Microbes Munching on Hydrogen

The hidden world of subsurface microbes

These microscopic inhabitants of the deep have been found in rocks several kilometers beneath the surface, with estimates suggesting there could be anywhere between 2 to 6 x 10²⁹ cells within the continental subsurface. These microbes can originate from natural transport or sedimentation processes, or they can be introduced by human activities such as drilling, pumping, or mining.

Human activities in the subsurface can alter conditions, leading to long-lasting changes in microbial diversity and activity. For these microbes to thrive, they need certain environmental parameters and basic substances for life, including water, an energy source, and essential elements like carbon, nitrogen, phosphorous, and various trace elements.

Life in the extreme

The conditions in which these microbes can thrive range from extreme to ambient, with cell numbers and diversity generally lower under extreme conditions. The temperature limits for life range from -15°C up to ~122°C, and there are no clear thresholds for pressure or brine salinity. Other factors, such as high concentrations of toxic chemicals, water activity, high radiation, and low permeability, can also significantly influence and reduce the presence of microbes.

Hydrogen: A microbial snack

One of the most important energy sources for these microbes is hydrogen (H₂). Microbes use special enzymes to break down hydrogen into protons and electrons. This energy can then be used to drive reactions within the cell, such as CO₂ and N₂ fixation. There is a large diversity within the hydrogen-related enzymes, such as different hydrogen affinities and properties adapted to naturally occurring hydrogen-containing environments and sources. In the subsurface, various electron acceptors are available for the oxidation of hydrogen, including CO₂/HCO₃^-, sulphate, sulfur, nitrate, ferric iron, or oxygen, leading to different energy gains for the cells.

Microbial side-effects of underground hydrogen storage

When microbes use hydrogen in their life processes, it can lead to a decrease of hydrogen around the cells and can cause changes in the gases around them. This can affect also our future energy systems!

We want to use renewable energy sources like solar or wind power to produce hydrogen through a process called electrolysis. This is where water (H₂O) is split into hydrogen (H₂) and oxygen (O₂). The hydrogen is then stored deep underground in geological formations, similar to how natural gas is stored. Why do we do this? Well, renewable energy sources like the sun and wind aren’t always available when we need them. The sun doesn’t shine at night, and the wind doesn’t always blow. But by converting their energy into hydrogen, we can store that energy and use it whenever we need it. When we’re ready to use the stored energy, we bring the hydrogen back to the surface.

But here’s where it gets really interesting: Some of the subsurface microbes love to munch on hydrogen so much that this can lead to changes in the gas mixture stored underground, which is something scientists are studying closely.

While microbes eat hydrogen, other gases like methane or hydrogen sulfide (H₂S) might be produced. This could affect how we can use the hydrogen that we get back from storage. Even a tiny bit of H₂S can cause problems with the quality of the gas and might even be a safety or health risk. So, we’d need to treat the gas to make it safe and useful again. Microbes can also trigger other effects like plugging or corrosion or geochemical changes. All factors which might change the subsurface environment and therefore the potential gas storages.

In simple terms, the tiny life forms deep within the Earth can interact with their environment in ways that can affect our human activities, including storing hydrogen underground. As we continue to learn more about this hidden world, we’ll keep finding new and amazing things about life on Earth. It’s a bit like discovering a whole new world, right beneath our feet!