Ammonia oxidizers in high-elevation rivers of the Qinghai-Tibetan Plateau display distinctive distribution patterns

Ammonia-oxidizing bacteria (AOB) and archaea (AOA) as well as Comammox catalyze ammonia oxidation. The distribution and biogeography of these ammonia oxidizers might be distinctive in high-elevation rivers which are generally characterized by low temperature, low ammonium concentration but strong solar radiation; however, it has rarely been documented. This study explored the abundance, community, and activity of ammonia oxidizers in the overlying water of five rivers in the Qinghai-Tibetan Plateau (QTP). Potential nitrification rates in these rivers ranged from 5.4 to 38.4 nmol-N L-1 h-1, and they were significantly correlated with ammonium concentration rather than temperature. Comammox were found in 25 of the total 28 samples and they outnumbered AOA in three samples. Contrary to most studied low-elevation rivers, average AOB amoA gene abundance was significantly higher than AOA, and AOB/AOA ratios increased with decreasing water temperature. The Simpson index of AOA community increased with elevation (p 0.56) and, owing to their elevational variations in source and concentration, suspended sediments facilitated distance-decay patterns for AOA and AOB community similarities. This study demonstrates distinctive biogeography and distribution patterns for ammonia oxidizers in high-elevation rivers of the QTP. Extensive research should be conducted to explore the role of these microbes in the nitrogen cycle of this zone. Importance Ammonia oxidizing archaea (AOA) and bacteria (AOB) as well as Comammox contribute to ammonia oxidation which plays significant roles in riverine nitrogen cycle and N2O production. Source regions of numerous rivers in the world lie in high-elevation zones, however, the abundance, community, and activity of ammonia oxidizers in rivers at high-elevation regions have rarely been investigated. This study revealed distinctive distribution patterns and community structures for ammonia oxidizers in five high-elevation rivers of the Qinghai-Tibetan Plateau, and the individual and combined effects of low temperature, low nutrient, and strong solar radiation on ammonia oxidizers were elucidated. The findings of this study are helpful to broaden our knowledge on the biogeography and distribution pattern of ammonia oxidizers in river systems. Moreover, this study would provide some implications to predict the performance of ammonia oxidizers in high-elevation rivers and its variations under global climate warming.