I have often wondered where the aquatic nitrifying bacteria come from, and assumed it was not the tap water because of the chlorine. But, I was/am wrong.
I did searches to try and find information on this subject, and the best one believe it or not was back in this forum (TFF) from 2010. The only member in that thread that I recognize as still being active here is
@kribensis12 [I was not on TFF in 2010, at least not actively]. He might remember this, but a couple of members had data that answers the question. Unfortunately the links they gave to sources don't work [all say site is gone or something], but the data is interesting.
First, something from my own knowledge. There are at least 10 species of
Nitrosomonas bacteria. These are the ammonia-oxidizing bacteria, using ammonia/ammonium and producing nitrite. There are distinct species found in, and only in, soil, marine (sea) water, and freshwater. They do not work cross-ecosystem, i.e., the marine and soil species cannot function in freshwater, and vice versa. The bacteria are simply present in these ecosystems, and they do not and cannot travel outside their respective ecosystem.
Nitrosomonas are non-motile bacteria, meaning they cannot move, they sort of "spread" along. All of this means that any sort of air transportation (as via spores as someone mentioned in the 2010 thread) is impossible. The bacteria are in and live in freshwater.
So, to the data from the 2010 thread. The bacteria are in the tap water. The chlorine used by municipal water authorities may (note, may) kill some, but certainly not all. This may be why it takes so long to establish the nitrifying bacteria in a new aquarium...there are few getting through. But clearly, some do, or the bacteria would never exist in a new aquarium unless added by seeding. Nitrifying bacteria are light phobic; they only live and function in darkness.
The above is not all that surprising, given that more recent studies have proved that rinsing filter media under the tap does not kill all the nitrifying. There just isn't enough chlorine to do so, and as far as chloramine, apparently that can't either. There are in fact bacteria species that consume part of the chloramine (came across this, now can't find it again!). The only thing that does kill them is drying out, freezing or boiling. They actually establish much faster and can multiply faster at temperatures inn the high 30's which is much too warm for fish.