Multiple heavy metals immobilization based on microbially induced carbonate precipitation by ureolytic bacteria and the precipitation patterns exploration.

Biomineralization to immobilize the toxic metal has great potential for the bioremediation of multiple heavy metal contamination. In this study, the efficiency of Microbially Carbonate Induced Precipitation (MICP) for several common heavy metals (Cu, Zn, Ni, Cd) in mining areas as well as their precipitation patterns were researched. After urease activity and precipitation ability comparison, Sporosarcina kp-4 and kp-22 were selected for subsequent studies. The removal of Cd was mainly based on the formation of cadmium carbonate induced by bacteria activity, while the removal of Cu was depended on the pH increase generated by the same process. Precipitation contributed to Zn and Ni removal was more complex, which was also based on the MICP process. Removal rates of Cu, Zn, Ni, and Cd (the concentration of all metals was 160 mg/L) reached 75.10%, 98.03%, 59.46% and 96.18%, respectively, within 2 h. For the immobilization of Cu, Zn, Ni and Cd at 160 mg/L, the optimal dosages of bacterial cultured solution were about 0.25 mL, 0.8 mL, 0.5 mL and 0.8 mL, respectively. Minimum inhibitory concentrations (MIC) revealed the toxicity of these heavy metals for MICP bacteria was arrange as: Cd > Zn > Ni > Cu. Our study confirmed that urease-producing bacteria could coprecipitate multiple heavy metals even without the ability tolerate them, and the MICP process was an effective biological approach that was worth investigating further to immobilize multiple heavy metals in ecological restoration.

Comparative transcriptome analysis reveals the differential response to cadmium stress of two Pleurotus fungi: Pleurotus cornucopiae and Pleurotus ostreatus.

Pleurotus has great potential for heavy metal mycoremediation. Using comparative transcriptome analysis, the response of Pleurotus ostreatus and Pleurotus cornucopiae under Cd contamination was evaluated. P. ostreatus and P. cornucopia accumulated 0.34 and 0.46 mg/g Cd in mycelium, respectively. Cd removal elevated with its concentration elevation, which reached 56.47% and 54.60% for P. ostreatus and P. cornucopia with Cd at 20 mg/L. Low-level Cd (≤ 1 mg/L) had no significant influence on either fungus, while varied response was observed under high-level Cd. 705 differentially expressed genes (DEGs) were identified in P. cornucopia at Cd1 and Cd20, whereas 12,551 DEGs in P. ostreatus. Differentially regulated functional categories and pathways were also identified. ATP-binding cassette transporters were involved in Cd transport in P. cornucopia, whereas the endocytosis and phagosome pathways were more enhanced in P. ostreatus. 26 enzymes including peroxisomal enzymes catalase and superoxide dismutase were upregulated in P. ostreatus, whereas only cytosolic catalase was overexpressed in P. cornucopia, suggesting their different Cd detoxification pathways. Also, the mitogen-activated protein kinase signaling pathway involved in Cd resistance in both species instead of glutathione metabolism, although more active in P. ostreatus. These findings provided new insight into the molecular mechanism of mycoremediation and accumulator screening.

Immobilization of cadmium by Burkholderia sp. QY14 through modified microbially induced phosphate precipitation.

Microbially induced phosphate precipitation (MIPP) is an advanced bioremediation technology to immobilize heavy metals. An indigenous bacterium QY14 with the function of mineralization isolated from Cd contaminated farmland soil was identified as Burkholderia ambifaria. The minimum inhibitory concentration value for QY14 was 550 mg/L for soluble Cd concentration. This study found that the addition of 10 mM Ca2+ during MIPP process could significantly increase the removal ratio of Cd, and the maximum removal ratio of Cd with 10 mM Ca2+ and without Ca2+ in solution was 99.97% and 76.14%, respectively. The increase of acid phosphatase activity and the formation of precipitate containing calcium caused by 10 mM Ca2+ addition contributed the increase of Cd removal efficiency. The results of SEM-EDS, FTIR and XRD showed that Cd was removed by forming Cd containing hydroxyapatite (Cd-HAP). In addition, the dissolution experiment showed the Cd release ratio of Cd-HAP (0.01‰ at initial pH 3.0 of solution) was lower than Cd-absorbed HAP, indicating that Cd was more likely removed by the formation of Ca10-xCdx(PO4)6(OH)2 solid solution. Our findings revealed MIPP-based bioremediation supplied with 10 mM Ca2+ could increase the Cd removal and could potentially be applied for Cd remediation.

Performance of microbial induced carbonate precipitation for immobilizing Cd in water and soil.

Microbial induced carbonate precipitation (MICP) is known as a significant process for remediating heavy metals contaminated environment. In this study, a novel Cd-resistant ureolytic bacteria was isolated and identified as Enterobacter sp. Its performances for immobilizing Cd in solution and soil were systematically discussed at different treatment conditions. Results showed that initial pH and Cd concentration were important parameters to influence Cd removal rate. The maximal Cd removal rate in solution reached 99.50 % within 7 days by MICP. The precipitation produced in Cd removal process were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometer to understand the removal mechanism. Analyses showed that Cd removal mechanism of CJW-1 was predominately via biominerals including calcites and vaterites to absorb Cd2+. Cd immobilization tests demonstrated that the highest Cd-immobilization rate in soil could reach 56.10 %. Although all treatments contribute to soil pH, fertility, and enzyme activities improvement, oyster shell wastes (OS) had a better effect on soil cation exchange capacity. All treatments had negative effects on soil respiration and bacterial community, but OS can alleviate such adverse influence. Our results emphasized that Cd-resistant ureolytic bacteria strain CJW-1 combined with OS had excellent ability and reuse value to remediate Cd-contaminated environment.

Insight into the mechanisms of plant growth promoting strain SNB6 on enhancing the phytoextraction in cadmium contaminated soil.

Plant growth-promoting rhizobacteria (PGPR) assisted accumulator has been proposed as a phytoextraction method to clean cadmium (Cd) in contaminated soil, while the mechanisms were few studied regrading PGPR-soil-accumulator as an assemble. In this study, we revealed the possible mechanisms of the plant growth-promotion strain SNB6 on enhancing the Cd phytoextration of vetiver grass by the analysis of the whole genome of SNB6, soil biochemical properties and plant growth response. Results showed that SNB6 encoded numerous genes needed for Cd tolerance, Cd mobilization and plant growth promotion. SNB6 increased HOAc-extractable Cd that showed a positive correlation with Cd uptake in accumulator. In addition, SNB6 improved the biochemical activities (bioavailability of nutritional substances, bacterial count, soil respiration and enzyme activity) in rhizosphere soil. Moreover, the antioxidative enzymes activities of accumulator were significantly enhanced by SNB6. Consequently, SNB6 promoted Cd uptake and biomass of accumulator, thus enhancing the Cd phytoextraction. The maximum Cd extractions in root, stem and leaf reached to 289.47 mg/kg, 88.33 mg/kg and 59.38 mg/kg, respectively. Meanwhile, the total biomass of accumulator was increased by 9.68-45.99% in SNB6 treatment. These findings could be conducive to the understanding the mechanisms of PGPR on enhancing the Cd phytoextraction of accumulator.

Mutation of alkyl hydroperoxide reductase gene ahpC of Xanthomonas oryzae pv. oryzae affects hydrogen peroxide accumulation during the rice-pathogen interaction.

Hydrogen peroxide (H2O2) is usually generated by normal aerobic respiration of pathogens and by the host defense response during plant-pathogen interactions. In this study, histochemical localization of H2O2 accumulation in rice inoculated with the wild-type strain (PXO99(A)) and the gene deletion mutant (ΔahpC) of alkyl hydroperoxide reductase subunit C (AhpC) of Xanthomonas oryzae pv. oryzae (Xoo), the bacterial blight pathogen of rice, was analyzed. The ΔahpC mutant displayed a significant decrease in endogenous H2O2 accumulation which was induced by the compensatory increase in H2O2 scavenging activity. The change in the bacterial endogenous H2O2 level affected the total amount of H2O2 accumulation during the interaction with rice plants. These results suggested that Xoo contributes to H2O2 accumulation in rice in a compatible interaction, and pathogen-driving H2O2 is in association with cell collapse of rice.