| As a form of land degradation, desertification causes the decline of soil fertility and the decrease of biodiversity and poses a severe threat on natural environment and social economy. In recent years, the quickening trend of desertification in the areas of arid, semi-arid and subhumid worldwide draw a wide range of concern. Many strategies for stabilizing sand are currently used in practice, of which the formation of microbial desert crust has a promising prospect for broad application in sand stabilizing due to the advantages in adaptability, low cost, quick achievement, etc. However, the microbes living in sand that is competent to stabilize sand remain to be further exploited.The Qaidam Basin is the highest altitude and harsh environment desert in China. The present studies aimed to isolate saline-alkali-tolerant bacterial strains from the Qaidam Basin Desert, assess their potential capability for fixing sand, and illustrate the physiological and biochemical characteristics for further insight into the mechenism of sand fixation. This study provided scientific support and microbial resource for sand fixation and management using microbial techniques. The main results can be drawn as follows.1. Using high alkali and low salt, and high salt and low alkali media, we isolated 19 bacterial strains with saline-alkali-tolerant from the desert soil crusts. These strains were identified as two phyla according to 16 S rRNA gene sequences aligment in NCBI database, i.e, Firmicutes and Proteobacteria. Among these, 16 strains(84.2% of the total) are assigned to Firmicutes, and 3 strains are belonged to the phylum Proteobacteria. These strains were further assigned to seven genera, that is, Marinococcus, Bacillus, Gracilibacillu, Planococcus, Planomicrobium, Salinicoccus, and Halomonas.2. The assays for salt tolerance of the strains showed that 15 strains could grow well in medium containing 0.85% of NaCl concentration, 19 strains could grow in NaCl concentration range from 5% to 10%, 13 strains could grow normally at 15% NaCl, and 3 strains AX14、AX17、AX18 could grow normally at 20% NaCl. All strains were unable to grow at the conditions of NaCl 25%. The optimal salt concentration for 2 strains AX8 and AX11 is 0.85%, for 2 strains AX4 and AX5, it is 10% of NaCl; and for the remaining strains, it is 5%. The measurement of alkaline tolerance showed that all strains isolated could grow at pH6-pH9 conditions. The optimum growth pH for 5 strains AX1、AX 2、AX 3、AX 6、AX 19 is 8 and for the others it is 7. The growth curves of these strains exhibited that 9 strains grown in logarithmic phase after 10 h of incubation, 8 strains after 15 h, 1 strain AX6 after 5 h, and 1 strain AX16 after 30 h.3. The BioLog results indicated that strains AX11 and AX18 could use 13 kinds of monosaccharides. Strains AX1 and AX2 could use 7 kinds of monosaccharides. 19 strains could use only 2 to 3 kinds of phosphohexose. Strain AX3 could use 6 kinds of carboxylic acids, esters and fatty acids.4. In order to characterize the sand-fixing of bacteria strains isolated, we further performed the experiments for sand-fixing by these strains. The result showed that all the liquid cultures of 19 strains had a certain effect for forming soil aggregates. The cultures of strains AX1 and AX10 could form more than 7 mm of sand crust in thickness after sprinkling on sand. The thickest sand crust was 7.456 mm on average, which was formed by strain AX1 cultures, indicating the strongest effect for sand-bonding. Additionally, 13 strains could form 5-7 mm crust in thickness and 2 strains could form less than 4 mm crust, among which the minimum(3.826 mm) was formed by AX9. The control experiment showed that the medium used in the strains cultrue had no effects for sand-bouding. Furthermore, the average weight diameters of sand aggregates formed by Halomonas strains AX4, AX5, and AX10 were 305.31 μm, 356.7 μm, and 288.29 μm, respectively. The minimum of average weight diameter was 41.574 μm formed by strain AX19.5. We further determined the content of bacterial extracellular polysaccharide produced by the strains isolated and the result showed that 6 strains can produce more than 800 mg/L extracellular polysaccharide, among which the highest is 1087 mg/L procuced by strain AX10; the content polysaccharide produced by strains AX6 and AX18 are less than 500 mg/L; and the lowest is the 417 mg/L produced by strain AX18.The correlation analysis between extracellular polysaccharide and sand crust thickness formed as well as the micro-aggregates stability of sand of 19 strains isolated showed that the content of extracellular polysaccharide of the strain is significantly correlated with the sand crust thickness(r=0.669, p=0.002) and is significantly correlated with the micro-aggregate stability(r=0.638, p=0.003). Therefore, we concluded that the extracellular polysaccharide secreted by strains play a critical role in binding sand particles to aggregate sand crust. |
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