacteria, the two groups are about as genetically different as a human and a tree.

These new microscopic extremists, found 3.2 kilometers (2 miles) in a South African gold mine, are called Hadesarchaea – roughly meaning “hellish ancient things.” Surviving at temperatures of up to 80°C (176°F), this name is certainly appropriate. This new study is the first to work out how these deep-dwelling, single-celled microbes are able to live within the Earth’s crust, at extremely high temperatures, lacking both oxygen and light.

Without the ability to photosynthesize, as many types of microorganisms do, scientists thought the Hadesarchaea may obtain their energy by taking advantage of certain chemical compounds in their surroundings. This process is known as chemosynthesis.

During chemosynthesis, carbon-containing molecules such as carbon dioxide or methane, lose an electron in a process called oxidation. These reactions produce small amounts of energy that the microorganisms use to make carbohydrates. Plenty of bacteria and archaea use this method to survive within areas like the Lost City Hydrothermal Field at the bottom of the Atlantic Ocean.

^{The Hadesarchaea were also found living beneath and within Yellowstone’s hot springs. Oliver S/Shutterstock}

These extremophiles are also found in the Yellowstone National Park hot springs in Wyoming and within the White Oak River estuary in North Carolina. By carefully sequencing the genome of these surface-dwelling Hadesarchaea, the researchers were able to identify key genes responsible for controlling metabolic processes.

“By comparing the metabolic genes of Hadesarchaea with those of other microorganisms, we figured out that Hadesarchaea had a rather versatile metabolic repertoire,” Thijs Ettema, an associate professor at Uppsala University, Sweden, told IFLScience. Genes associated with the oxidization of carbon monoxide were found, meaning that they may use a considerably rare form of chemosynthesis.

Ettema notes that the Hadesarchaea appear to have adapted to make do with whatever forms of carbon seep down to these nearly-inhospitable depths, making them “scavengers” in a manner of speaking. In addition, their relatively small genome means that they use very little energy in producing nutrients or replicating themselves.

All in all, they are certainly well-adapted to living in such challenging conditions. Life that may have evolved in comparably extreme environments elsewhere in the Solar System – and beyond – may perhaps gain energy by using the same type of chemosynthesis. However, Ettema cautions that “before being able to provide a more firm, educated guess about [this], we would first need to understand better how other microbes of Earth’s deep biosphere make a living.”