Different responses of Sinorhizobium sp. upon Pb and Zn exposure: Mineralization versus complexation

Lead (Pb) and zinc (Zn) have been discharged into environment and may negatively impact ecological security. Rhizobia has gained attention due to their involvement in the restoration of metal polluted soils. However, little is known about the responses of rhizobia under Pb and Zn stress, especially the roles played by extracellular polymeric substances (EPS) in the resistance of these two metals. Here, Sinorhizobium sp. C10 was isolated from soil around a mining area and was exposed to a series of Pb/Zn treatments. The cell morphology and surface mineral crystals, EPS content and fluorescent substances were determined. In addition, the extracellular polysaccharides and proteins were characterized by attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS). The results showed that Zn stress induced the synthesis of EPS by C10 cells. Functional groups of polysaccharides (CO) and proteins (C-O/C-N) were involved in complexation with Zn. In contrast, C10 resisted Pb stress by forming lead phosphate (Pb3(PO4)2) on the cell surface. Galactose (Gal) and tyrosine played key roles in resistance to the Zn toxicity, whereas glucosamine (N-Glc) was converted to glucose in large amounts during extracellular Pb precipitation. Together, this study demonstrated that C10 possessed different strategies to detoxify the two metals, and could provide basis for bioremediation of Pb and Zn polluted sites.

Amelioration of Organic Carbon and Physical Health of Structurally Disturbed Soil through Microbe–Manure Amalgam

Less precipitation, high temperature, and minimal natural vegetation are characteristic of regions having an arid climate. The harsh environment massively destructs the soil structure of that area by burning soil organic carbon, leading to deteriorated soil nutritional quality, creating a significant threat to agricultural production and food security. Direct application of organic wastes not only substitutes lost organic carbon but also restores soil structure and fertility. This study was conducted to assess the impact of organic amendments, i.e., farm manure (FM), poultry manure (PM), molasses (MO), and Exo-Poly Saccharides (EPS) producing rhizobacterial strains i.e., M2, M19, M22 amalgams as treatments. To assess the impact of treatments on soil carbon and structure restoration to hold more water and nutrients, a 42-day incubation experiment using a completely randomized design (CRD) under the two-factor factorial arrangement was conducted. Macro aggregation (0.25 to >1 mm), carbon retention in macro aggregates, active carbon (dissolved organic carbon, a mineral-associated organic carbon, microbial biomass carbon), total organic carbon, the carbon mineralization activities, and water retention capacities were observed to be highest in soils that were treated with (FM + M2, FM + M22, PM + M19, and MO + M19). Finally, we conclude that organics mineralization by microbial actions releases organic glues that not only impart particle aggregation but also conserve organics as aggregate entrapped carbon. Amalgamated application of microbe–manure combinations directly impacts soil structure and organic carbon contents, but in an indirect scenario, it improves the fertility and productivity of the soil. Therefore, it is strongly recommended to use organic manures and microbes in combination to restore structurally degraded lands.