Ecophysiological niche expansion driven by biofilm lifestyle in an archaeal soil nitrifier

As key drivers of nitrification, ammonia-oxidizing archaea (AOA) play a central role in the global nitrogen cycle and contribute significantly to the emissions of the potent greenhouse gas nitrous oxide (N ₂ O). However, the ecological implications of AOA growth as biofilms, remain poorly understood. Since nitrite production can be used to follow cellular activities directly we were able to compare biofilms with planktonic cells of the terrestrial model AOA Nitrososphaera viennensis at ecologically and agriculturally relevant conditions. Biofilms were more resistant across nearly all tested conditions and remained active at lower temperatures, acidic pH, and high ammonium concentrations. Collectively, activities in biofilm help reconcile discrepancies between earlier laboratory and environmental observations of soil AOA. Additionally, biofilms showed a high general resilience and lowered sensitivities to nitrification inhibitors. Although in situ biofilms grown in microrespiratory chambers exhibited activity and ammonia affinity similar to planktonic cells, biofilm cultures produced only half as much N₂O. The enhanced fitness of biofilms across all tested conditions vastly expands the potential ecophysiological niche of AOA and supports the hypothesis that biofilm growth represents the in situ phenotype of AOA in soil environments.