A global team of researchers has found that comammox bacteria, which they initially identified in 2015, are capable of thriving on guanidine, an organic compound rich in nitrogen, as their only energy and nitrogen source. This remarkable capability paves the way for focused cultivation of these mysterious microorganisms and might play a significant role in lessening nitrous oxide emissions from agriculture. These research findings have recently been published in the journal Nature.
Nitrification is the process where ammonia is transformed into nitrite and then into nitrate, a task performed by specialized microorganisms known as nitrifiers. This process is crucial for the global nitrogen cycle across nearly all ecosystems, yet it has a complicated impact on global environmental changes. On the positive side, nitrification contributes to the release of nitrous oxide, a powerful greenhouse gas and harmful to the ozone layer, leading to significant fertilizer loss in agriculture and causing eutrophication in water bodies. Conversely, nitrification is vital for nutrient removal in wastewater treatment systems, helping to safeguard water bodies from excessive nitrogen resulting from wastewater. The authors of the study have now identified methods to encourage nitrifiers in the ecosystem that produce lower amounts of nitrous oxide.
“Green” Nitrifiers
Comammox bacteria are referred to as “green” nitrifiers because they produce only minimal amounts of nitrous oxide as a byproduct of their metabolic processes and effectively eliminate nitrogen compounds in wastewater treatment facilities. Historically, since the discovery of nitrifiers in the 19th century, it was believed that these microorganisms could solely use ammonia and urea for respiration. However, in 2015, researchers led by Michael Wagner and Holger Daims revealed that certain nitrifiers could also utilize cyanate, which is chemically unstable, for energy. “In our recently published research, we have now demonstrated that comammox bacteria can also thrive on guanidine, a less conventional substrate,” says Marton Palatinszky, the lead author of the study. “Comammox bacteria possess a transporter and an enzyme, both of which we meticulously characterized, enabling them to convert guanidine into ammonium with high energy efficiency inside the cell.”
Guanidine is a metabolic byproduct from microorganisms and plants. Its role in human and animal metabolism is not well understood. It is produced in soils during the breakdown of synthetic fertilizer additives and appears in wastewater due to the decomposition of the commonly prescribed medication, metformin. However, the distribution and further processes of guanidine in the environment remain poorly understood. The international research team, which includes microbiologists from the Helmholtz Centre for Environmental Research in Leipzig, Germany, and Aalborg University in Denmark, has confirmed that guanidine is found not only in human urine but also in animal waste and that comammox bacteria utilize guanidine in wastewater treatment facilities. They further established that nitrifiers in agricultural soils can metabolize guanidine as well.
New Opportunities for Cultivation and Nitrous Oxide Reduction
The microbiologists in Vienna are now working to enrich and isolate comammox bacteria from environmental samples using guanidine, as currently, only one strain exists in pure culture worldwide. “This endeavor appears particularly promising because none of the other tested nitrifier strains were able to grow using guanidine as their sole source of energy and nitrogen,” remarks Katharina Kitzinger, a Senior Scientist at CeMESS. The team is also exploring whether adding guanidine to agricultural fertilizers could enhance the population of comammox bacteria in farm soils, potentially decreasing nitrous oxide emissions from agriculture.
“This research was made possible through the collaborative efforts of many researchers in the ‘Microbiomes Drive Planetary Health’ Cluster of Excellence initiated in 2023. We extend our heartfelt gratitude to the Austrian Science Fund (FWF) for their exceptional support,” states study leader Michael Wagner.