@jaypeecee , the paper has rather corrected my opinion. For obvious reasons, the authors stressed the importance of photoinhibition in oceans, with 'unlimited' depth. Canister filters provide perfect darkness, so it's a solution to the problem. Yet I have no filters and nitrification is running just fine in the substrate. So, it seems that it occurs in a layer where oxygen still penetrates but light does not.
In any case, thanks for sharing.
There is an embarrassing irony in all of this. Somewhere in my head I had a distant memory of having read about nitrifying bacteria's sensitivity to light. But I couldn't track it down. It turns out that I'd mentioned it in an article that I wrote four years ago. And that article referenced the very paper that @John q has provided in his post above!