Thioalkalimicrobium aerophilum gen. nov., sp nov and Thioalkalimicrobium sibericum sp nov., and Thioalkalivibrio versutus gen. nov., sp nov., Thioalkalivibrio nitratis sp nov and Thioalkalivibrio denitrificans sp nov., novelobligately alkaliphilic and obligately chemolithoautotrophic sulfur-oxidizing bacteria from soda lakes

Forty-three strains of obligately chemolithoautotrophic sulfur-oxidizing ba cteria were isolated from highly alkaline soda lakes in south-east Siberia (Russia) and in Kenya using a specific enrichment procedure at ph 10. The m ain difference between the novel isolates and known sulfur bacteria was the ir potential to grow and oxidize sulfur compounds at ph 10 and higher. The isolates fell into two groups that were substantially different from each o ther physiologically and genetically. Most of the Siberian isolates belonge d to the group with a low DNA G+C content (48.0-51.2 mol%). They were chara cterized by a high growth rate, a low growth yield, a high cytochrome conte nt, and high rates of oxidation of sulfide and thiosulfate. This group incl uded 18 isolates with a DNA homology of more than 40 %, and it is described here as a new genus, Thioalkalimicrobium, with two species Thioalkalimicro bium aerophilum (type species) and Thioalkalimicrobium sibericum. The other isolates, mainly from Kenyan soda lakes, fell into a group with a high DNA G+C content (61.0-65.6 mol%). In general, this group was characterized by a low growth rate, a high molar growth yield and low, but relatively equal, rates of oxidation of thiosulfate, sulfide, elemental sulfur and polythion ates. The group included 25 isolates with a DNA homology of more than 30%. It was less compact than Thioalkalimicrobium, containing haloalkalophilic, carotenoid-producing, nitrate-reducing and facultatively anaerobic denitrif ying strains. These bacteria are proposed to be assigned to a new genus, Th ioalkalivibrio, with three species Thioalkalivibrio versutus (type species) , Thioalkalivibrio denitrificans and Thioalkalivibrio nitratis. Phylogeneti c analysis revealed that both groups belong to the gamma -Proteobacteria. T he Thioalkalimicrobium species were closely affiliated with the neutrophili c chemolithoautotrophic sulfur bacteria of the genus Thiomicrospira, formin g a new alkaliphilic lineage in this cluster. In contrast, Thioalkalivibrio was not related to any known chemolithoautotrophic taxa, but was distantly associated with anaerobic purple sulfur bacteria of the genus Ectothiorhod ospira.