Microbes that thrive in acidic environments seem to have "cost free" ATP energy available to them due to the pre-existing H gradient across the cytoplasmic membrane. However, this is not entirely true because .

The microbes thriving in acidic environments are termed as acidophiles, and these range from eukaryotes to bacteria and archaea, which are mainly found in diverse acidic surroundings like sulfuric geysers and pools, in the human stomach, and in the regions that get polluted by acid mine drainage.  

The mentioned case is not entirely correct as the protons found in the acidic surroundings are not utilized for the generation of ATP as they are not originating from within the cell. In order to sustain their internal acidic pH, the acidophiles exhibit adaptations like the presence of the negatively charged proteins on the surfaces of their membranes so that they can prevent deterioration due to acidic surroundings.