Sulfur-dependent respiration under extremely haloalkaline conditions in soda lake 'acetogens' and the description of Natroniella sulfidigena sp. nov.

Microbial sulfidogenesis is the main dissimilatory anaerobic process in anoxic sediments of extremely haloalkaline soda lakes. In soda lakes with a salinity >2 M of the total Na(+) sulfate reduction is depressed, while thiosulfate- and sulfur-dependent sulfidogenesis may still be very active. Anaerobic enrichments at pH 10 and a salinity of 2-4 M total Na(+) from sediments of hypersaline soda lakes with thiosulfate and elemental sulfur as electron acceptors and simple nonfermentable electron donors resulted in the isolation of two groups of haloalkaliphilic bacteria capable of dissimilatory sulfidogenesis. Both were closely related to obligately heterotrophic fermentative homoacetogens from soda lakes. The salt-tolerant alkaliphilic thiosulfate-reducing isolates were identified as representatives of Tindallia magadiensis, while the extremely natronophilic obligate sulfur/polysulfide-respiring strains belonged to the genus Natroniella and are proposed here as a novel species Natroniella sulfidigena. Despite the close phylogenetic relation to Natroniella acetigena, it drastically differed from the type strain phenotypically (chemolithoautotrophic and acetate-dependent sulfur respiration, absence of acetate as the final metabolic product). Apparently, in the absence of specialized respiratory sulfidogens, primarily fermentative bacteria that are well adapted to extreme salinity may take over an uncharacteristic ecological function. This finding, once again, exemplifies the importance of isolation and phenotypic investigation of pure cultures.