Cyanobacteria are regarded as critical in the formation of the atmosphere that we have today, even though they were not the first organisms capable of photosynthesis. Some have a vital role within ecology, for example Heterocysts which produce enzymes capable of fixing Nitrogen. Other species can be endosymbiotic and some are known to produce toxins know as cyanotoxins.
A Biology of the Algae, Sze, 3ed, McGraw-Hill, 1998, New York
Microbiology, Prescott et al, 6ed, McGraw-Hill, 2005, New York