Changes of bacterial community structure,monosaccharide composition and CO2 exchange along the successional stages of biological soil crusts.

Biological soil crusts (BSCs) are a dominant ecological landscape of drylands, which have a significant impact on global biogeochemical flux. However, it is unclear how bacterial community and physiological characteristics vary along the BSCs successional stages. In this study, bacterial community composition, physiological characteristics, and monosaccharide composition of extracellular polysaccharides (EPSs) were compared among different successional stages. Our findings demonstrated that besides the dominant bacterial species, the bacterial communities also showed considerable differences between these two stages. Cyanobacteria were keystone taxa in the early stage, while heterotrophic bacteria (Proteobacteria, Actinobacteria and Acidobacteria) were keystone taxa in the later stages. According to the results of CO2 exchange, cyanobacterial crusts accumulated net carbon faster than moss crusts, while moss crusts had a significantly higher respiration rate. The monosaccharide analysis indicated that the EPSs components also varied depending on BSCs' successional stages. Specifically, the contents of rhamnose and arabinose were higher in the cyanobacterial crusts than other types of crusts, while the contents of fucose, xylose, mannose and glucose were the highest in cyanobacterial-lichen crusts, and galactose content was highest in the moss crusts. Altogether, our results stress the heterogeneous variation of BSCs along with succession, and this work offered a fresh viewpoint for a deeper comprehension of the interactions between the monosaccharide components of EPS and the networks of bacterial communities in BSCs.

The living forms of Microcoleus vaginatus and their contributions to the aggregate structure of biocrusts.

Microcoleus vaginatus has been regarded as the important contributor for biocrust formation and ecological services. However, little is known about its living forms in biocrusts, and whether the living form is related to biocrust structure. Therefore, in this study, natural biocrusts collected from the Gurbantunggut Desert were divided into different aggregate/grain fractions, aiming at investigating the living forms of M. vaginatus in biocrusts at fine scale, and exploring its roles in aggregate structure and ecological functions of biocrusts. The results showed that two distinct living forms of M. vaginatus had been identified from the biocrusts. The non-bundling M. vaginatus was mainly distributed in the fractions of > 0.5 mm, forming aggregate structure by cementing sand particles firmly; while the bundling M. vaginatus, distributed mainly among the free sand particles with diameter < 0.5 mm, and easily migrated up to biocrust surface after hydration. Furthermore, the aggregate structure formed by non-bundling M. vaginatus supported a higher biomass, nutrient contents, and enzyme activities. Altogether, our results suggest that the strong migrating ability of bundling M. vaginatus contributes to the environmental adaptation and light resource acquirement, while non-bundling M. vaginatus acts as the constructor of the aggregate structure in biocrusts.

Physical Disturbance Reduces Cyanobacterial Relative Abundance and Substrate Metabolism Potential of Biological Soil Crusts on a Gold Mine Tailing of Central China.

As the critical ecological engineers, biological soil crusts (biocrusts) are considered to play essential roles in improving substrate conditions during ecological rehabilitation processes. Physical disturbance, however, often leads to the degradation of biocrusts, and it remains unclear how the physical disturbance affects biocrust microorganisms and their related metabolism. In this study, the photosynthetic biomass (indicated by chlorophyll a), nutrients, enzyme activities, and bacterial communities of biocrusts were investigated in a gold mine tailing of Central China to evaluate the impact of physical disturbance on biocrusts during the rehabilitation process of gold mine tailings. The results show that physical disturbance significantly reduced the photosynthetic biomass, nutrient contents (organic carbon, ammonium nitrogen, nitrate nitrogen, and total phosphorus), and enzyme activities (ß-glucosidase, sucrase, nitrogenase, neutral phosphatase, and urease) of biocrusts in the mine tailings. Furthermore, 16S rDNA sequencing showed that physical disturbance strongly changed the composition, structure, and interactions of the bacterial community, leading to a shift from a cyanobacteria dominated community to a heterotrophic bacteria (proteobacteria, actinobacteria, and acidobacteria) dominated community and a more complex bacterial network (higher complexity, nodes, and edges). Altogether, our results show that the biocrusts dominated by cyanobacteria could also develop in the tailings of humid region, and the dominants (e.g., Microcoleus) were the same as those from dryland biocrusts; nevertheless, physical disturbance significantly reduced cyanobacterial relative abundance in biocrusts. Based on our findings, we propose the future work on cyanobacterial inoculation (e.g., Microcoleus), which is expected to promote substrate metabolism and accumulation, ultimately accelerating the development of biocrusts and the subsequent ecological restoration of tailings.

Nitrogen concentration acting as an environmental signal regulates cyanobacterial EPS excretion.

As an important carbon (C) storage in biological soil crusts (BSCs), exopolysaccharides (EPSs) are not only a part of the desert C cycle, but also the key materials for cyanobacteria to resist desert stress. In this study, the influence of initial N concentrations (10, 25 and 50 mg L-1 designated as N10, N25 and N50 respectively) on Microcoleus vaginatusis growth and the excretion of EPSs including RPS (released exopolysaccharides) and CPS (capsule exopolysaccharides) were evaluated at different growth periods. In logarithmic period, higher ratio of biomass to EPSs indicated by (DW-CPS)/EPSs was observed in the N50 group with the highest N concentration (about 40 mg L-1) in the medium, while no difference was observed among the three groups in stationary period when the N concentrations of medium were lower than 25 mg L-1. The CPS/RPS showed similar results with (DW-CPS)/EPSs, and stayed higher than 1 in each group. Notably, obvious difference displayed in the monosaccharidic composition and morphologies between CPS and RPS, but not the N levels. The changes of C/N in cells at different growth period indicate that the excretion of EPSs, a mechanism that maintains the balance of cell C/N ratio, only works when the N in the environment is sufficient. Our results showed that, as the raw material and environmental signal, environmental N concentration regulates the elements (C and N) percentage of cyanobacterial cells and its EPSs excretion pattern, but not the monosaccharidic composition or the morphologies. These results also implied that, as the essential self-protecting materials, more EPSs with higher proportion of CPS would be excreted to response the low N environment.

[Seasonal dynamics of the physicochemical properties of biological crusts exopolysaccharides and the microbial community structure]. / 生物结皮胞外多糖理化特性及菌群结构的季节动态.

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.