Laughing Gas Kills Soil Microbes That Help Plants Grow

Dentists use it to take the edge off a tooth extraction. Drag racers inject it to boost engine power. And every time a farmer waters a fertilized field, nitrous oxide bubbles up from the soil - a byproduct of microbial metabolism that has spent decades in the climate conversation without anyone asking what it does to the bacteria it surrounds.

A new study from MIT suggests the answer is: quite a lot, and not in a good way.

Vitamin B12 is the target

Nitrous oxide's toxicity in humans has been known for decades. Researchers discovered it can deactivate vitamin B12, which the body requires for a range of metabolic functions. That fact mostly stayed in medical contexts, applied to cases of recreational overuse or occupational exposure. Nobody thought much about what it was doing in the rhizosphere - the zone of soil surrounding plant roots, where millions of bacteria compete and cooperate to help plants access nutrients and fight off pathogens.

Darcy McRose, MIT's Thomas D. and Virginia W. Cabot Career Development Professor in the Department of Civil and Environmental Engineering, and PhD student Philip Wasson thought it was worth a look. Their findings, published in mBio, a journal of the American Society for Microbiology, suggest the assumption that N2O "doesn't interact with organisms" in agricultural soil needs to be reconsidered.

The experiment that demonstrated the mechanism

Cells need the amino acid methionine to grow. Bacteria can make it through two different pathways: one that requires vitamin B12, and one that does not. Many bacterial species carry genes for both. McRose and Wasson started with Pseudomonas aeruginosa, a well-studied bacterium, and genetically removed the B12-independent pathway. What they were left with was a microbe that had to rely on B12 to make methionine - and was therefore vulnerable to anything that deactivated it.

That microbe became sensitive to nitrous oxide. Its growth was impaired even by the N2O it produced itself during normal metabolism.

Next, they tested a synthetic community of bacteria collected from the roots of Arabidopsis thaliana, the small flowering plant used as a model organism in plant biology. Many of these root-dwelling bacteria were also sensitive to N2O. When they co-cultured sensitive bacteria with N2O-producing bacteria, the sensitive ones struggled to grow.

"This suggests that N2O-producing bacteria can affect the survival of their immediate neighbors," Wasson explained.

How many bacteria are at risk?

The researchers went further. They surveyed bacterial genomes to estimate how many species rely only on the B12-dependent methionine pathway - and are therefore potentially vulnerable to N2O. Their estimate: roughly 30 percent of bacteria with sequenced genomes. That is a substantial fraction of the microbial diversity underpinning soil health and plant productivity.

"This suggests that in the environment, exposure to N2O is going to select for certain types of organisms based on their genomic content, which is a highly testable hypothesis," McRose said.

Agricultural soil is where the stakes are highest

In farmed fields, nitrous oxide concentrations spike - sometimes for days or weeks - following nitrogen fertilizer application, rainfall, soil thawing, and other events. These spikes are precisely when the microbial communities around plant roots are under the most pressure anyway. If N2O is selectively suppressing certain bacterial species during those spikes, it could be shifting the composition of rhizosphere communities in ways that affect plant health, nutrient uptake, or disease resistance.

"This work suggests N2O production in agricultural settings is worth paying attention to for plant health," McRose said. "It hasn't been on people's radar, but it is particularly harmful for certain microbes. This could be another knock against N2O in addition to its climate impact."

A proof of concept, not a field result

The researchers are careful about the limits of what they have shown. Lab experiments with Pseudomonas aeruginosa and synthetic microbial communities are controlled environments quite different from agricultural fields with their variable temperatures, moisture levels, and vastly more complex microbial ecosystems. Whether the effect observed in vitro shows up as a detectable signal in genome sequencing studies of agricultural soil is the next question.

Wasson calls the paper a proof of concept and plans to study actual agricultural soil next. "We want to see if we can detect a signature for this N2O exposure through genome sequencing studies, where the only microbes sticking around are not sensitive to N2O," he said.

If that signature shows up, it would open a new angle on how farming practices - the timing of irrigation, fertilization, and other events that drive N2O spikes - might be managed to protect the microbial communities that crops depend on. The work was supported by the MIT Research Support Committee and a MIT Health and Life Sciences Collaborative Graduate Fellowship.