They lived before oxygen appeared on Earth. Below the surface are the remains of very ancient life forms

Beneath the surface of the Earth PO s important to the vast diversity of life. Recent analyzes of the two main groups of subsurface microorganisms indicate that their evolutionary way of adapting to life in the dark is more interesting than might have been expected.

For the first 2 billion years of Earth's existence, there was no oxygen in our planet's atmosphere. As the air on the surface of our planet changed, only some life forms adapted to it. Others, instead of adapting, decided to find a place where the conditions would be more suitable for them, where there would be simply less oxygen.

Bacteria enter the stage

Patescibacteria and DPANN are two common groups of such microorganisms living below the Earth's surface - they are bacteria and archaea with very simple genomes. Many scientists believed that without the ability to breathe oxygen, these microorganisms must rely on complex interactions with others to support their relatively simple "lifestyle".

Scientists may have underestimated them. Recent research shows that, instead of symbiotic dependence on other major groups of organisms, most Patescibacteria and DPANN live as completely independent cells.

These microorganisms are truly unique and exciting examples of the very early evolution of life on Earth. Researchers suspect that these may be remnants of ancient life forms that existed in the young Earth's anaerobic atmosphere and then hid beneath the Earth billions of years ago, says Ramunas Stepanauskas, who studies microbial biology and evolution at the Bigelow Laboratory for Ocean Sciences.

Scientists collecting samples of microorganisms in Death Valley

Search for microorganisms in Hades

Previous work on these two groups of microorganisms has collected a few examples of organisms taken from a shallow depth below the Earth's surface, mainly in North America. The latest research is much more detailed. Scientists analyzed nearly 5,000 individual microbial cells from 46 different locations around the world, including a volcano at the bottom of the Mediterranean Sea, hydrothermal vents in the Pacific and gold mines in South Africa.

Our biophysical observations of individual cells do not support the common view that Patescibacteria and DPANN are dominated by symbiotes. Their tiny genomes, tiny cell sizes may be the result of an ancient, primitive energy metabolism that relies solely on fermentation

- the authors write.

Fermentation as a way of life? Definitely yes

Fermentation is one of the metabolic options that living organisms use to break down glucose without the help of oxygen. Many life forms use fermentation to produce energy, especially microorganisms that do not interact with air at all.

Nevertheless, the use of fermentation is less efficient than respiration, which means that this type of metabolism forces the microorganisms to live a rather slow and relaxed life. The two groups of microorganisms described here do not mind at all. None of them contain any traces of the electron transport chain, a metabolic process that produces energy by attaching electrons to oxygen. As it turns out, they are relatively simple, the potentially very old ways of surviving do not need it.

Examination of the genomes and direct experiments on samples from both groups showed no signs of respiration, and more careful studies of the intercellular connections showed that neither of them connected to the others.

The authors of the study point out that some symbiotic compounds may have disappeared due to sampling, but when sorting the cells, scientists tried to manipulate them with the utmost care.

Moreover, genome content and laboratory analysis of cell physiology indicate that these groups of microorganisms have little, if any, ways to produce energy other than fermentation.

Research results indicate that Patescibacteria and DPANN are former life forms that never learned to breathe. These two major branches of the evolutionary tree of life make up a huge part of the total microbial diversity on our planet - yet they lack the abilities we normally expect from any form of life.

- says Stepanauskas.

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They lived before oxygen appeared on Earth. Below the surface are the remains of very ancient life forms