Structural characterization of the uncommon polysaccharides obtained from Peltigera canina photobiont Nostoc muscorum

Cellular and extracellular polysaccharides of the lichenized fungus Peltigera canina photobiont Nostoc muscorum (UTEX-2493) were evaluated. From EPS a linear (1 → 4)-linked β-d-xylan was chemically characterized, and because of this and other findings involving lichenized fungi it could be proposed that this polymer may be typical of photobionts isolated from lichen thalli. From cellular biomass extracts a complex polysaccharide formed mainly of l-arabinose (2,3-O-Me2-, 24.8%) and d-xylose (2,3-O-Me2-, 10.6%) both units (1 → 4)-linked was described. A smaller part of this structure has β-l-Arap and β-d-Xylp units branched at O-3 by α-d-Manp units, which in turn presents O-3 or O-2 single-unit substitution by α-d-Manp or α-l-Fucp as non-reducing ends. Arabinosyl residues were found to be in the β-l-arabinopyranosyl form, different from the most common form described in plant cell wall polysaccharides, α-l-arabinofuranosyl (Araf). Both polymers characterized in this work were not previously described when analyzing Nostoc sp. or cyanobacteria in general.