Heterocyst-forming species are able to "fix" nitrogen gas, which cannot be absorbed by plants, into ammonia (NH3), nitrites(NO2) or nitrates (NO3), which can be absorbed by plants and converted to protein and nucleic acids. The rice paddies of Asia, which feed about 75% of the world's human population, could not do so were it not for healthy populations of nitrogen-fixing cyanobacteria in the rice paddy waters.
Found in almost every conceivable habitat, from oceans to fresh water to bare rock to soil, cyanobacteria produce the compounds responsible for "earthy" odors we detect in soil and some bodies of water (such as those being cyanobacterially cleaned at water treatment plants). The greenish slime on the side of your damp flower pot, the wall of your house or the trunk of that big tree is more likely to be cyanobacteria than anything else. Cyanobacteria have even been found on the fur of polar bears, to which they impart a greenish tinge!
I added light and mine went bye bye.