Nitrifying bacteria
a)	Convert free nitrogen to nitrogen compounds
b)	Convert proteins into ammonia
c)	Reduce nitrates to free nitrogen
d)	Oxidize ammonia to nitrates

(d)

Nitrifying bacteria (one of the chemosynthetic bacteria) oxidize ammonia to nitrates and obtain energy for the preparation of food. This oxidation occurs in two steps. In the first step, ammonia is oxidized to nitrite by nitrite bacteria (e.g., Nitrosomonas and Nitrosococuus). In the second step, nitrate is oxidised to nitrate by nitrate bacteria (e.g., Nitrocystis and Nitrobacter).