Microbes in Groundwater Reservoirs in Canada Produce ‘Dark Oxygen’ Without Light [Study]

Many subterranean regions are considered oxygen-poor dead zones where only simple bacteria can survive by scrounging resources and slow-moving metabolisms. It was assumed that once those supplies ran out, the underground habitat would eventually die at deeper depths. However, a new study proves it isn't exactly the case.

Microbes Produce Dark Oxygen Underground

Researchers found several bacteria that create significant amounts of oxygen even without light in groundwater reservoirs 200 meters below Alberta, Canada's fossil fuel resources. It's like discovering "the scale of oxygen coming from the photosynthesis in the Amazon rainforest," according to Karen Lloyd, a subsurface microbiologist at the University of Tennessee who was not involved in the work, Quanta magazine reported.

The microorganisms produce and release so much of what the researchers call "dark oxygen." So much gas escaping from the cells fosters the growth of oxygen-dependent life in the strata and groundwater nearby.

According to Barbara Sherwood Lollar, a geochemist from the University of Toronto who was not engaged in the research, it is a significant study. The formation of oxygen-containing molecules has generally been disregarded because oxygen seems to be so dependent on photosynthesis and the existence of light, even though past studies have looked at methods that could produce hydrogen and certain other essential chemicals for underground life. No study has until now brought it all together as well as this one, she continued.

Scientists typically observe that the number of microbial cells declines with depth in surveys of the sediment beneath the seafloor, for instance: Older, deeper samples cannot support as much life because they are more cut off from the nutrients produced by photosynthetic plants and algae at the top. However, Emil Ruff's team was shocked to find that the older, deeper groundwaters contained more cells than the newer fluids.

Using molecular methods to find the distinctive marker genes of the bacteria in the samples, the researchers began to identify them. Many were methanogenic archaea, simple, single-celled bacteria that create methane by eating carbon and hydrogen found in decomposing organic materials or leaking from rocks. Numerous microorganisms that consume methane or minerals in the water were also found.

It was puzzling that so many bacteria were aerobes, which need air to metabolize substances like methane and other gases. Since photosynthesis is impossible, how can aerobes survive in groundwaters that shouldn't have any oxygen? According to chemical studies, the groundwater samples from 200 meters below the surface also contained a significant amount of dissolved oxygen.

How Oxygen Forms Without Light?

The dissolved oxygen in the groundwater may have come from geological processes, bacteria, or plants, in theory. The researchers used mass spectrometry, a method that can assess the mass of atomic isotopes, to discover the solution.

Typically, geologically derived oxygen is heavier than biologically derived oxygen. Light oxygen indicated that it must have come from a living thing because it was in the groundwater. The most likely contenders were the microorganisms.

The complete community of bacteria in the groundwater was sequenced, and the researchers then identified the metabolic events and pathways most likely to produce oxygen.

The new study's senior author and the director of the lab where Ruff was working, Marc Strous of the University of Calgary, kept finding fresh solutions that led him back to a discovery he had made more than ten years prior.

In the late 2000s, Strous saw a peculiar style of life in various methane-feeding bacteria often found in lake sediments and wastewater sludges while working in a lab in the Netherlands. The bacteria produced its oxygen by using enzymes to break down the soluble chemicals known as nitrites (which have a chemical group of nitrogen and two oxygen atoms), unlike other aerobes that take in oxygen from their environment. The bacteria split methane for energy by using the oxygen they produce.

This type of chemical breakdown by bacteria is referred to as dismutation. It was previously believed that few natural processes could produce oxygen. The oxygen created by dismutation can, however, leak out of the cells and into the surrounding medium to benefit other oxygen-dependent species, in a sort of symbiotic process, according to recent laboratory tests employing artificial microbial communities. According to Ruff, this may be why diverse populations of aerobic microbes flourish in the groundwater and possibly the nearby soils.

The study was published in Nature Communications.

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