Catalyzes the reduction of nitrite to ammonia, consuming six electrons in the process (PubMed:22382353). Has very low activity toward hydroxylamine (PubMed:22382353). Has 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. kcat is 824 sec(-1) with nitrite as substrate. kcat is 2380 sec(-1) with hydroxylamine as substrate. Nitrogen metabolism; nitrate reduction (assimilation). Homodimer. Belongs to the cytochrome c-552 family.