There's no question that humans are driving long-term changes in the amount of carbon in the atmosphere. But the human influence is taking place against a backdrop of natural carbon fluxes that are staggering in scale. Each year, for example, the amount of CO 2 in the atmosphere cycles up and down by over a percent purely due to seasonal differences in plant growth.

The effectiveness of biological activity provides the hope that we could leverage it to help us pull some of our carbon back out of the atmosphere at an accelerated pace. But the incredible scale of biology hides a bit of an ugly secret: the individual enzymes and pathways that are used to incorporate CO 2 into living organisms aren't that efficient. These pathways are also linked to a complex biochemistry inside the cell that doesn't always suit our purposes.

Fed up with waiting for life to evolve a solution to our industrialization problem, a German-Swiss team of researchers has decided to roll its own. In an astonishing bit of work, they've taken enzymes from nine different organisms in all three domains of life and used them to build and optimize a synthetic cycle that can use carbon dioxide with an efficiency 20 times that of the system used by plants.

Breaking up CO 2

The problem with carbon dioxide is that it's a very stable molecule. It requires a fair bit of energy to break it down, but unless we can figure out how to break it down efficiently, we can't use atmospheric CO 2 for any of the many things we use carbon for, like the polymers in our plastics or the graphite in our electrodes. While various ideas have been floated for incorporating atmospheric CO 2 into usable chemicals, none of them has managed to scale economically yet.

Living organisms, however, do this trick all the time. More than 90 percent of the carbon removed from the atmosphere ends up being made into sugars by photosynthetic organisms, and there are at least five other minor pathways through which organisms build complex molecules starting from CO 2 . All of these processes have issues when it comes to how we might want to use them. Many of them are relatively inefficient; others will only work in environmental conditions that are inconvenient; all of them are plugged into a complex cellular biochemistry that often results in lots of side products or a final product that's not easy to turn into something useful.

All of those annoying features are what you might expect from evolution, which is tuning the carbon reactions for the environments and needs of specific organisms. So, the team behind the new work decided to do what evolution hasn't: bring together enzymes from organisms that would never come in contact with each other and build a pathway that's designed for efficient use of CO 2 .

To do so, they started by focusing on the limiting enzyme in most known pathways: the one that breaks down CO 2 in the first place. The team searched the databases for all enzymes belonging to this class and identified ones that had the properties they were looking for. They settled on a group of enzymes called enoyl-CoA carboxylases/reductases, or ECRs.

ECRs were only discovered fairly recently, and they typically aren't even the main route for obtaining carbon in the organisms that have them. But for the purposes here, ECRs have a lot of good properties: they're highly efficient, don't undergo side-reactions with oxygen, and don't require any unusual chemicals to make the reaction work.

Building a pathway

But the reaction that ECRs catalyze is only the first step, and it would require a constant feed of chemicals to react the CO 2 with. Most organisms obtain carbon dioxide as part of a cycle. They get it to react with a larger chemical, then break off a smaller carbon-containing molecule, and then use a few further reactions to re-form the original chemical. (You can see an example of this in a Calvin Cycle diagram.) So, the team decided to build an entire cycle that incorporates the ECR enzyme.

Rather than adapt an existing cycle, the researchers started from scratch, building hypothetical pathways that use biologically plausible molecules and then evaluating them for energy efficiency. Only once a cycle was identified did they search databases to find out whether any enzymes existed that could catalyze the reaction. They ended up with a 13-step cycle that incorporated CO 2 at two different steps and ended by combining the two resulting carbons with acetic acid to form a four-carbon molecule called malic acid. A number of chemical co-factors and energy in the form of ATP would need to be added along the way, but on paper, it all worked out.

And that's when the real work began.

In total, 12 of those 13 steps required a distinct enzyme to work, so the authors had to obtain the genes for all of these, make proteins, and then purify them. Once they had that, the team showed that adding the enzymes for each step ended up producing the products expected. Once all the enzymes were added, the expected end product (malic acid) was produced.

This process let researchers identify any inefficiencies in the process. For example, things tended to bog down at step 10 of the cycle, leading to the accumulation of the chemical produced by step nine. So, they looked at the enzyme involved and determined the reaction would be more efficient if it used oxygen instead of the chemical it typically required. The team looked at the structure of the enzyme and redesigned it to use oxygen. It worked.

They kept tweaking the pathway. The overall design was replaced with one that used a somewhat different reaction pathway. Some of the enzymes ended up spitting out a bunch of side products that were unusable dead-ends; those were engineered to stop this. In other cases, new enzymes were added to do what the researchers call "proofreading"—when a dead-end side product was made, they converted it back to a useful one.

By the time the team was done, the system used 17 different enzymes from nine different organisms, including bacteria, archaea, plants, and humans. The final system was truly impressive, using carbon dioxide with an efficiency 20 times that of the system used in photosynthesis.

The big picture

Take a moment to appreciate the scale of this accomplishment. In four billion years of evolution, life has only managed to evolve six known pathways that start with carbon dioxide and build more complex molecules. In just a few years, a bunch of grad students in Zurich added a seventh.

There are some pretty obvious limitations to this system as it now stands. A variety of biochemical co-factors need to be added to the reaction to get it to work, and the output—malic acid—is currently only used as a food additive. But malic acid undergoes a variety of reactions within cells, and there's no reason to think that some of these couldn't direct it into a useful industrial chemical. Or, there's no reason to believe we couldn't find other ways of using malic acid if there was suddenly a surplus of it.

The other thing is that the entire pathway can now be put inside cells, either normal bacteria like E. coli or the synthetic cells with a minimal genome that researchers are working on. If that's the case, then the need to supply all the chemical co-factors should go away, since the cells should be producing them anyway. More importantly, if the cell is made to depend on this pathway as its only source of carbon, evolution would have the chance to optimize it even further.

The paper also comes at an interesting time. International climate negotiations are taking place as nations start to grapple with the fact that the Paris Agreement isn't sufficient to keep the planet under the goal of 2 degrees Celsius warming. The US has submitted its plans for the mid-century, which include extensive use of carbon capture and storage to make its energy system carbon neutral. Even then, however, it's likely that we'll need to pull carbon directly out of the atmosphere before this century is out to limit warming.

Something like this, which could make atmospheric carbon into an industrial feedstock, might be essential to enabling that future. The same goes for a separate paper in the same issue of Science that describes re-engineering trees to get them to photosynthesize more efficiently under variable light conditions. We're probably going to need some sort of technology like this, so it's nice to see the fundamental science that could enable it getting done.

Science, 2016. DOI: 10.1126/science.aah5237 (About DOIs).

Updated to clarify the need for external energy supply.