Corrosive extracellular polysaccharides of the rock-inhabiting model fungus Knufia petricola

Melanised cell walls and extracellular polymeric matrices protect rock-inhabiting microcolonial fungi from hostile environmental conditions. How extracellular polymeric substances (EPS) perform this protective role was investigated by following development of the model microcolonial black fungus Knufia petricola A95 grown as a sub-aerial biofilm. Extracellular substances were extracted with NaOH/formaldehyde and the structures of two excreted polymers studied by methylation as well as NMR analyses. The main polysaccharide (similar to 80%) was pullulan, also known as alpha-1,4-; alpha-1,6-glucan, with different degrees of polymerisation. Ilpha-(1,4)-linked-Glcp and alpha-(1,6)-linked-Glcp were present in the molar ratios of 2:1. A branched galactofuromannan with an alpha-(1,2)-linked Manp main chain and a beta-(1,6)-linked Galf side chain formed a minor fraction (similar to 20%). To further understand the roles of EPS in the weathering of minerals and rocks, viscosity along with corrosive properties were studied using atomic force microscopy (AFM). The kinetic viscosity of extracellular K. petricola A95 polysaccharides (ae 0.97 x 10(-6) m(2) s(-1)) ranged from the equivalent of 2% (w/v) to 5% glycerine, and could thus profoundly affect diffusion-dominated processes. The corrosive nature of rock-inhabiting fungal EPS was also demonstrated by its effects on the aluminium coating of the AFM cantilever and the silicon layer below.