Catalyzes the six-electron reduction of nitrite to ammonium. 2 H2O + NH4(+) + 6 oxidized [2Fe-2S]-[ferredoxin] = 8 H(+) + nitrite + 6 reduced [2Fe-2S]-[ferredoxin] Binds 1 siroheme per subunit. Binds 1 [4Fe-4S] cluster per subunit. Nitrogen metabolism; nitrate reduction (assimilation). Monomer. By nitrate and by light. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.