There are algae and fungi that can happily sustain temps produced by engines. Alcohols contained in engine coolants do the job combined with extremely limited amount of light and no oxygen (most of it is bonded after some time by metals and other elements).
That's why I use closed loop with no reservoirs which runs slightly pressurized in working conditions and high performance car coolant additives (ethanol mainly).
If UV CCs sold for case modding market were to have biocidal properties it would be damaging to human health as well (cancer and the like).
If you want to use UV to sterilize your loop you need to get your mitts on hospital grade emitters
