Could mutant plants save us from global warming?

Atmospheric carbon dioxide levels are rising, triggering global climate change, scientists agree. Researchers have been searching for ways to scrub some of this damaging gas from the atmosphere, and the answer may have been right in front of them.

"We actually have taken our inspiration from nature itself," says Tobias Erb, a biochemist at the Max Planck Institute for Terrestrial Microbiology in Germany, in a phone interview with The Christian Science Monitor.

Plants and other photosynthesizing organisms can turn carbon dioxide into biomass. And now Dr. Erb and his team have built a synthetic pathway to do that more efficiently – at least in a test tube, and perhaps someday in plants or other organisms. Their results are published in a paper published Thursday in the journal Science.

Microscopic organisms first solved the problem of how to "fix" atmospheric carbon dioxide about 2.5 billion years ago. Photosynthetic bacteria and algae created today's oxygen-rich air by "breathing in" carbon dioxide (CO2), locking carbon (C) into their biomass, and "exhaling" oxygen gas (O2). After they died, if they were buried beneath sediment instead of decomposing at the surface, heat and pressure eventually turned some of their carbon into the fossil fuels human use today to power cars, heat homes, and make anything plastic. 

So although those ancient organisms made it possible for us to breathe by locking away the excess carbon from the atmosphere, humans are now pumping that very same carbon back into the atmosphere, and much faster than any modern photosynthesizer can suck it out. 

Some environmentalists have suggested an increase in plants might help remedy the situation. But today, photosynthesizing organisms convert just 25 percent of humans' carbon emissions into biomass each year. In other words, humans are releasing four times as much carbon as these plants and bacteria can take in. 

Part of the problem is that the plant enzyme involved in carbon dioxide fixation, as the process is called, is actually a very slow catalyst. So that's why Erb and his team decided to build their own.

"There are actually a lot of bacteria and microorganisms that can also fix CO2. And they use different mechanisms," Erb says, "which tells us that nature itself has the potential to find other ways to fix CO2."

After examining the options, Erb and his team created their pathway for carbon dioxide fixation using 17 different enzymes taken from 9 different organisms, ranging from bacteria to plants to humans. And when they had fit all the puzzle pieces together, the resulting system was much speedier at fixing carbon dioxide – at least in a test tube.

"They have done this in a totally cell-free system," points out F. Robert Tabita, a microbiologist at Ohio State University who was not part of the research team, in a phone interview with the Monitor. "When you think about putting it into an organism, there are a lot questions that would have to be answered," including whether it is compatible, whether the pathway will actually be expressed at its optimal, test tube level, and whether the process is sustainable over long periods of time.

"This is an exciting outcome for systems biology, demonstrating that novel theoretical CO2 fixation pathways can indeed be realized," writes Lisa Ainsworth, a plant biologist at the University of Illinois at Urbana-Champaign who was not part of the research, in an email to the Monitor. "Whether this pathway or another novel pathway could be engineered into plants is an open question, but this research certainly advances the possibility."

Erb agrees that a lot more needs to be figured out before this pathway, a reaction networked called the crotonyl–coenzyme A (CoA)/ethylmalonyl-CoA/hydroxybutyryl-CoA (CETCH) cycle, can be transplanted into any organisms. 

When the researchers are ready for that stage, it might be easier to start with photosynthetic bacteria, as they are easier to cultivate and manipulate than plants, Erb says. 

Dr. Ainsworth adds that an engineered system like the new CETCH cycle probably won't replace the pathway that naturally exists in plants, the Calvin Benson Bassham (CBB) cycle, as the CBB cycle also produces compounds essential for plants' growth.

"A more efficient CO2 fixation cycle is exciting, and there may be the potential to add this to plants to supplement the CBB cycle, but primary and secondary metabolism co-evolved with the CBB cycle and are intimately dependent upon it," says Ainsworth.

Jeffrey Way, senior staff scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard University who was not involved in the research, is also skeptical about how readily this pathway will be able to be slipped into a living organism.

He agrees with Erb that bacteria would be a good place to start, but, he says, "it's impressive that they got this to work at all, even in a test tube."

Erb is cautious about the implications of his work for mitigating climate change. "I don't want to sell myself as solving the problem of climate change," he says. Instead, he is more focused on the potential of employing this pathway, or building others like it, that could be used to harvest CO2 from the atmosphere as a way of building useful, carbon-based materials.

Dr. Tabita agrees that scaling such a mechanism up to have a global impact on the CO2 levels could "get to be a little dicey." But Dr. Way thinks these sorts of incremental technological advances might be just the key in fighting global climate change.

"It's going to be a long hard slog" that will require many scientists to come up with incremental solutions from different angles over decades, Way says in a phone interview with the Monitor. And combining those pieces may help, he says.

One other breakthrough that might be combined with Erb's CETCH cycle is the so-called bionic leaf, Way says. Some of his colleagues at Harvard University have designed an artificial photosynthesis system that harnesses sunlight to produce fuels. Perhaps the CETCH cycle can be incorporated into this system too, Way and Erb both suggest.

Another study in Science this week also looked at improving the efficiency of photosynthesis in plants. That study focused on improving crop yields, but the modified plants were also better at carbon fixation. 

Researchers looking for ways to pull carbon dioxide out of the atmosphere haven't just looked at biology for solutions. Suggestions have ranged from turning CO2 into stone to disposing of the gas in the deep ocean. And, of course, there's always the option of not putting the greenhouse gas in the atmosphere in the first place.