Ammonia oxidizers in river sediments of the Qinghai-Tibet Plateau and their adaptations to high-elevation conditions.

Ammonia-oxidizing bacteria (AOB) and archaea (AOA) as well as complete ammonia oxidizers (comammox) aerobically catalyze ammonia oxidation which plays essential roles in riverine nitrogen cycle. However, performances of these ammonia oxidizers in high-elevation river sediments have rarely been documented. This study investigated the abundance, community, and activity of ammonia oxidizers in five high-elevation rivers of the Qinghai-Tibet Plateau (QTP). Comammox were dominant ammonia oxidizers in 23% of studied samples and the clade B was principal comammox type. amoA gene abundances of AOA and AOB in these high-elevation rivers were comparable to those in low-elevation rivers. However, in contrast to most studied low-elevation rivers, AOB amoA gene abundance outnumbered AOA in 92% samples, which might be caused by the lower temperature and more intense solar radiation of the QTP. Potential nitrification rates (PNRs) ranged from 0.02 to 2.95 nmol-N h g dry sediment. Ammonia concentration was the limiting factor to PNRs at some sites, and when ammonia was not limiting, the PNR: ammonia ratio was greater at higher temperatures. There was no apparent variation in ammonia oxidizer community compositions along the elevation gradient due to the high elevation (2687 to 4223 m) of our entire study area. However, compared with low-elevation rivers, the lower temperature, huge diurnal temperature change, and lower nutrient conditions in the QTP rivers shaped distinctive communities for ammonia oxidizers; the unique community characteristics were significantly correlated to PNRs. These results suggest that ammonia oxidizers in the five high-elevation rivers have adapted to high-elevation conditions; more research should be conducted to study their adaptation mechanisms and their roles in riverine nitrogen cycle.