Bacteria Consumes Nitrous Oxide When Oxygen Disappears—We Can Use It to Tackle Climate Change

The most infamous greenhouse gases are carbon dioxide (CO2) and methane, and there is much international focus on reducing emissions of these. However, nitrous oxide (N2O) now ranks alongside these as a cause for great concern.

For around 15 years, we have been researching the potential for bacteria to be used as a "sink" to remove N2O from the atmosphere.

Historically, N2O is best known to the general public as "laughing gas"—the anesthetic used in a dentist's surgery. Although it only accounts for around nine percent of total greenhouse gas emissions, it has around 300 times the global warming potential of carbon dioxide and stays in the atmosphere for about 120 years. N2O also destroys the ozone layer with a similar potency to chlorofluorocarbons (CFCs) and is considered the single biggest cause of ozone depletion over the Arctic.

More than 80 percent of N2O emissions globally are associated with the agricultural and waste treatment industries. Throughout the 20th century and continuing into the 21st century, N2O in the environment has increased by 50 parts per billion and this is rising by around 0.25 percent each year. Even a small fraction of N2O emitted into the atmosphere can have far-reaching consequences for the environment, and it is arguably one of the most important anthropogenic substances emitted in the 21st century.

While there have been efforts to address CO2 emissions, N2O is now emerging as a pressing global concern. It is vital that we begin to predict the environmental impact of N2O and mitigate its release. This requires researchers with different skill sets to come together from around the world to prevent the next wave of climate change.

To realise the 1.5 degree Centigrade limit enshrined in the U.N. global climate agreement, emission of greenhouse gases must decrease significantly and rapidly.

The U.N. sustainable development Goal 13 is "to transform our world by taking urgent action to combat climate change and its impacts." To achieve this, changes in agricultural practices are of course essential. But new strategies are also needed to mitigate and control anthropogenic N2O emissions, without negatively impacting food security to sustain expanding populations globally.

Our team, working as the Nitrous Oxide Research Alliance (NORA), comprises groups from around Europe funded under the auspices of a European Union Marie Curie Sklodowski International Training Network.

We have been researching a process called "denitrification"—which is a major global source and sink for N2O.

When faced with a shortage of oxygen, many bacterial species (exemplified by Paracoccus denitrificans) are able to switch from oxygen respiration to using nitrates to support respiration. During this denitrification process, water-soluble nitrates are converted to gases including N2O, that are emitted into the atmosphere.

Denitrification is central to the biogeochemical cycling of nitrogen and has well-established industrial applications in water purification and waste water treatment. Our latest findings, published in PNAS, show that some soil bacteria are primed ready to consume N2O when there is no oxygen present.

It was previously thought that bacteria had to first sense N2O before they could breathe and consume it in place of oxygen. But we discovered that bacteria "hedge their bets" and gamble on N2O being present in their environment, and so keep the systems for N2O destruction active—and even deliberately distribute them within new cells—to give them a chance to survive low oxygen levels within the soil.

In this study, we tagged the systems for nitrous oxide production and destruction so that we could see them by microscopy and follow their synthesis within new bacteria in the laboratory without changing the natural activity of the cells.

Our findings show that bet-hedging is prominent below 20 degrees Centigrade and may be widespread in soil organisms, so this natural phenomenon could be harnessed to our advantage to control N2O emissions and combat climate change.

This work will help inform policy makers of the potential to exploit bacteria as sinks to remove this powerful climate-active gas from the atmosphere.

So can we mitigate N2O release? Despite decades of research into N2O emissions, no plausible large-scale mitigation options have been put forward to date—apart from reducing nitrogen inputs from fertilizers and nitrogen fixation. But this will not work because fertilizers are needed to feed the increasing world population.

So we must find other solutions. We acknowledge that this will not be easy, but we are certain that the chances of finding such management options will be greatly enhanced by integrated research teams that bring together microbiologists, biochemists and soil scientists with those in the agricultural industry, such as fertilizer manufacturers.

It is clear that many factors influence N2O release from soil bacteria, including nitrogen input, pH, copper content and oxygen levels. Managing this release is complex. But having a better understanding of the factors that impact release at a bacterial level creates opportunities for better management of agricultural practice to minimize emissions.

From our studies, we can speculate that soils all around the world contain high populations of bacteria "armed" with the enzyme required to destroy N2O and this enzyme is primed to operate as soon as oxygen is depleted from the soil environment.

The question now is how to harness this bacterial resource that is already present in the ground beneath our feet to tackle the global problem of N2O emissions.

Professor David Richardson and Dr Andrew Gates are from the School of Biological Sciences at the University of East Anglia, U.K.

The views expressed in this article are the authors' own.