Co‐substrate for DsrAB in sulfite reduction. Catalyzes the reduction of sulfite to sulfide. This is the terminal oxidation reaction in sulfate respiration, a process catalyzed by the sulfate-reducing bacteria. [DsrC protein]-disulfide + 2 A + 3 H2O + hydrogen sulfide = [DsrC protein]-dithiol + 2 AH2 + H+ + sulfite.