Hydrogen-Eating Bacteria in Soil that Work Like Batteries

Image Source: https://www.nature.com

Many people know about bacteria when in comes to our health – some bacteria might cause us to get sick if we eat them, while other bacteria help our bodies, for instance, by breaking down food and making vitamins and nutrients available to us. But did you know that bacteria also play a role in our atmosphere?

Aerobic bacteria found in soil are responsible for eating, and thus removing, hydrogen from our air, which is crucial for regulating the chemical reactions that happen naturally in our atmosphere. As governments attempt to shift toward a hydrogen economy, these bacteria might become even more vital for capturing the extra hydrogen that humans produce (which might otherwise contribute to global warming in new ways).

To learn more about these hard-working bacteria, we interviewed Professor Chris Greening (CG) with Monash University’s Department of Microbiology. Dr. Greening helped lead a study investigating one enzyme called Huc that allows soil bacteria to effectively snack on atmospheric hydrogen.

CTS: What roles do bacteria play in our atmosphere’s regulation, and why is hydrogen an important part of this?

CG: Just as there’s a global nitrogen and carbon cycle, there’s a global hydrogen cycle too with important environmental and ecological ramifications. While a wide range of processes produce atmospheric hydrogen (including human activities), most of the atmospheric hydrogen (approximately 75%) is taken up by soil bacteria containing high-affinity hydrogenases like Huc, which are enzymes that split hydrogen into a proton and an electron. They’re responsible for the net consumption of about 60 million tonnes of atmospheric hydrogen. 

This process helps to regulate climate change given the levels of hydrogen in the atmosphere influence the levels of greenhouse gases such as methane through effects on atmospheric chemistry. It also means that, if we transition more towards a hydrogen economy, soil bacteria will help to remove most of the hydrogen that will sometimes leak into the atmosphere. 

CTS:  I’ve heard this process described as the bacteria eating hydrogen. Is this what they’re doing? 

CG: Hydrogen is an excellent source of energy. Whereas plants and animals can’t eat hydrogen, many bacteria can. This is because they’ve evolved a special enzyme called hydrogenase to bind and extract energy from hydrogen. Bacteria can grow and survive by eating hydrogen as an energy source and transferring the derived electrons to oxygen. This creates an electrical current that powers the cell.

A hybrid stick and electrostatic surface representation of the central cavity Huc, showing that the surface of the chamber is lined with hydrophobic residues.

Image Source: https://www.nature.com

CTS: You mentioned that bacteria are responsible for 75% of the removal of hydrogen from the atmosphere. Why haven’t we known about the major role of these bacteria in oxidizing atmospheric hydrogen until recently?

CG: It was always known that soils consume most of the hydrogen from the atmosphere each year. But it was assumed this was an abiotic process because nobody could isolate a bacteria that could grow on atmospheric levels of hydrogen alone. It turns out many bacteria that eat atmospheric levels of hydrogen are out there, but they survive on hydrogen rather than grow on it.

CTS: It sounds like hydrogenases are good at surviving on very small concentrations of hydrogen. How do they do this?

CG: The enzyme has a large electrochemical overpotential (voltage), which makes it electrically fine-tuned to use atmospheric concentrations of hydrogen. This depends on them using a different type of “wire” of iron-sulfur minerals to pass the electrons from hydrogen to oxygen.

CTS: What are some things we use biocatalysts for in man-made systems?

CG: We’ve characterised the first catalyst that can extract energy from air, and this is a major breakthrough. The dream is to translate this into air-powered devices that don’t require any other charge. However, the relatively low energy available in the air means that any devices would need to be small with low energy requirements, for example, certain watches or sensors. Nevertheless, Huc is extraordinarily catalytically efficient, oxygen insensitive, and temperature resistant compared to other hydrogen catalysts like platinum so it has a broad use across wide range of hydrogen levels.

This study was recently published in Nature

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