This　study　investigated　the　colonization　process　of　ammonia-oxidizing　microorganisms　(AOMs)　on　five　common　types　of　microplastics　(MPs)　as　well　as　the　potential　nitrification　niches　of　MPs　in　water　supply　systems,　with　the　methods　of　biofilm　simulation　reactors,　physicochemical　characterization　of　MPs　and　microbiological　analysis　techniques.　The　results　indicated　that　chlorination　and　microbial　colonization　led　to　an　increase　in　the　crystallinity　of　microplastics　and　the　formation　of　hydrophilic　groups　such　as　carbonyl　and　hydroxyl　moieties.　The　abundances　of　amoA　genes　of　AOMs　on　microplastics　ranged　from　(1.54±0.02)　×　10　to　(3.07±0.64)　×　101　copies/g.　Comammox　was　predominant　on　most　of　the　microplastics.　Ammonia-oxidizing　archaea　(AOA),　which　was　highly　chlorine-resistant,　was　found　to　be　predominant　on　the　MPs　only　after　chlorination　for　42days.　The　total　abundance　of　AOMs　exhibited　a　positive　correlation　with　microplastic　crystallinity　on　unchlorinated　microplastics,　which　was　particularly　obvious　on　higher　intrinsic　crystallinity　MPs,　i.e.,　polyethylene　(PE)　and　polypropylene　(PP).　However,　this　correlation　was　not　observed　after　chlorination　treatment.　Diversity　analysis　of　AOMs　colonizing　on　microplastics　revealed　that　AOA　on　unchlorinated　MPs　were　more　closely　related　to　soil-derived　AOA.　On　the　contrary,　AOA　on　chlorinated　MPs　was　more　closely　to　aquatic　AOA.　©　2025　Chinese　Society　for　Environmental　Sciences.　All　rights　reserved.