Scientists have engineered a bacterium that can take carbon dioxide from the air and turn it into fuel in a single enzymatic step.
The process draws on sunlight to produce methane and hydrogen inside the bacterium Rhodopseudomonas palustris, in essence reversing combustion. These engineered bacteria could guide scientists toward better carbon-neutral biofuels.
Researchers published their results yesterday in the journal Proceedings of the National Academy of Sciences.
Co-author Caroline Harwood, a professor of microbiology at the University of Washington, said the report blossomed from her work studying an enzyme called nitrogenase.
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“We’re really interested in the enzyme nitrogenase because it does a phenomenally difficult reaction,” she said.
In nature, the enzyme serves as a catalyst to help certain bacteria turn inert atmospheric nitrogen gas into reactive ammonia in a process called nitrogen reduction, or nitrogen fixation. The enzyme uses adenosine triphosphate (ATP), a compound that serves as an energy currency in cells.
Without the enzyme, the nitrogen reduction reaction has a huge energy barrier and rarely occurs on its own.
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Researchers wondered if they could tweak nitrogenase to work with other stable and inert molecules. “It’s been sort of recently appreciated that this enzyme is kind of promiscuous and can do other reactions, as well, only not as efficiently,” Harwood said.
Some of her collaborators managed to isolate and alter nitrogenase to use the most oxidized form of carbon, carbon dioxide, as its starting material and produce the most reduced form of carbon — methane. But this modified enzyme was tediously produced in test tubes at small scales, which isn’t good enough for a process that might one day produce industrial quantities of biofuels.
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“We wanted to see if we could get an actual living organism to do this conversion,” Harwood said.
The team prepared a version of the R. palustris bacterium that was modified to crank out the engineered nitrogenase at full blast. In its natural state, the bacterium absorbs sunlight to produce ATP, so light helped generate the energy to power the enzyme in the modified cells.
The researchers found that the modified nitrogenase could no longer fix nitrogen, but it could produce methane and hydrogen when the bacteria were illuminated.
However, the new nitrogenase isn’t anywhere near as efficient at producing methane from carbon dioxide as it is at making ammonia from nitrogen gas. “The normal enzyme makes about two hydrogens for every [molecule of] ammonia,” Harwood said. “The altered enzyme makes a thousand hydrogens for every molecule of methane.”
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Daniel Lessner, an associate professor in the department of biological sciences at the University of Arkansas, Fayetteville, who was not involved in the study, said the findings chalk out a clearer pathway to produce methane, the major component of natural gas, from living organisms.
“It’s exciting,” he said of the new report.
Lessner studies a class of bacteria called methanogens that naturally produce methane. However, they use different starting materials, like acetate.
“The methanogens require other microbes to provide them with other electron donors,” he said. “What you need then is not just one microorganism but multiple microorganisms.”
On the other hand, the new engineered nitrogenase in R. palustris converts carbon dioxide into methane on its own in a single step, simplifying the process. And since it occurs in a living organism, the reaction takes place at ambient temperatures, reducing the energy required to produce a biofuel.
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“The process that’s naturally occurring is still more efficient, but because of the simplicity of this engineered organism, it would make it easier to manipulate the process,” Lessner said.
Harwood said her team is now investigating whether they can tweak the enzyme to improve its efficiency in reducing carbon dioxide, as well as looking for other useful chemicals they could make.
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Reprinted with permission from E&E Publishing.
