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

Thioalkalivibrio denitrificans is the first example of an alkaliphilic, obl igately autotrophic, sulfur-oxidizing bacterium able to grow anaerobically by denitrification. It was isolated from a Kenyan soda lake with thiosulfat e as electron donor and N2O as electron acceptor at pH 10. The bacterium ca n use nitrite and N2O, but not nitrate, as electron accepters during anaero bic growth on reduced sulfur compounds. Nitrate is only utilized as nitroge n source. In batch culture at pH 10, rapid growth was observed on N2O as el ectron acceptor and thiosulfate as electron donor. Growth on nitrite was on ly possible after prolonged adaptation of the culture to increasing nitrite concentrations. In aerobic thiosulfate-limited chemostats, Thioalkalivibri o denitrificans strain ALJD was able to grow between pH values of 7.5 and 1 0.5 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 f or 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 r eduction by sulfide, the bacterium was able to grow in sulfide-limited cont inuous culture with N2O as electron acceptor at pH 10. The highest anaerobi c growth rate with N2O in continuous culture at pH 10 was observed with pol ysulfide (S-8(2-)) as electron donor. Polysulfide was also the best substra te for oxygen-respiring cells. Washed cells at pH 10 oxidized polysulfide t o sulfate via elemental sulfur in the presence of N2O or O-2. In the absenc e of the electron accepters, elemental sulfur was slowly reduced which resu lted in regeneration of polysulfide. Cells of strain ALJD grown under anoxi c conditions contained a soluble cd(1)-like cytochrome and a cytochrome-aa( 3)-like component in the membranes.