The ability to reduce nitrate (NO3^-) to ammonia (NH3) is common to virtually all plants, fungi, and bacteria. It occurs in the following four steps:

NO3^- -> NO2^- -> NO^- -> NH2OH -> NH3.

The first step, conversion of NO3^- to NO2^-, is catalyzed by a large and complex enzyme called nitrate reductase.

The last three steps in the reduction of nitrate to ammonia are carried out by nitrite reductase. It contains one Fe2S2 center and one molecule of siroheme, a partially reduced iron porphyrin. The electron donor for each step is ferredoxin.