Water absorption coefficient drives Nostoc sp. colonization on mineral substrates

Nostoc is a genus of filamentous Cyanobacteria common in both terrestrial and aquatic habitats. On land, Nostoc species can also form biofilms on stone monuments, due to their ability to remain desiccated for months or years, and fully recover metabolic activity within hours to days after rehydration with liquid water. We performed in vitro experiments inoculating a Nostoc sp. strain collected from the UNESCO site of archaeological park of Pompeii on five different stone materials, namely tuff, brick, limestone, porphyry, and glass paste to assess the role of water availability on early stage of biofilm formation on stones. The stone tiles, incubated in sealed glass chambers at four different relative humidity values (100%, 92%, 85%, 76%), were observed weekly in a 12-weeks interval to determine surface colonization and growth rate. Confocal laser scanning microscopy (CLSM) was then used to assess thickness, cellular volume and exopolysaccharides (EPS) production on each material. We found that in humidity driven in vitro colonization of Nostoc sp. water absorption coefficient (WAC) determines the propagation of the colonies; in particular, tuff and brick showed the best bioreceptivity, and a greater Nostoc sp. colonization rate, which was attributed to the higher values of WAC.