Final answer:
Nitrifying bacteria play a crucial role in the nitrogen cycle by converting ammonium into nitrites and nitrates in a process called nitrification, which is vital for plant growth.
Step-by-step explanation:
Organisms called nitrifying bacteria convert ammonium ions into nitrites and nitrates in a process known as nitrification. When plants and other organisms die or release wastes, decomposers break down the organic matter and, through ammonification, release nitrogen in the form of ammonium ions. The nitrifying bacteria, such as Nitrosomonas, then oxidize these ammonium ions to nitrites and subsequently, other bacteria convert the nitrites to nitrates. These nitrates can be absorbed by plants and are essential for their growth. The nitrogen cycle is completed by denitrifying bacteria, which convert nitrates back into nitrogen gas and release it into the atmosphere.