Sulfate reducers are prokaryotic microorganisms that are able to reduce sulfate or partially oxidized sulfur compounds, such as sulfite and thiosulfate, in a non-assimilatory manner in order to obtain energy through anaerobic respiration. Sulfate reducers couple the oxidation of propionate, butyrate, lactate, and ethanol among other electron donors with the reduction of sulfate to sulfide. They are obligate anaerobes and include heterotrophs as well as authotrophs. The best known are Gram-negative bacteria, but Desulfotomaculum is a Gram-positive spore-forming sulfate-reducer genera closely related to Clostridium. There is a phylum which includes thermophilic sulfate reducers, including Thermodesulfobacteria. Different archaea, e.g., Archaeoglobus, are able to use sulfate as an electron acceptor for anaerobic respiration. Sulfate-reducing microorganisms are fundamental elements of the sulfur cycle.