The thermophilic microbial community of a hydrotherm at East-Tuvinian upland is described. High-throughput profiling of the 16S rRNA gene and sequencing of the complete metagenome were used to characterize the phylogenetic and metabolic diversity of the microbial community of the Dikij Arjaan alkaline thermal spring of Ush-Bel'dir mineral water deposit in Tyva, Russia. Members of the uncultured phylum ``Acetothermia'' (candidate division OP1) predominated in the microbial community of the spring. While this phylum has already been detected in many thermal ecosystems, it never formed the main component of their microbial communities. Apart from ``Acetothermia'', members of Nitrospirae and Chloroflexi comprised a significant part of the microbial community. Comparative analysis of our geochemical data on the spring and the previously published data on in silico reconstruction of Acetothermia metabolism led us to the suggestion that carbonate and bicarbonate can serve as the major carbon sources for the dominant bacterial group and that Acetothermia act as the primary producers in this ecosystem. Analysis of the functional genes also revealed the presence of physiological groups of denitrifyers, iron reducers, carboxydotrophs and diazotrophs. This is the first report on a thermophilic microbial community of a hydrotherm from the south-western extremity of the Baikal rift zone. Moreover, microbial communities with predominance of uncultured Acetothermia have not been reported previously in terrestrial hydrotherms.