Catalyzes the reduction of sulfite to sulfide. This is the terminal oxidation reaction in sulfate respiration. [DsrC protein]-disulfide + 2 A + 3 H2O + hydrogen sulfide = [DsrC protein]-dithiol + 2 AH2 + H(+) + sulfite [DsrC protein]-S-sulfanyl-L-cysteine/L-cysteine + 3 A + 3 H2O = [DsrC protein]-disulfide + 3 AH2 + 2 H(+) + sulfite Binds 2 [4Fe-4S] clusters per subunit. Binds 1 siroheme per subunit. Highly thermostable. Inactive towards methylviologen below 55 degrees Celsius. Heterotetramer of two alpha and two beta subunits. Although the protein complex is found in the soluble fraction it may be membrane-associated in vivo.