After some more thought, it's also just plain hard to believe in a pure atmospheric bacterium. I'm not sure we even have such a thing on planet Earth. Plenty that survive in the atmosphere better than expected, may even actively use it to spread, but nothing that lives up there. (Links to corrections solicited.) We don't routinely experience green days because of a mass of atmospheric bacteria blowing in above us and blocking the sun as they photosynthesize.

Energy it has in abundance, but where is it getting the atoms it's made out of? Carbon and oxygen are readily available, but you need more than that. Water is minimal in the atmosphere, and mere presence isn't enough, you need to be able to gather it against the strong osmotic pressures pulling it back out. (Earth life seems to have needed hundreds of millions of years just to grow on Earth land, a wildly more friendly environment than an atmosphere that still has puddles of nearly pure water to work with. Even a cloud is, by comparison, water-poor.) How do you get any nitrogen? Lightning may fixate it as it does on Earth, but then it immediately dissipates. It's very difficult to use N2 directly, although some Earth life manages. After that what's left in the atmosphere is some vicious acids and some noble gasses. So in the basic CHON equation, C & O are covered, N is exceedingly chemically difficult, and H is almost missing. Trace elements are also not available because they don't float, except sulphur (we use this ourselves, but it's not the most useful one), and while bacteria may get by without using as much of them as we do, I'm not sure you can make a life form out of nothing but CHON, let alone CO(Nish).

My bet is this will turn out to be one or more of the basic abiotic phosphine processes accumulating over long, long time because whatever processes that would consume phosphine are for some reason suppressed (e.g. lack of oxidants?).