Hydrochemistry Predominantly Shapes the Unique Nitrogen-Fixing Bacterial Communities in Tibetan Hot Springs.

The Tibetan Plateau has a large number of hot springs with varying temperatures and hydrochemistry, high elevation, and limited nitrogenous nutrition. Nitrogen-fixing bacteria (NFB) can fix N2 to form ammonia and thus provide bioavailable nitrogen. However, there is limited knowledge about the distribution of NFB and its influencing factors in Tibetan hot springs. Here, we measured hydrochemical variables of the hot springs with a wide temperature range (32–77 °C) in the Qucai and Daggyai geothermal zones on the Tibetan Plateau and investigated the composition of NFB using high-throughput sequencing of 16S rRNA and nifH genes. The Cl−/SO42− ratio in Qucai hot springs was higher than that in Daggyai, indicating that Qucai and Daggyai hot springs were more affected by the supply of liquid and gaseous phases, respectively. The NFB communities consisted predominantly of Nirtospirae, Chloroflexi, Deltaproteobacteria and an unidentified clade, with the last three acting as the main NFB with over 42% of the communities (the proportions are significantly larger than those found in hot springs of other geothermal regions). This demonstrates the uniqueness of NFB communities in Tibetan hot springs. NFB richness was limited by temperature in the studied Tibetan hot springs and was significantly lower than in low-elevation geothermal regions. The NFB community was predominantly affected by hydrochemistry, in contrast to the entire prokaryotic community, which was primarily influenced by temperature. This study expands our current understanding of NFB distribution and diversity as well as biogeochemical process in geothermal spring environments. [ABSTRACT FROM AUTHOR]