A great new discovery to extract electricity from the air

The hydrogen present in low concentrations in the air we breathe is consumed by processes that occur mainly in the soil and which, for a long time, were considered inorganic. For about a decade, however, it has been known that over two-thirds of atmospheric hydrogen is consumed by bacteria of different groups that live in the soil, which are able to use this gas – present at very low concentrations in the air – to reduce oxygen present in much higher concentrations.

Until now, the process by which this phenomenon occurred has remained a mystery; now, with a new, beautiful work in Nature, an Australian research group has demonstrated that bacteria consume atmospheric hydrogen through special enzymes, with very particular properties and incredible efficiency, to feed an electron transport chain similar to that which we feed when we breathe into our mitochondria, until we reduce the oxygen in water.

In other words, bacteria literally create electric currents starting from the air, through which they are able to fuel their energy metabolism, especially in conditions of scarcity of other sources of energy.

It is an admirable phenomenon, of which it is worth giving some details, whose importance also for future applications will be clear at the end of this brief summary. Many bacteria, in anaerobic conditions, are capable of using hydrogen as an electron donor to feed electrochemical currents and finally reduce an acceptor, which can be for example sulfur, but also other types of chemical substances; as in a microscopic battery, a flow of electrons, i.e. an electric current, starts from a tiny anode, i.e. from the site of a specific protein where the oxidation of molecular hydrogen takes place, to reach a different site on a different protein which it acts as a cathode, where the reduction of the final acceptor of the transported electrons takes place. The transport of electrons is exploited to create, for example, an electrochemical potential, transporting ions out of the cell (or from the inside of the mitochondria), a potential which will then feed a specific molecular machine capable of producing molecules useful for fueling the energy metabolism of the cell.

The problem is that, in aerobic conditions, atmospheric oxygen normally prevents the oxidation of traces of atmospheric hydrogen present, “poisoning” the enzymes assigned to it.

Instead, in the new work it was found that a specific type of bacterial enzyme, called Huc, is able to selectively capture molecular hydrogen inside itself, through a sort of sieve that prevents the entry of larger oxygen, but leaves spread the gas of interest; all this, thanks to the strong affinity of the active site inside the enzyme, with such a high efficiency as to be able to extract hydrogen from the environment even at much lower concentrations than those at which it is naturally present in the air. Once inside the enzyme, a special oxidation reaction takes place, which splits the molecular hydrogen into ions, generating two electrons per hydrogen molecule; these enter a transport circuit, and the microscopic current then reaches its nanometric “cathode”, where it reduces the oxygen of the air into water.

The working process of this tiny air cell is wonderful in itself; but, and here we come to the applicative interest, the truly remarkable thing is that it works equally well even outside the bacterial cell of origin. Specifically, the researchers put part of the purified enzyme in a small vial with hydrogen and an electron acceptor that changes color when reduced, demonstrating that in the test tube the hydrogen was consumed until it was no longer detectable, while the color of the ‘electron acceptor changed; all in a temperature range from zero to 80 degrees, and without the enzyme degrading over time. They also built simple electrical circuits with Huc attached to an electrode and found that they could thus generate small currents, in which electrons were transferred to a metal cathode.

Huc could then be used in fuel cells or for generators powering low-energy devices, if it could be produced in large enough volumes – something yet to be proven. However, the detail with which the functioning mechanism of the enzyme has been described makes it possible to design similar artificial enzymes with even better properties, especially in terms of industrialization; and perhaps, in this way, after billions of years since the process has evolved naturally, we too will be able to exploit it to our advantage to literally extract electricity from the air.