Complex role of the polymeric matrix in biological soil crusts

Extracellular polymeric matrix (EPM) is a complex component of the organo-mineral assemblages created by biological soil crusts (BSCs). Mainly of polysaccharidic origin, embeds soil and sediments and provides key benefits to the crust community. Services provided include: sediment cohesion and resistance to erosion, moisture provision, protection from external harmful factors, as well as a possible natural plant fertilizing effect. EPM is the product of BSC microbial community, and it is constituted by exopolysaccharides (EPS) associated to other substances, organized in a three-dimensional structure having different gelification grades, and degrees of condensation. This review aims at focusing scientific attention, for the first time, on the characteristics and the roles of three operationally defined EPM fractions, one water soluble, one more adherent to cell and sediment, and one firmly attached to microbial cells. The latest results obtained by analyzing EPM of natural and induced (i.e, the result of cyanobacteria inoculation) BSCs are outlined, and the optimized extraction methodology is described in details. The review underlines the complexity of investigating the characteristics and the role of microbial EPS, and its supra-structure (EPM), in natural conditions (as opposed to unialgal culture in laboratory conditions), where the matrix is subjected to continuous microbial rearrangement due to biosynthetic, self- and cross-feeding processes, and to microbial activity affected by environmental parameters.