Bacterial tetraethers from Tibetan hot springs: Implications for nitrogen metabolism and biological sources

Branched glycerol dialkyl glycerol tetraethers (bGDGTs) are considered to be produced by bacteria that are predominantly found in soils and peat bogs. Recently, however, in situ production of bGDGTs is reported from a terrestrial hot spring in the Great Basin. In this study, we analyzed water chemistry, bacterial lipids, and pyrosequencing data from 37 Tibetan hot springs in order to evaluate the linkage between biological sources, metabolic processes and the distribution of bGDGTs. Analyses of absolute and relative concentrations of intact polar- and core bGDGTs (IP-bGDGTs and C-bGDGTs) suggest that the bGDGTs are predominantly produced in situ in Tibetan hot springs. Cluster analysis separated the hot spring samples into three major groups, which showed significant relationships between bGDGTs and concentrations of ammonia, nitrite or nitrate. The nirS gene abundance also correlated significantly with bGDGTs. These results indicate that the bGDGT-producing organisms may be involved in nitrogen metabolism in the Tibetan hot springs. Pyrosequencing analysis identified eight phyla of Bacteria (Acidobacteria, Bacteroidetes, Chlorobi, Firmicutes, Nitrospirae, Proteobacteria, Verrucomicrobia and Spirochetes) that may be potential sources of bGDGTs based on significant correlations of these organisms with the distribution of different bGDGTs. Representatives of these phyla have been implicated in nitrogen oxide transformations in many diverse environments including hot springs. Overall, our results suggest that bacteria producing bGDGTs may play an important role in nitrogen cycle in the Tibetan hot springs.