Catalyzes the reduction of nitrite to ammonia, consuming six electrons in the process (PubMed:7999130). Has very low activity toward hydroxylamine, and even lower activity toward sulfite. Sulfite reductase activity is maximal at neutral pH (By similarity). 6 Fe(III)-[cytochrome c] + 2 H2O + NH4(+) = 6 Fe(II)-[cytochrome c] + 8 H(+) + nitrite Binds 1 Ca(2+) ion per monomer. Binds 5 heme c groups covalently per monomer. Nitrogen metabolism; nitrate reduction (assimilation). Homodimer (PubMed:7999130). Probably also exists as a membrane-associated heterooligomeric complex (PubMed:10440380). X-ray crystallographic analysis includes a sulfate ion that is thought to be at the substrate-binding site. Belongs to the cytochrome c-552 family.