Carbon assimilation pathways in sulfate-reducing bacteria II. Enzymes of a reductive citric acid cycle in the autotrophic Desulfobacter hydrogenophilus

Abstract

The strict anaerobe Desulfobacter hydrogenophilus is able to grow autotrophically with CO2, H2, and sulfate as sole carbon and energy sources. The generation time at 30°C under autotrophic conditions in a pure mineral medium was 15 h, the growth yield was 8 g cell dry mass per mol sulfate reduced to H2S. Enzymes of the autotrophic CO2 assimilation pathway were investigated. Key enzymes of the Calvin cycle and of the acetyl CoA pathway could not be found. All enzymes of a reductive citric acid cycle were present at specific activities sufficient to account for the observed growth rate. Notably, an ATP-citrate lyase (1.3 μmol · min-1 · mg cell protein-1) was present both in autotrophically and in heterotrophically grown cells, which was rapidly inactivated in the absence of ATP. The data indicate that in D. hydrogenophilus a reductive citric acid cycle is operating in autotrophic CO2 fixation. Since other autotrophic sulfate reducers possess an acetyl CoA pathway for CO2 fixation, two different autotrophic pathways occur in the same physiological group.