Final answer:
Certain chemoautotrophic bacteria transform ammonia and ammonium into nitrate through nitrification, making it more available to plants.
Step-by-step explanation:
In the nitrogen cycle, certain chemoautotrophic bacteria transform ammonia and ammonium into nitrate through a process called nitrification. This process is carried out by bacteria such as Nitrosomas, Nitrobacter, and Nitrospira. Nitrification makes nitrate more available to plants, which can then absorb it through their roots.