John Timmer

Image of strings of small green cells

The price of photovoltaic power has plunged, making it competitive with fossil fuel-powered electrical generation. But there is still a range of applications, like ships and aircraft, where electrical power doesn’t help much. And storing the electricity produced by solar power so that it can be used at night remains an unsolved problem. For those reasons, there’s been continued interest in converting solar power to a fuel that can be stored, either through the use of electricity generated by photovoltaics or by using light to directly power fuel generation.

There’s obviously a means of generating fuel through light that’s been in use for roughly 3 billion years: photosynthesis. But photosynthesis requires a large and complex suite of proteins that’s hard to maintain outside of cells. And inside of cells, the products of photosynthesis are quickly put to use to help the cells grow. So, engineering a version of photosynthesis that might be useful for fuel production has been challenging.

Earlier this week, however, researchers from the University of Kiel described how they’ve rearranged some photosynthetic proteins to make bacteria that emit hydrogen when exposed to light.

Hold the oxygen

In certain photosynthetic bacteria called cyanobacteria, it’s normal to produce hydrogen in short spurts. It’s part of a process that the cyanobacteria use to shut down photosynthesis when things go dark. This typically leaves the cyanobacteria with spare electrons in their photosynthetic systems, which they combine with some of the hydrogen ions left over from splitting water, resulting in a molecule of hydrogen. But this only happens for a very short burst before these electrons are used up.

Once light is restored, photosynthesis starts up again, and electrons become plentiful. But photosynthesis also leads to the production of oxygen, as the proteins that produce the hydrogen turn out to be oxygen sensitive. So, once the oxygen is produced, these enzymes are shut down, and hydrogen production stops again. Combined, these things ensure that the window for hydrogen production is very brief.

Ideally, to produce any sort of useful fuel, we’d like the system to be active all the time. So, the researchers set out to engineer a version that could be.

The primary limitation of the system is the fact that the hydrogen-producing enzyme is a backup—it’s only active if the electrons have nowhere else to go. That, in part, is based on its interactions with the complex that uses light to liberate electrons. The movement of electrons is optimal at certain distances from their source, and the hydrogen-producing enzyme is usually docked off in an awkward location.

So, the researchers completely re-engineered the proteins. They deleted docking sites that allowed other protein complexes to interact with the electron producing one. And they altered the docking site of the hydrogen-producing complex so that it was moved closer to where the electrons are produced. Ideally, these changes should make the hydrogen-producing complex the primary target of electrons liberated 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 engineered ones. The resulting cyanobacteria grew considerably more slowly than their non-engineered cousins, but they did continue to grow. This indicated that electrons still got to where they were needed often enough to power the cyanobacteria’s normal metabolic activity—the changes hadn’t redirected everything towards making hydrogen.

As in normal cyanobacteria, illuminating them resulted in a short burst of hydrogen production, which quickly went away in both strains as oxygen built up. If they shifted the cyanobacteria to an oxygen-free environment and added a system to scavenge free oxygen atoms, however, it was possible to get the engineered strain to keep on producing hydrogen. They could get rid of the oxygen scavenging by simply flowing nitrogen gas over the bacterial cultures, eliminating the oxygen from the environment.

The engineered cyanobacteria produced the highest levels of hydrogen yet seen in these organisms, and they could continue producing hydrogen for hours. Presumably, they’d eventually scavenge enough hydrogen ions from the solution they were growing in to change its pH, but this didn’t seem to be a problem during several hours of illumination.

The researchers behind the work say there are a number of ways they can potentially improve the flow of electrons within their engineered complex. And, ultimately, it would be ideal to make the process less sensitive to oxygen in general.

But they argue that their approach provides a big benefit over previous efforts in this area. Many of these have focused on removing the photosynthesis components from a living cell in order to precisely control the pathways that are active in order to bias production toward hydrogen or other fuels. But, outside the cell, these components quickly pick up damage and can’t be replaced. Alternatives that operate in an intact cell face the challenge of keeping the cell from diverting energy into the pathways it needs for rapid reproduction. This work, the researchers argue, confirms that you can have the benefits of working in living cells while at the same time engineering away some of those competing pathways.

Nature Energy, 2020. DOI: 10.1038/s41560-020-0609-6  (About DOIs).



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