Denitrification at extremely high pH values by the alkaliphilic, obligately chemolithoautotrophic, sulfur-oxidizing bacterium Thioalkalivibrio denitrificans strain ALJD

Abstract.

Thioalkalivibrio denitrificans is the first example of an alkaliphilic, obligately autotrophic, sulfur-oxidizing bacterium able to grow anaerobically by denitrification. It was isolated from a Kenyan soda lake with thiosulfate as electron donor and N₂O as electron acceptor at pH 10. The bacterium can use nitrite and N₂O, but not nitrate, as electron acceptors during anaerobic growth on reduced sulfur compounds. Nitrate is only utilized as nitrogen source. In batch culture at pH 10, rapid growth was observed on N₂O as electron acceptor and thiosulfate as electron donor. Growth on nitrite was only possible after prolonged adaptation of the culture to increasing nitrite concentrations. In aerobic thiosulfate-limited chemostats, Thioalkalivibrio denitrificans strain ALJD was able to grow between pH values of 7.5 and 10.5 with an optimum at pH 9.0. Growth of the organism in continuous culture on N₂O was more stable and faster than in aerobic cultures. The pH limit for growth on N₂O was 10.6. In nitrite-limited chemostat culture, growth was possible on thiosulfate at pH 10. Despite the observed inhibition of N₂O reduction by sulfide, the bacterium was able to grow in sulfide-limited continuous culture with N₂O as electron acceptor at pH 10. The highest anaerobic growth rate with N₂O in continuous culture at pH 10 was observed with polysulfide (S₈^2–) as electron donor. Polysulfide was also the best substrate for oxygen-respiring cells. Washed cells at pH 10 oxidized polysulfide to sulfate via elemental sulfur in the presence of N₂O or O₂. In the absence of the electron acceptors, elemental sulfur was slowly reduced which resulted in regeneration of polysulfide. Cells of strain ALJD grown under anoxic conditions contained a soluble cd₁-like cytochrome and a cytochrome-aa₃-like component in the membranes.