Exopolysaccharides produced by cyanobacteria of Induced Biological Soil Crusts positively affect seed germination and seedlings fitness of desert shrubs, contributing to the restoration of desertified areas

Biological Soil Crusts (BSCs) are complex, cyanobacteria-dominated microbial communities, commonly found in arid and semiarid areas of the world. The resilience of crustal organisms under constrained conditions such as drought and high solar irradiation is ensured by microbial-produced extracellular polysaccharides (EPSs), which also accomplish for a wide array of key ecological roles. Most notably, EPSs represent a huge carbon source that is directly available to heterotrophic organisms. As an essential component of dry land ecosystems, BSCs impinge on soil characteristics, water regimes, and establish two-way complex interactions with plants. The removal (or the immission) of BSCs in a given ecosystem can effectively shift the state of the ecosystem itself (Bowker, 2007) so that their induction on degraded soils is considered a feasible approach to maintain, and possibly amend, land fertility. In Hobq desert, Inner Mongolia, China, different experimental sites constituted by moving sandy dunes were inoculated with cyanobacterial cultures in different years (Chen et al., 2006). As a result, these sites provide a unique and valid study setting, at present characterized by the presence of induced BSCs at different developmental stages associated to grass and shrubs. Within this frame, this study was aimed at assessing the effects of cyanobacterial inoculation in terms of abundance of phototrophic microorganisms and of C pool increase. A special attention on the degradation processes of the microbial-released EPS was given by evaluating the enzymatic activities related with their degradation in the soil. Under laboratory conditions, the effect of the presence of cyanobacterial-produced EPS on seed germination, seedling-ion uptake, photosynthetic activity and oxidative system of Caragana korshinskii were also tested. C. korshinskii is a desert sub-shrub widely diffused in the area under study and was used as a model to assess the fertilizing potential of cyanobacterial-produced EPS towards vegetation growing in desert soils. Results showed that BSC induction resulted beneficial in enhancing SOC (Soil Organic Carbon) and in increasing the abundance of phototrophic organisms and vegetation cover. Notably, cyanobacterial-EPS resulted gainful to seedling growing and metabolism, contributing also a defensive effect against the damaging effects of reactive oxygen species (ROS), generated under UV-irradiation, salt stress and desiccation. An improvement in photosynthetic activity and nutrient content of C. korshinskii seedlings, stimulated by cyanobacterial EPS, was also observed (Xu et al., 2013). In general, evaluated parameters confirmed the reliability of employing cyanobacteria to address land rehabilitation and improvement, pointing the attention on the key role played by cyanobacterial polysaccharides and their biofertilizing effect for the restoration of desertified areas