Microorganisms have many talents. Some of them produce hydrogen from sunlight or biomass, others produce electricity from hydrogen. With their help, metabolic processes from the earliest days of the planet could become an integral part of a modern energy economy.
Blue-green algae do not have a good reputation. When they emerge in lakes visited by bathers, their toxic metabolites can cause dizziness and breathing difficulty. But they are the basis for all life on earth. And these special microbes are not actually algae at all, but bluish bacteria – today, therefore, they are also known as cyanobacteria.
Billions of years ago, they developed the ability to convert sunlight into energy and to store it. It is only thanks to this process, photosynthesis, that more complex forms of life have been able to develop.
Today, researchers are trying to use photosynthesis to produce hydrogen in an environmentally friendly way. For this, they are focusing on certain enzymes, specifically hydrogenases, that can originate from blue-green algae or “real” algae.
Hydrogen via photosynthesis
The process of photosynthesis occurs in several steps. In so-called photosystem I, sunlight sets energetic electrons free. Normally, the cell would use these to store energy in the form of sugars in further steps. The enzyme hydrogenase can capture these electrons and bind them to H+ ions instead, which are available everywhere in the cell. This is how hydrogen is biologically produced from sunlight.
This process is a relic from times when completely different conditions prevailed on earth. “We can encourage this metabolic pathway by putting the algae on a type of sulfur diet in an airtight container. After they have consumed the oxygen, they begin to produce hydrogen, which rises in small bubbles,” illustrates Christina Marx of the photobiotechnology working group at Ruhr-Universität Bochum (RUB).
The search for the perfect enzyme
Also Kirstin Gutekunst, professor in molecular plant physiology at Universität Kassel, emphasizes, “No organism has an interest in primarily producing hydrogen for humans.” To promote hydrogen production, they therefore have to artificially join the hydrogenase to photosystem I. A major challenge in this is that the hydrogenase is sensitive to and reacts with oxygen, which also evolves from the water splitting during photosynthesis.
Marx, Gutekunst and other researchers are therefore in the laboratory searching for microorganisms, enzymes and other biological components that produce as much hydrogen as possible yet at the same time are not destroyed by oxygen.
In 2020, Gutekunst had led the research group at the Christian-Albrechts-Universität zu Kiel (CAU) that succeeded in inducing the process in a living cyanobacterium for the first time. The advantage of this is that the bacterium can repair itself, so the process is more stable. Also the H2 yield ended up being significantly higher than in earlier projects. However, the cyanobacteria got the electrons not only from water splitting, but also from sugar. “Either the organism must produce the sugar itself beforehand or we must supply it externally. What we really want is to produce hydrogen exclusively with water and sunlight,” explains Gutekunst.
As part of her professorship in Kassel, she is continuing the research towards finding suitable hydrogenases. “Right now, we’re studying an enzyme from knallgas bacteria. It is fairly resistant to oxygen. Unfortunately, it takes up H2 rather than producing it,” says Gutekunst. That’s why her team is working in parallel with different mutations – always in search of the all-rounder.
The Arbeitsgruppe Photobiotechnologie team around Prof. Thomas Happe at the RUB, to which Marx belongs, is also looking for the perfect enzyme for hydrogen production. Together with the University of Osaka, the researchers at the RUB want to understand the structures and mechanisms even better, by looking at cryogenic enzyme samples and other biological building blocks under the electron microscope. Their goal is not only to make the enzymes more active and stable, but also to develop simpler structures, which are easier to use technically.
“We are working on so-called mini-enzymes. These have the function of a hydrogenase, but are smaller and simpler in structure. They contain practically only the active center and the necessary structure to be able to catalytically produce hydrogen as well as split hydrogen. This way, they will also be easier to commercially manufacture and use later on,” says Happe.
A challenge is still the sensitivity to oxygen. Some enzymes, like the CbA5H being studied at the RUB, can shield themselves against oxygen. “This is an important step, because then the active center stays intact,” says Marx. “But as soon as oxygen is present in the environment, it’s kind of like the pause button is pressed and the enzyme doesn’t produce more hydrogen, although like practically all other enzymes is not destroyed by oxygen. Our goal is to develop an enzyme that does not allow oxygen to penetrate into the active center and at the same time still produces hydrogen.”
In order for these enzymes to be used technically, they must be applied to surfaces, and in such a way that they stay a long time and can work as efficiently as possible. The RUB team intend to approach this task in further projects.
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