Lightning, Lichen, Water tanks and rainwater harvesting
During lightning storms some nitrogen in very small quantities is dissolved into the rain. This quantity is far too small to do any harm.
Lichen is a symbiotic relationship between a fungus and algae, and the one cannot live without the other. This tuft of very precious growth on a tiled roof is one of the best indicators of pollution. So long as lichen grows on a roof the water that falls on that roof is safe for household use. Beware if all the lichen dies. Many people think that this growth is unsightly and should be removed and the roof painted. This is not true and ill advised. Leave the lichen alone.