Phototrophic microorganisms (algae, cyanobacteria) can grow on stone surfaces or may penetrate the pore system of the stone itself. Phototrophic microorganisms have a direct effect on the deterioration of stone due to their pigments which cause an aesthetically detrimental effect.
Filamentous cyanobacteria (Nostoc sp, Leptolyngbia sp., Stigonema ocellatum) and green algae (Desmococcus olivaceus and Haemaotococcus pluvialis) are capable of forming dense mucous-like layers on surfaces which produce characteristic colourations on the substrata.
Nostoc. sp can produce blackish-green pigmentation which leaches into the substrate on which it is growing. The algal species Desmococcus olivaceus produces bright green colouration of buildings as shown below.