Diversity and Distribution of Nitrogen-Fixing Bacteria in Two Geothermal Channels in Tengchong Geothermal Zone, Yunnan~Province

To investigate the composition and diversity of nitrogen-fixing bacteria community and their responses to temperature gradients along two hot spring channels in Tengchong, Yunnan Province, China. Five sampling locations in the Hmzfj channel and four sampling locations in the Srbz channel were selected along outflows of each corresponding channel. The temperature and pH of the sampled waters were measured. The diversity of nitrogen-fixing bacteria community was studied through phylogenetic analyses of {\it nifH} genes. Temperatures of the nine sampling sites in the two hot spring channels in Tengchong range from 40.7 $^\circ$\hspace{-0.8pt}C to 86.8 $^\circ$\hspace{-0.8pt}C with a pH gradient of 6.5-8.7. Seven {\it nifH} gene clone libraries were constructed, and a total of 255 clones were randomly picked and subjected to phylogenetic analysis. The obtained {\it nifH} gene clones were grouped into 25~OTUs (operational taxonomic unit) at the cutoff of 97\% similarity. The nitrogen-fixing bacteria are mainly composed of {\it Chloroflexi}, {\it Cyanobacteria} and {\it Proteobacteria} in the two studied geothermal channels. Diversity indices of nitrogen-fixing bacteria community show no distinct differences among the studied sites. The PCoA (principal coordinates analysis) and cluster analyses illustrate that temperature played an important role in shaping the community structures of nitrogen-fixing bacteria in the studied samples. Temperature was the major factor influencing the composition of nitrogen-fixing bacteria community in Tengchong hot~springs