The Effect Of Polysaccharide-producing Rhizobium Sp. Q32 On Silicate Minerals Weathering And Its Mechanism

The interaction between minerals and microorganisms is a common geological process in nature. Microorganisms can promote the dissolution of minerals by direct and indirect mechanisms. The direct mechanism refers to the formation of microenvironment in bacteria-mineral interface when microbes and minerals contract directly, which can promote the dissolution of minerals. The indirect mechanism means the chemical degradation effect of metabolites of microbes on minerals. In this study, the role of microbial polysaccharides produced by Rhizobium sp. Q32 in the silicate mineral weathering was investigated.In this paper, the effect of polysaccharide-producing Rhizobium sp. Q32 on silicate minerals weathering was studied in shaking-flask condition. The result showed that during 21 days of culturing, Rhizobium sp. Q32 could promote the dissolution of K-feldspar, biotite, phlogopite and illite. After 21 days of incubation, the concentration of silicon in solution was increased by 138-162% compared with the control, indicating that the strain Q32 could promote the tested silicate mineral weathering. By using the media containing silicate mineral as the only potassium source, the strain Q32 excreted expolysaccharide, which reached the highest concentration-9.46 mg/L on the 7th day of incubation.The mineral samples were collected and analzyzed by means of scanning electron microscope (SEM), it was observed that the surfaces of minerals were coated with biofilm and corroded pits were formed on mineral surfaces. Energy dispersive spectronmeter (EDS) images showed that the relative concentrations of Si and Al on the mineral surfaces were lower than those on raw mineral after 21 days of incubation, which demonstrated that metal ions were released from silicate minerals surfaces.Isolation, purification and analysis of the monosaccharide compositions of expolysaccharide were carried out by the means of UV, IR and GC. The concentration of expolysaccharide produced by strain Q32 in the mineral-containing medium increasd by 5.8-17.3% compared with those in the mineral-free medium, which indicated that minerals can stimulate the the production of expolysacchrides. Rhamnose, Xylose, Galactose, Mannose and Glucose constituted the monosaccharide composition of the expolysacchrides. Characteristic absorption peaks of the polysaccharides could be found from the IR spectra. Analysis of monosaccharide composition of expolysaccharides showed that the structure of the expolysaccharide produced under different contions had not changed.Purified expolysaccharide could dissolve silicate minerals, mainly due to chelation caused by acid group of the expolysaccharide such as carboxyl. Organic acids such as gluconic acid, tartaric acid and malic acid were aborbed in the expolysaccharide, the concentration of them were only 0.31 mM,0.27 mM and 0.29 mM, respectively. But mineral weathering could not occurred with low concentrations of organic acids, so it indicated that extracellular polysaccharides produced by the strain Q32 played the main role in the mineral weathering in this study.