Polysaccharides as a key step in stone bio-erosion

Abstract

Terrestrial cyanobacteria and microalgae in Roman hypogea are possibly the first colonizers of the exposed surfaces and their extensive development is supported by the mineral composition of the substrata and the porous nature of calcareous surfaces. The strategies for the adhesion to the stone are based on the production and secretion of mucilaginous compounds at the cell surface. Exopolymeric substances (EPS) secreted by the microorganisms) act in sticking the cells to the substratum, and their adhesive properties contribute to the formation and cohesion of biofilms. A polysaccharide matrix containing cell debris and significant amounts of inorganic material adsorbed from the substrate makes up most of the cyanobacterial films to which airborne particles, bacteria, and spores adheres, which increases masking effects, transformation, and corrosion of the surfaces. This chapter describes studies performed on polysaccharide fractions of selected cyanobacterial species from Roman hypogea to gain preliminary information on their physicochemical characteristics by using circular dichroism. Cyanobacterial polysaccharides were studied in Leptolyngbya sp. that was isolated from Catacombs of St. Sebastian in Rome. After harvesting of cultures and culture media by centrifugation, polysaccharide extraction was performed to obtain three fractions: the polysaccharides in solution, released polysaccharides (RPS) and the envelope polysaccharides, capsular polysaccharides (CPSc= cold extract and CPSh= hot extract). The contribution of released (RPS) and capsular (CPS) polysaccharides to total biomass was calculated and circular dichroism spectra of the three extractable polysaccharide fractions were analyzed. All fractions showed the presence of uronic acids and CPS of sulfated residues. No evidence of temperature dependence was found, while low pH values caused an irreversible change in the polysaccharide molecular conformation.