Bory de Saint-Vincent
Audouinella is a widespread genus of red algae, found in marine and freshwater environments. It grows as small tufts of red, brown, or black hairlike filaments on any solid surface - most dramatically in freshwater on the edges of slow-growing leaves. Often tolerant of high levels of pollution, acidity, Audouinella thrives on dissolved phosphate and nitrates. It reproduces via spores.
The form known as Black Brush Algae is a particular nuisance in aquaria as few fish, even those widely promoted as algivores, will eat it. In natural ecosystems, this genus that infests aquariums is found in unpolluted lotic systems.
It has been tested for germination and new growth using NO3 and PO4 fertilizers and such results came out negative for a decade's worth of observations. It has been shown to be inducible by limiting and varying the CO2 concentration in planted aquariums. While other possible inducement mechanisms may exists, this is the most consistent and has been shown in many test by aquarists.
-  contains more taxonomic information on this alga.