Catalyzes the reduction of nitrite to ammonium in the nitrate assimilation pathway, using ferredoxin as the electron donor. Can use reduced methyl viologen but neither NADPH nor NADH as electron donors. 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. Inhibited by cyanide and azide. In the presence of 3.2 M NaCl. Optimum pH is 7.5 at 60 degrees Celsius. At 40 degrees Celsius (temperature of the natural environment for H.mediterranei) the optimum pH is 6.5. Optimum temperature is 60 degrees Celsius. Nitrogen metabolism; nitrate reduction (assimilation). Monomer. Repressed by the presence of ammonium as nitrogen source. Enzyme stability and activity depend upon the salt concentration. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.