Algae: Algae are often treated as one group of organisms, but in reality the group is a collection of the true algae and the oxygenic phototropic bacteria group: Cyanobacteria. The two groups of organisms grow at the same general environmenal locations because their moisture and metabolic needs are similar. To the untrained observer, these two groups look very much alike.
True algae: Algae are divided into about 10 Divisions containing groups such as stoneworts, brown algae, dinoflagellates, red algae and green algae. Most algae spend all their life in water, however, one group has members with the ability to live both in water and on land. These are the “green algae”.
Green algae: Green algae are photosynthetic plants containing both chlorophyll a and chlorophyll b in which way they differ from all other algae. Green coverings on buildings are often caused by thick layers of “air algae” which are single to few celled green algae often belonging to the large family of Chlorococcaceae. Some green algae can develop colonies or grow in single celled strings such as Apathococcus and Stichococcus.
Cyanobacteria: Cyanobacteria are still often called blue green algae because they often appear very dark blue green to almost black. When black they can be confused with fungal growth, and a microscopic examination may be required to tell the difference. Some Cyanobacteria are single to few celled and can develop a thick slime layer around the cells. Some consist of a single row of cells that can move around in the environment. Cyanobacteria prefer higher temperatures and are often an annoying problem in sub-tropical and tropical regions.
Blue stain: This group of fungi belongs systematically to the same subdivision of Ascomycetes as the mold fungi, and thus there are no major differences between blue stain and mold. The only reason for treating them as a separate group is that they often penetrate deep into the wood, which they stain bluish to black with an abundance of dark hypha.