Effects of light intensity on components and topographical structures of extracellular polymeric substances from Microcoleus vaginatus (Cyanophyceae)

Abstract

:Extracellular polymeric substances (EPS) acting as cementing substance (chelator) of biological soil crusts (BSCs) are a considerable amount of metabolic investment for cyanobacteria. However, their partition mechanism has long been overlooked. In this paper, we investigated the effects of light intensity on production, distribution pattern, carbohydrate and protein contents, monosaccharide composition, and topographical structures of EPS from Microcoleus vaginatus, the first dominant cyanobacterium of BSCs. The results showed that the amounts of released polysaccharides (RPS), capsular polysaccharides (CPS), and total EPS increased with enhanced light intensity. The weaker the light, the higher the ratio of CPS to EPS and the lower the ratio of RPS to EPS. Although the highest light intensity significantly reduced the charge distribution in RPS, it had no effect on the charge distribution of CPS. For monosaccharide composition, although light intensity did not influence either RPS or CPS, the proportions of main components were still different. Carbohydrate, glucose, galactose, and arabinose were most abundant in RPS; whereas, glucose and galactose were highest in CPS. Comparison with protein content in RPS, carbohydrate was dominant in water eluates under 40 μE m−2 s−1, whereas protein was dominant in 1.0 M NaCl eluates. Carbohydrate was dominant in both eluates under 80 μE m−2 s−1. Protein content of the same fraction in CPS generally decreased with increasing light intensity. The images from atomic force microscopy showed that light intensity significantly affected the molecular weight of EPS, as well as the topographical structures, namely, long chains only found in RPS at 80 μE m−2 s−1 and CPS at 4 μE m−2 s−1. Short chains and irregular lumps occurred under all test light intensities. The above results strongly suggested that light intensity not only affects the EPS yield but also its partition pattern, component, and spatial structure. Therefore, light intensity also likely influences BSCs gluing cohesion.