The production of extracellular polymeric substances (EPS) by cyanobacteria isolated from biocrusts in Sejzi Plain, Iran, and their contribution to UVB stress tolerance

Cyanobacterial extracellular polymeric substances (EPS) are secondary metabolites with high physiological importance for survival under ultraviolet radiation (UVR). This is particularly important in drylands, where cyanobacteria are the main components of biological soil crusts (biocrusts), which are subjected to strong natural solar UVB (280-315 nm) irradiation. This research aims to assess the capability of cyanobacteria isolated from biocrusts from Sejzi Plain, Iran, to produce EPS under UVB treatment. Using a top-down approach, two cyanobacterial species were isolated from biocrusts, and identified as Microcoleus vaginatus and Oscillatoria sp. Both taxa were exposed to controlled UVB radiation for 24, 48 and 72 h. Chlorophyll a content, more soluble EPS fraction (loosely bound EPS, LB-EPS), and less soluble EPS fraction (tightly bound EPS, TB EPS) production were measured. M. vaginatus and Oscillatoria sp. produced equal amounts of both EPS under the UVB treatment. A significant relationship was found for decreasing LB-EPS after 24 h of exposure to UVB, while the amount of TB EPS did not decrease, and even increased significantly after 48 and 72 h. The chlorophyll a content of treated cyanobacteria and their control culture was not significantly different. Therefore, under prolonged UVB up to 72 h, these cyanobacteria produced more TB-EPS than LB-EPS. Consequently, TB-EPS might contribute to higher survival under UVB than LB-EPS.