Catalyzes the reduction of nitrite to ammonia, consuming six electrons in the process. 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). Belongs to the cytochrome c-552 family.