Which bacteria are primarily responsible for the first step of nitrification, converting ammonia to nitrite?

• A. Nitrosomonas and Nitrobacter
• B. Denitrifying bacteria
• C. Methanogens
• D. Halophiles

Answer: (A)

Nitrification happens in two steps: first, ammonia is oxidized to nitrite by ammonia-oxidizing bacteria such as Nitrosomonas. This step is driven by bacteria that specialize in converting ammonia into nitrite, a form that can continue to be oxidized to nitrate by nitrite-oxidizing bacteria like Nitrobacter. The question asks which bacteria handle the first step, so the correct choice centers on Nitrosomonas, the classic ammonia-oxidizer. The other options don’t perform this nitrification step: denitrifying bacteria reduce nitrate to nitrogen gases under low-oxygen conditions, methanogens produce methane, and halophiles are salt-loving microbes not specifically tied to nitrification. Including Nitrosomonas in the option makes it the best choice because it names the organism responsible for the initial conversion of ammonia to nitrite.