Complex role of the exopolysaccharidic matrix in biological soil crusts

In arid and semiarid environments, soil carbon sequestration (CO2 fixation) by cyanobacteria and by biological soil crusts (BSC) is considered an eco-friendly and natural process to increase soil C content and a viable pathway to contrast desertification and to favor soil rehabilitation. Within this context, inoculation-based techniques with exopolysaccharideproducing cyanobacteria have proved to be a viable and sustainable pathway to increase soil biomass, soil stabilization, and to increase soil fertility. In this presentation, a particular focus will be given on the role of the extracellular polysaccharidic matrix (EPM) synthesized by cyanobacteria in giving the structure to natural or induced BSCs and to enhance their water trapping and retaining capability. EPM was extracted with methods aimed at separately removing the tightly bound exopolysaccharidic fraction (TB-EPS) and the loosely bound exopolysaccharidic fraction (colloidal EPS; C-EPS) from BSCs having different ages. The fractions were analyzed in terms of monosaccharidic composition, and molecular weight (MW) distribution. We observed that the relative amounts of uronic acids increase in the EPM with the age of the crusts, implying advantages for the community-water relations. In addition, we also Abstracts – Third International Workshop on Biological Soil Crusts 14 observed significant differences in MW distribution between the two EPS fractions, being TBEPS mostly composed by one molecular fraction having high MW, while C-EPS showed to be also composed by low MW fractions. This difference suggests distinct roles of TB-EPS and CEPS fractions within the crust system. Indeed, TB-EPS most likely affects BSC structure and water-retaining properties, while C-EPS most likely contributes to the intake of C in the soil, thus favoring the growth of the chemoheterotrophic microbial community. The role of EPM in water capture from non-rainfall sources, water maintenance at the topsoil, and in maintaining a high water potential was also shown.