Unexpected numbers of microbes are fighting nitrous oxide

An international team of researchers has discovered that the range of microorganisms that combat nitrous oxide (N₂O) is broader than expected. N₂O – a greenhouse gas which is 300 times more potent than carbon dioxide (CO₂) – is released into the atmosphere through agricultural practices, sewage treatment and fossil fuel combustion, and has the capacity to destroy sections of the Earth’s ozone layer. The researchers, however, whose results have been published in the journal Proceedings of the National Academy of Sciences (PNAS), discovered that nature wields an unexpectedly large arsenal capable of fighting this damaging pollutant.

Some microorganisms, known as denitrifiers, help to mitigate the harmful effects of N₂O by transforming it into innocuous nitrogen gas (N₂). However, previous studies have identified an inconsistency between the predicted and actual levels of N₂O being emitted into the atmosphere. Essentially, lower quantities of the gas are being released than had been expected, indicating the existence of an unknown N₂O ‘sink’.

The latest study, conducted by researchers from the University of Tennessee, Knoxville (UT), the University of Illinois at Urbana-Champaign, Georgia Institute of Technology (Georgia Tech), the US Department of Agriculture, the University of Puerto Rico (UPR) and India’s National Institute of Abiotic Stress Management (NIAM), seems to account for this apparent discrepancy. Moreover, the team’s findings could facilitate efforts to further reduce N₂O emissions.

To find out more about this natural army of denitrifiers, I spoke to lead researcher Dr Frank Loeffler, Oak Ridge National Laboratory Governor’s Chair for Microbiology at UT. I began by asking how global emissions of N₂O compared with those of other greenhouse gases such as CO₂.

"The most important thing to note is that N₂O is much more potent than CO₂," Dr Loeffler answered. "Its potential to trap solar radiation is far greater; small concentrations of N₂O in the atmosphere can have a significant impact on global warming. Another issue to consider is that N₂O – unlike CO₂ – contributes to the destruction of the ozone layer; it poses a threat on two levels. One of the most pressing concerns, therefore, is that N₂O emissions are likely to increase even more rapidly than those of CO₂. The main source of N₂O is the use of agricultural fertilisers. The global human population is growing, and in order to feed all of these extra people, agricultural activity will have to increase. Fertilisers are also used to produce crops for bioenergy. As the utilisation of fertiliser increases, N₂O emissions will rise."

In light of this worrying assessment, Dr Loeffler and his colleagues were pleasantly surprised to discover that the number of microorganisms capable of denitrification was larger than previously anticipated. After screening the microbial genomes of enzyme systems that catalyse the reduction of N₂O to N₂, the team uncovered an unexpectedly broad distribution of denitrifying enzymes across different groups of microbes.

"Scientists have studied the nitrogen cycle for a long time, and denitrifiers were the only group of organisms known to be capable of reducing N₂O to N₂," explained Dr Loeffler. "It came as quite a surprise, therefore, to find that many other microorganisms outside of this group act as N₂O reductases."

Of course, the fact that a larger natural army than expected is combating this harmful greenhouse gas comes as welcome news. However, whilst it accounts for the aforementioned discrepancy, it does not help our current situation. I asked Dr Loeffler whether in the future, his findings might be used to further reduce N₂O emissions.

"I think that this is, at least in theory, a possibility," he replied. "N₂O fluxes are being measured in fields and forests across the globe. A better understanding of the organisms that are able to catalyse the reduction of N₂O to N₂ could open up new opportunities within the arena of agricultural soil management. For example, we could potentially increase the activity of these types of organism and thus precipitate larger levels of N₂O consumption before the gas is released into the atmosphere."

To conclude our conversation, I asked Dr Loeffler whether he had further research planned in this area. As he explained, he and his colleagues are currently investigating some of the more perplexing observations that have been made.

"We have funding from the US Department of Energy to continue our research," he explained. "Denitrification is thought to be an anoxic process – it requires a dearth of oxygen in order to take place. However, somewhat inexplicably, researchers have observed N₂O production and consumption under conditions in which oxygen has been available. If these observations are corroborated, they may help us to devise a strategy to manage soil in a way that reduces N₂O emissions. There is more work to be done, but I think that our findings will motivate others to study the abundance and activity of the atypical enzyme systems that we have identified."