What is the biological process called when nitrifying bacteria oxidize ammonia to nitrite and nitrate under aerobic conditions?

• A. Denitrification
• B. Ammonification
• C. Assimilation
• D. Nitrification

Answer: (D)

Nitrification is the biological oxidation of ammonia first to nitrite, then to nitrate, carried out by nitrifying bacteria under aerobic conditions. In wastewater systems, these microbes use oxygen to convert the toxic ammonia into nitrite and then nitrate, a two-step process that prepares nitrogen for further removal or utilization. This oxidation in the presence of oxygen is what sets nitrification apart from other nitrogen transformations: denitrification reduces nitrate to nitrogen gas in low-oxygen environments; ammonification converts organic nitrogen to ammonia during decomposition; and assimilation is the uptake of inorganic nitrogen into biomass without oxidation. So the described process—oxidizing ammonia to nitrite and nitrate with oxygen—the best match is nitrification.