Balancing the nitrification reaction $($oxidation of ammonium to nitrate by O$2)$: Ammonium $($NH$4+)$ is released into water in the decomposition of natural organic matter $($e$.$g$.,$ cellular material containing proteins$)$ and is oxidized to nitrate $($NO$3-)$ by bacteria using dissolved oxygen $($DO$)$ in the process of nitrification. Although the reaction does not occur abiotically, it can be written as a chemical reaction. Write the balanced chemical reaction.