'Candidatus Desulfonatronobulbus propionicus': a first haloalkaliphilic member of the order Syntrophobacterales from soda lakes.

Propionate can be directly oxidized anaerobically with sulfate as e-acceptor at haloalkaline conditions either incompletely to acetate (an example is Desulfobulbus alkaliphilus), or completely (for example by the members of genus Desulfonatronobacter). An enrichment with propionate at methanogenic conditions (without sulfate) inoculated with mixed sediments from hypersaline soda lakes in Kulunda Steppe (Altai, Russia) resulted in a domination of a new member of Syntrophobacteraceae (Deltaproteobacteria) in a consortium with the haloalkaliphilic lithotrophic methanogen Methanocalculus alkaliphilus. Transfer of this culture to a medium containing propionate as e-donor and sulfate as e-acceptor resulted in a disappearance of the methanogen and sulfide formation by the bacterial component, finally isolated into a pure culture at these conditions. Strain APr1 formed a distinct phylogenetic lineage within the family Syntrophobacteraceae, being equally distant from its members at the genus level. Phenotypically, strain APr1 resembled the species of the genus Syntrophobacter with substrate spectrum restricted to propionate and propanol utilized with sulfate, sulfite and thiosulfate as the e-acceptors. Propionate is oxidized incompletely to acetate. It is a moderately salt-tolerant (max. 1.2 M Na+) obligate alkaliphile (pH opt. 10). The isolate is proposed to be classified as a new candidate genus and species 'Candidatus Desulfonatronobulbus propionicus'.