The price of photovoltaics has dropped and is therefore competitive with electricity generation from fossil fuels. But there are still a number of applications, such as ships and planes, where electrical energy doesn't help much. The storage of electricity generated by solar energy for nighttime use remains an unsolved problem. For these reasons, there is still interest in converting solar energy into a fuel that can be stored, either by using electricity from photovoltaics or by using light to generate electricity directly.
There is obviously a means of producing fuel by light that has been used for about 3 billion years: photosynthesis. However, photosynthesis requires a large and complex set of proteins that are difficult to maintain outside of cells. And inside cells, photosynthesis products are quickly used to promote cell growth. Developing a version of photosynthesis that could be useful for fuel production was therefore a challenge.
Earlier this week, researchers at Kiel University described how they rearranged some photosynthetic proteins to make bacteria that emit hydrogen when exposed to light.
Hold the oxygen
With certain photosynthetic bacteria, the so-called cyanobacteria, it is normal to produce hydrogen in short jumps. It is part of a process by which the cyanobacteria turn off photosynthesis when it gets dark. This typically leaves the cyanobacteria with spare electrons in their photosynthesis systems, which they combine with some of the hydrogen ions that remain when water is split, resulting in a hydrogen molecule. However, this only happens for a very short burst before these electrons are used up.
As soon as the light is restored, photosynthesis starts again and the electrons become abundant. However, photosynthesis also leads to the production of oxygen, since the proteins that produce the hydrogen turn out to be sensitive to oxygen. As soon as the oxygen is produced, these enzymes are switched off and hydrogen production stops again. Together, these things ensure that the window for hydrogen production is very short.
Ideally, we want the system to be constantly active to produce any kind of useful fuel. So the researchers set out to develop a version that it could be.
The main limitation of the system is the fact that the hydrogen-producing enzyme is a backup – it is only active if the electrons cannot go anywhere else. This is due in part to its interactions with the complex, which uses light to release electrons. The movement of the electrons is optimal at certain distances from their source, and the hydrogen-producing enzyme is usually docked in an unfavorable place.
The researchers have therefore completely redesigned the proteins. They deleted docking sites that allowed other protein complexes to interact with the electron-producing one. And they changed the docking point of the hydrogen-producing complex so that it was brought closer to the place where the electrons are produced. Ideally, these changes should make the hydrogen-producing complex the primary target of the electrons that are released when light strikes the resulting complex.
Let there be light
The researchers then deleted the normal versions of the proteins that make up these complexes and replaced them with the constructed ones. The resulting cyanobacteria grew significantly slower than their undeveloped cousins, but they continued to grow. This indicated that electrons still arrived often enough where they were needed to drive the normal metabolic activity of the cyanobacteria – the changes hadn't redirected everything to producing hydrogen.
As with normal cyanobacteria, lighting led to a brief burst of hydrogen production, which quickly disappeared in both strains when oxygen built up. However, if they relocated the cyanobacteria to an oxygen-free environment and added a system to trap free oxygen atoms, it was possible to get the constructed strain to continue producing hydrogen. You could remove the oxygen binding by simply flowing nitrogen gas over the bacterial cultures and thus removing the oxygen from the environment.
The manipulated cyanobacteria produced the highest levels of hydrogen previously observed in these organisms, and they were able to produce hydrogen for hours. Eventually, they would likely trap enough hydrogen ions from the solution in which they grew to change their pH, but this didn't seem to be a problem during several hours of lighting.
The researchers behind the work say that there are a number of ways to improve the flow of electrons in their technical complex. Ultimately, it would be ideal to make the process generally less sensitive to oxygen.
However, they argue that their approach offers a great advantage over previous efforts in this area. Many of these have focused on removing the photosynthetic components from a living cell to precisely control the active pathways to target production to hydrogen or other fuels. Outside the cell, however, these components quickly take damage and cannot be replaced. Alternatives that work in an intact cell face the challenge of preventing the cell from diverting energy into the paths it needs for rapid reproduction. This work, the researchers argue, confirms that while you work out some of these competing routes, you can have the benefits of working in living cells.
Nature Energy, 2020. DOI: 10.1038 / s41560-020-0609-6 (About DOIs).