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In some aquifers deep underground, microbes use a chemical trick to produce oxygen that sustains entire underground ecosystems. Not only does this disrupt our basic understanding of underground ecosystems, but it could also help us search for extraterrestrial life.

Wen| Chen Qiang

Scientists have realized that there is a vast biosphere in the soil and rocks beneath our feet. Although little is known about these underground life forms, they make up a significant portion of the total mass of life on Earth and may be more diverse than life on the surface.

However, scientists generally assume that many places underground are oxygen-deprived zones, and that only primitive microorganisms can maintain slow metabolism and survive on trace nutrients. Scientists also believe that with increasing depth, nutrient resources will become scarcer and the number of life will become less and less.

A recent discovery, however, debunks this view. Scientists have discovered a large number of microorganisms in groundwater more than 200 meters above the surface in Canada. And surprisingly, even in the absence of sunlight, these microbes can produce large amounts of oxygen. So much oxygen that scientists felt it came from the Amazon rainforest. Scientists also believe that this oxygen creates favorable conditions for oxygen-dependent life in groundwater and surrounding formations.

Deep groundwater holds large amounts of dissolved oxygen The new study investigated a deep aquifer in Alberta, Canada. Because livestock and agriculture there rely heavily on groundwater, the local government actively monitors changes in the pH and chemical composition of the water. However, no one has systematically studied microorganisms in groundwater.

Emil Ruff, then a postdoctoral researcher in microbiology at the University of Calgary in Canada, chose to survey the area in 2015. He thought it would be "a piece of cake," but he never thought it would cost him six years of hard work.

After collecting groundwater from 95 wells in Alberta, Ruff and his colleagues began performing basic microscopic examinations. They stained the microbial cells in the groundwater samples with a nucleic acid dye and counted them under a fluorescent microscope. By radioactively dating the organics in the samples and examining the depth at which they were collected, the researchers were able to determine the age at which the groundwater aquifers they extracted were formed.

However, a pattern appeared in the data that puzzled them. For example, scientists typically survey seafloor sediments and find that the number of microbial cells decreases with depth. Older, deeper samples do not contain much life because they are further away from the nutrients produced by the plants and algae above. But here, to the researchers 'surprise, older, deeper groundwater tends to contain more microbial cells.

Ruff et al. then identified the microbes in the sample by using molecular tools to discover their signature genes. The results showed that many of the samples were methanogenic archaea, single-celled microbes that produce methane after consuming hydrogen and carbon from rocks or decaying organic matter. They also found many bacteria that feed on methane or minerals in water.

Paradoxically, however, many bacteria are aerobe, requiring oxygen to digest methane and other compounds. In the absence of photosynthesis, there should be no oxygen in groundwater, so how do aerobic bacteria thrive in groundwater?

Chemical analysis of the groundwater samples, which came from more than 200 meters deep, revealed a large amount of dissolved oxygen. This was something unheard of. "We must have ruined the sample! "Ralph's initial reaction was this. He first tried to prove that the dissolved oxygen in the sample was due to improper handling. However, hundreds of samples contained large amounts of oxygen, which mishandling did not seem to explain.

If dissolved oxygen is not coming from external pollution, where does it come from? Ruff realized that his research could be an unprecedented discovery that could shatter our basic understanding of underground ecosystems.

Microbes break down compounds to produce oxygen In theory, dissolved oxygen in groundwater could come from plants, microbes or geological processes. To find out, the researchers used mass spectrometry, a technique that measures the mass of atomic isotopes. Generally speaking, geological processes produce heavier oxygen atoms than biological ones. They found that the oxygen atoms in the groundwater samples were light, meaning it must have come from living things. The most likely candidates are microorganisms.

The researchers sequenced the genome of the entire microbial community in the groundwater and traced the pathways most likely to produce oxygen. The answer has always pointed to a phenomenon previously discovered by Marc Strous of the University of Calgary. Strauss was the senior author of the new study and head of the lab where Ruff was working at the time.

Strauss discovered more than a decade ago that a bacterium that is common in lake sediments and sewage sludge has a special way of life. The bacterium feeds on methane, but instead of taking oxygen from its surroundings, as other aerobic bacteria do, it uses enzymes to break down nitrites (chemical groups consisting of one nitrogen atom and two oxygen atoms) to produce oxygen for itself. The bacteria then use the oxygen they produce to digest methane for energy.

Scientists thought this method of breaking down compounds to produce oxygen was rare in nature. But in 2022, a team of scientists examining artificial microbial communities in the laboratory discovered that oxygen from decomposition could be released from cells into the surrounding medium, allowing other oxygen-dependent organisms to survive. Ruff thinks this may be why aerobic microbial communities thrive in groundwater and surrounding soil layers.

The discovery could help in the search for extraterrestrial life-a discovery that not only reveals how decomposition participates in the cycling of matter in the biosphere, but also helps us understand how the subsurface biosphere evolves. The mere possibility that groundwater is rich in oxygen "changes our understanding of the underground past, present and future. "Raf said.

In addition, understanding what kind of life exists beneath the surface of our planet can also help us search for extraterrestrial life. For example, under the thick ice layers of Europa and Enceladus, there may be oceans of liquid water that sunlight may not penetrate, but oxygen may be produced by the decomposition of microorganisms there. If we ever find life on a planet like this, it's likely that it survived by decomposition to produce oxygen.

References:

https://doi.org/10.1038/s41467-023-38523-4

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