Growth physiology and competitive interaction of obligately chemolithoautotrophic, haloalkaliphilic, sulfur-oxidizing bacteria from soda lakes

Two different groups of haloalkaliphilic, obligately autotrophic, sulfur-oxidizing bacteria belonging to the genera Thioalkalimicrobium and Thioalkalivibrio have recently been discovered in highly alkaline and saline soda lakes. To understand response to their extreme environment and different occurrence in soda lakes, the growth kinetics and competitive behavior of several representatives have been characterized in detail using batch and pH-controlled continuous cultivation. The bacteria belong to the true alkaliphiles, growing within the pH range 7.5-10.6 with maximum growth rate and maximum growth yield at pH 9.5-10. On the basis of their response to salt content, three groups can be identified. All the Thioalkalimierobium strains and some of the Thioalkalivibrio strains belonged to the moderate halophiles. Some of the Thioalkalivibrio strains from hypersaline soda lakes were extremely salt-tolerant and capable of growth in saturated soda brines. The Thioalkalimicrobium strains demonstrated relatively high specific growth rates, low growth yield, high maintenance, and extremely high rates of thiosulfate and sulfide oxidation. In contrast, the Thioalkalivibrio strains, in general, were slow-growing, high-yield organisms with lower maintenance and much lower rates of oxidation of sulfide and thiosulfate. Moreover, the latter survived starvation much better than Thioalkalimicrobium. Different growth characteristics and salt resistance appear to determine the outcome of the enrichment cultures from different soda lakes: Thioalkalimicrobium dominated in the enrichments with freshly obtained samples from diluted soda lakes at low-medium salinity, while Thioalkalivibrio was the predominant organism in enrichments from aged samples and at hypersaline conditions. In mixed thiosulfate-limited chemostat cultures at low salinity, Thioalkalimicrobium strains (mu(max)=0.33 h(-1)) out-competed Thioalkalivibrio strains (mu(max) = 0.15 h(-1)) at D > 0.02 h(-1). The overall results suggest that Thioalkalimicrobium and Thioalkalivibrio represent two different ecological strategies.