The black stains are also very likely to harbor a number of molds or fungi. Unlike algae, the fungi on our roofs lacks chlorophyll and are unable to manufacture food from raw materials. The algae must get nutrition from some form of organic matter. To thrive, they also require a warm, humid environment.
Fungi normally begin their life cycle as an airborne spore that settles on roofs. The spores on the northern exposure of the roof stand a better chance of survival because that portion of the roof is the last part to dry after a rain or morning dew. This raises the humidity on the surface of the shingle, thus creating a perfect feeding and breeding ground.