All sulfate-reducing organisms are strict anaerobes.  Because sulfate is energetically stable, before it can be metabolized it must first be activated by adenylation to form APS (adenosine 5’-phosphosulfate) thereby consuming ATP.  The APS is then reduced by the enzyme APS reductase to form sulfite () and AMP.  In organisms that use carbon compounds as electron donors, the ATP consumed is accounted for by fermentation of the carbon substrate.  The hydrogen produced during fermentation is actually what drives respiration during sulfate reduction.