Preferential temperature and ammonia concentration for in-situ growth of Candidatus Nіtrоѕосоѕmісuѕ ammonia oxidising archaea

Among cultivated and characterised ammonia oxidising archaea (AOA), representatives of Candidatus Nitrosocosmicus are unique in their ability to grow at high ammonia concentration (up to 100 mM), at concentrations that are tolerated by many ammonia oxidising bacteria (AOB). These strains also grow at a wide range of incubation temperatures (4–45 °C), with highest ammonia oxidation rates at relatively high temperature (28–40 °C). In addition, ammonia oxidiser growth is often promoted by reduced competition for ammonia, such as in the presence of a specific inhibitor against ammonia oxidiser competitors. Therefore, this study aimed at assessing the optimal conditions (temperature and ammonia concentration) of Ca. Nitrosocosmicus in soil by determining the nitrification rate and the growth of Ca. Nitrosocosmicus AOA and AOB in pH 7.5 soil microcosms amended with inorganic ammonia and octyne and incubated at a range of temperatures (15–35 °C). It demonstrated that growth of Ca. Nitrosocosmicus AOA increases with incubation temperature in soil, with an optimum of 25 °C. In addition, growth of Ca. Nitrosocosmicus is greater when AOB are inhibited, especially under high NH4+ concentration. This study indicates that Ca. Nitrosocosmicus is tolerant to high NH4+ concentration in soils, which contradicts the accepted belief that AOA growth is inhibited in soil with high NH4+ concentration, and it also confirms the role of a near-neutrophilic AOA in nitrification activity in soil with higher nitrogen content. This study also shows the relevance and limitations of cultivated strains in predicting microbial growth in natural environments.