Seasonal dynamics of the physicochemical properties of biological crusts exopolysaccharides and the microbial community structure

Abstract

Exopolysaccharides (EPS), an important substance of cyanobacteria in resisting stresses, are the main form of carbon storage in biocrusts and play an important role in material cycling and stability of biocrusts. In this study, the biocrusts in different seasons (January, April, July, October) were collected from Gurbantunggut Desert, and the dynamics of EPS content, composition, morphological characteristics and microbial community structures were analyzed. The results showed that: 1) The excretion of EPS showed obvious seasonal dynamics. The EPS contents in January, April, July and October were 81.72, 52.46, 76.77, 70.54 μg·cm-2, and the chlorophyll a contents were 2.7, 4.94, 4.2 and 5.98 μg·cm-2, respectively. Cyanobacteria allocated more fixed organic carbon to EPS in winter and summer, and more to their own biomass accumulation in spring and autumn. 2) EPS in biocrusts of each season was composed of seven kinds of monosaccharides. The sum of relative mole percentages of glucose and galactose was 46%-56%, much higher than the other five monosaccharides. The monosaccharide compositions of EPS were significantly affected by temperature and precipitation. There was no significant difference in the Fourier infrared spectra of EPS in biocrusts across different seasons. 3) The observation results of atomic force microscope showed that more filamentous and thick rope-like structures occurred in EPS in July and October, while the EPS showed block-like morphology in January and April. 4) The results of 16S rDNA high-throughput sequencing showed that Cyanobacteria and Microcoleus were the dominant bacterial phyla and genus in biocrusts in all the four seasons, with significantly higher relative abundance than other bacterial phyla and genera. The relative abundance of Proteobacteria was significantly positively correlated with the relative mole percentages of fucose and galactose, indicating that the composition of monosaccharides affected heterotrophic bacteria in crusts. In deserts, environmental factors such as temperature and moisture changed significantly across seasons. The physicochemical properties of biocrust exopolysaccharides and the seasonal dynamics of bacterial communities were controlled by multiple factors, such as temperature, moisture, and light.