Algae

EPA's framing starts in water: algae are natural and important primary producers in aquatic ecosystems. Cyanobacteria, which EPA says are also known as blue-green algae, are microscopic organisms that use sunlight to produce their own food.

UF/IFAS Extension's EDIS adds the taxonomy: an alga is one of a large group of photosynthetic organisms that store food as starch, while cyanobacteria are true bacteria, neither plant nor fungus. The word algae, in other words, names a category, not one organism.

ARMA writes that all the algae species behind roof discoloration are transported through the air and tend to collect and grow on roofing structures and other outdoor surfaces.

ARMA also notes that the resulting discoloration, usually brown to black, is observed across the United States and Canada and may be mistaken for fungus growth, soot, dirt, moss or tree droppings.

The organism ARMA names as probably the most prevalent behind that staining has its own glossary entry: Gloeocapsa magma. How the spores travel once they are airborne, and whether that means algae spreads from one house to the next, is a question of its own.

ARMA's algae and moss bulletin says algae can form where moisture tends to dwell on roof surfaces, and that coastal or humid climates create the most conducive conditions for algae growth. What that means for a Naples roof is its own answer, why algae grows faster here than up north. ARMA's other bulletin says the discoloration is most readily visible on light-colored roofs and not so visible on darker shades, and whether that changes anything is taken up at do white roofs need cleaning more often.

The Tile Roofing Industry Alliance widens the surface list beyond roofing: many climates are conducive to mildew, algae and moss growth on everything from glass, stucco, siding and driveways to concrete and clay roofing tiles. Whether growth from a roof reaches the pavers below is its own answer, roof algae on pavers, and what a paver cleaning removes is set out on the paver page.

ARMA's bulletins sort the look-alikes by pattern: moss and tree droppings typically produce only localized discoloration.

The neighboring words have glossary pages of their own, including mold and lichen, a fungus and an alga growing together. How algae differs from mold on a roof is answered at algae versus mold on a roof.