Abstract: The present invention relates to the biologically catalyzed, anaerobic generation of sulfide species as sulphide, hydrosulfide or hydrogen sulfide in anaerobic bioreactors from the reduction of tetravalent sulfur derived from one or more sources including sulfur dioxide containing combustion flue gas, or the reduction of divalent or pentavalent sulfur containing liquors such as thiosulfate or dithionate containing liquors. Flue gas sources of sulfur dioxide also contain one or more bio-nutrients or energy sources. The generated sulfide is useful for numerous applications including waste treatment and metals recovery as sulfides.