Microbial secreted exopolysaccharides affect the hydrological behaviour of natural and induced biological soil crusts in desert soils

Biological crusts (BSCs) are complex microbial associations, resistant to drought and adverse environmental conditions, widely distributed on the surface of arid and semi-arid soils. BSCs are constituted by cells and microbial filaments embedded in a polysaccharidic matrix (EPS) that binds them together and with soil particles. EPSs of BSCs play a key role in hydrological processes taking place at the topsoil in desert environments. Indeed, the hydrophobicity caused by the non-polar constituents of the exudates and the clogging of micropores, which is conversely caused by polar interactions of the hydrophilic constituents, determines the final water content and distribution of the soil. The formation of the BSCs starts from the primary colonization of nutrient-poor soils carried out by filamentous cyanobacteria, followed by the proliferation of micro-algae, bacteria and chemoheterotrophic fungi. The presence of BSCs facilitates the uptake of moisture from the atmosphere and contributes to enriching the soils with organic matter. The role of the EPSs in affecting the hydrological behaviour of BSCs also depends on the texture of the soils, as it was evident by comparing the results obtained with natural and artificially induced BSCs. In this lecture, the studies carried out on the role of microbial secreted EPSs in affecting the hydrological behaviour of natural BSCs in Arizona deserts and of artificially induced BSCs in Chinese deserts will be presented.