Alkalo-thermophilic iron-reducing bacteria of the Goryachinskoe thermal water deposit

A study of the metabolic group of iron-reducing prokaryotes that use the weakly crystalline mineral ferrihydrite as an electron acceptor has revealed its widespread distribution on Earth. However, the ability of iron reducers to develop in polyextreme conditions – at elevated pH values and temperatures – has not yet been demonstrated. To prove the existence of alkalithermophilic iron reducers, we studied water and sediment samples from the Goryachinsk nitrogen springs. These springs are confined to young tectonic faults of the Baikal rift zone, and their waters are characterized by low mineralization, high pH (8.8‒9.2) and temperature (53‒55°C). By adding synthesized ferrihydrite to water and bottom sediment samples from the Goryachinsk springs and incubating them under anaerobic conditions at temperatures of 50 and 55°C, enrichment cultures were obtained that reduce this mineral. Profiling of water and sediment samples and primary enrichment cultures by the 16S rRNA gene revealed their high phylogenetic diversity, represented almost exclusively by bacterial taxa. Further transfers with ferrihydrite and organic acids or hydrogen added as an electron donor allowed us to obtain stable enrichment cultures of organo- and lithotrophic alkalithermophilic iron reducers. From 30 to 50% of their total representation were representatives of the genus Parvivirga , one of the first cultivated representatives of the OPB41 group (now the order Anaerosomatales) of the phylum Actinomycetota.