Scientists have been working to analyze all of the various factors that contribute to climate change, and microbes are one influence whose effect is largely still a mystery. Researchers have now found a class of enzymes that allow bacteria to utilize nitrate when they're in an environment with low oxygen levels. Microbes can sometimes employ unusual or alternative biochemical pathways when their survival depends on it. When this class of enzymes breaks down nitrate, nitrous oxide gas is produced, which is a potent greenhouse gas. The findings have been reported in the Proceedings of the National Academy of Sciences (PNAS).
Nitrous oxide is the third most significant greenhouse gas after carbon dioxide and methane. However, methane does not last as long in the atmosphere as carbon dioxide, and nitrous oxide is even more short-lived than methane. Reducing nitrous oxide emissions could, therefore, have immediate benefits for the environment.
One way to do that might be to lower the amount of certain fertilizers that are frequently used in agriculture. These fertilizers can lead to excess nitrate levels in soil, which is then metabolized by soil microbes, while generating nitrous oxide in the process. If fertilizer is used more sparingly, it could be beneficial for everyone, and might save farmers money.
"Nitrous oxide is a much more difficult greenhouse gas to monitor than carbon dioxide, but with this research we now know there are way more sources producing nitrous oxide than previously thought," said senior study author Woody Fischer, Professor of Geobiology at Caltech.
"Understanding where and when this gas is released into the atmosphere can help us make smarter decisions. There's a not-too-distant future in which a farmer has information about the communities of microbes present in their soil, enabling informed decisions about how and when to use fertilizer for landscape health."
In this study, the researchers analyzed thousands of microbial genomes, looking for microbial genes that encode for enzymes enabling microbes to 'breathe' or perform cellular respiration. While most microbes can utilize oxygen for this purpose, some microbes can produce enzymes that use nitric acid instead, and which produce nitrous oxide gas.
Bacteria can switch from respiring oxygen to nitric oxide in various environments such as wetlands, soils, or lakes, for example, when oxygen levels are ten percent lower than what is normally found in the atmosphere.
"We've missed large regions of the biosphere where nitrous oxide was being produced because these proteins were undiscovered," noted Fischer. "Now we can much more accurately predict, through genomic sequence information, which organisms in which environments are producing nitrous oxide. There are way more than we thought."
Sources: California Institute of Technology, Proceedings of the National Academy of Sciences (PNAS)
