Stabilization and mechanism of uranium sequestration by a mixed culture consortia of sulfate-reducing and phosphate-solubilizing bacteria.

In this study, a highly efficient phosphate-solubilizing bacteria (PSB) (Pantoea sp. grinm-12) was screened out from uranium (U) tailings, and the carbon and nitrogen sources of mixed culture with sulfate-reducing bacteria (SRB) were optimized. Results showed that the functional expression of SRB-PSB could be promoted effectively when glucose + sodium lactate was used as carbon source and ammonium nitrate + ammonium sulfate as nitrogen source. The concentration of PO43- in the culture system could reach 107.27 mg·L-1, and the sulfate reduction rate was 81.72%. In the process of biological stabilization of U tailings by mixed SRB-PSB culture system, the chemical form of U in the remediation group was found to transfer to stable state with the extension of remediation time, which revealed the effectiveness of bioremediation on the harmless treatment of U tailings. XRD, FT-IR, SEM-EDS, high-throughput sequencing, and metagenomics were also used to assist in revealing the microstructure and composition changes during the biological stabilization process, and explore the microbial community/functional gene response. Finally, the stabilization mechanism of U was proposed. In conclusion, the stabilization of U in U tailings was realized through the synergistic effect of bio-reduction, bio-precipitation, and bio-adsorption.

Lab-scale evaluation of the microbial bioremediation of Cr(VI): contributions of biosorption, bioreduction, and biomineralization.

Bioremediation of Cr(VI) by microorganisms has attracted immense research interests. There are three different mechanisms for bioremediation of Cr(VI): biosorption, bioreduction, and biomineralization. Identifying the relative contributions of these different mechanisms to Cr(VI) bioremediation can provide valuable information to enhance the final result. This article explores the corresponding contributions of different mechanisms in the Cr(VI) bioremediation process. To obtain a deeper understanding of each bioremediation mechanism, the corresponding precipitation products were analyzed via different methods. Fourier transform infrared spectrometer (FTIR) analysis showed that Cr(VI) was adsorbed by functional groups in EPS to form a chelate compound. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis determined that the stable Cr(III) compounds and mineral crystals which contain chromium gradually formed during the bioremediation process. High-throughput sequencing technology was applied to monitor microbial community succession. The results showed that the total removal rate of Cr(VI) reached 77.64% in 56 days in 100 mg/L Cr(VI). Bioreduction was the major contributor to the final result, followed by biosorption and biomineralization; their proportions are 69.61%, 19.16%, and 11.23%, respectively. Besides, the high-throughput sequencing data indicated that reductive microorganisms were the dominant flora and that the relative abundance of different reductive microorganism types changes significantly. This work has clarified the contributions of different mechanisms during Cr(VI) bioremediation process and provided a new enhancement strategy for Cr(VI) bioremediation.Graphical abstract.

Optimization of Environmental Conditions for Microbial Stabilization of Uranium Tailings, and the Microbial Community Response.

Uranium pollution in tailings and its decay products is a global environmental problem. It is of great significance to use economical and efficient technologies to remediate uranium-contaminated soil. In this study, the effects of pH, temperature, and inoculation volume on stabilization efficiency and microbial community response of uranium tailings were investigated by a single-factor batch experiment in the remediation process by mixed sulfate-reducing bacteria (SRB) and phosphate-solubilizing bacteria (PSB, Pantoea sp. grinm-12). The results showed that the optimal parameters of microbial stabilization by mixed SRB-PSB were pH of 5.0, temperature of 25°C, and inoculation volume of 10%. Under the optimal conditions, the uranium in uranium tailings presented a tendency to transform from the acid-soluble state to residual state. In addition, the introduction of exogenous SRB-PSB can significantly increase the richness and diversity of endogenous microorganisms, effectively maintain the reductive environment for the microbial stabilization system, and promote the growth of functional microorganisms, such as sulfate-reducing bacteria (Desulfosporosinus and Desulfovibrio) and iron-reducing bacteria (Geobacter and Sedimentibacter). Finally, PCoA and CCA analyses showed that temperature and inoculation volume had significant effects on microbial community structure, and the influence order of the three environmental factors is as follows: inoculation volume > temperature > pH. The outcomes of this study provide theoretical support for the control of uranium in uranium-contaminated sites.

Isolation and Identification of Uranium Tolerant Phosphate-Solubilizing Bacillus spp. and Their Synergistic Strategies to U(VI) Immobilization.

The remediation of uranium (U) through phosphate-solubilizing bacteria (PSB) is an emerging technique as well as an interesting phenomenon for transforming mobile U into stable minerals in the environment. While studies are well needed for in-depth understanding of the mechanism of U(VI) immobilization by PSB. In this study, two PSB were isolated from a U-tailing repository site. These bacterial strains (ZJ-1 and ZJ-3) were identified as Bacillus spp. by the sequence analysis of 16S ribosomal RNA (rRNA) genes. Incubation of PSB in liquid medium showed that the isolate ZJ-3 could solubilize more than 230 mg L-1 P from glycerol-3-phosphate and simultaneously removed over 70% of 50 mg L-1 U(VI) within 1 h. During this process, the rapid appearance of yellow precipitates was observed. The microscopic and spectroscopic analysis demonstrated that the precipitates were associated with U-phosphate compound in the form of saleeite-like substances. Besides, scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR) analysis of the precipitates confirmed that the extracellular polymeric substances (EPS) might also play a key role in U sequestration. Furthermore, SEM and FTIR analysis revealed that part of U(VI) was adsorbed on the bacterial surface through cellular phosphate, hydroxy, carboxyl, and amide groups. This study provides new insights into the synergistic strategies enhancing U immobilization rates by Bacillus spp. that uses glycerol-3-phosphate as the phosphorus source, the process of which contributes to harmful pollutant biodegradation.

Effects of Heavy Metals/Metalloids and Soil Properties on Microbial Communities in Farmland in the Vicinity of a Metals Smelter.

Microorganisms play a fundamental role in biogeochemical cycling and are highly sensitive to environmental factors, including the physiochemical properties of the soils and the concentrations of heavy metals/metalloids. In this study, high-throughput sequencing of the 16S rRNA gene was used to study the microbial communities of farmland soils in farmland in the vicinity of a lead-zinc smelter. Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, and Gemmatimonadetes were the predominant phyla in the sites of interest. Sphingomonas, Gemmatimonas, Lysobacter, Flavisolibacter, and Chitinophaga were heavy metal-/metalloid-tolerant microbial groups with potential for bioremediation of the heavy metal/metalloid contaminated soils. However, the bacterial diversity was different for the different sites. The contents of heavy metal/metalloid species and the soil properties were studied to evaluate the effect on the soil bacterial communities. The Mantel test revealed that soil pH, total cadmium (T-Cd), and available arsenic played a vital role in determining the structure of the microbial communities. Further, we analyzed statistically the heavy metals/metalloids and the soil properties, and the results revealed that the microbial richness and diversity were regulated mainly by the soil properties, which correlated positively with organic matter and available nitrogen, while available phosphorus and available potassium were negatively correlated. The functional annotation of the prokaryotic taxa (FAPROTAX) method was used to predict the function of the microbial communities. Chemoheterotrophy and airborne chemoheterotrophy of the main microbial community functions were inhibited by soil pH and the heavy metals/metalloids, except in the case of available lead. Mantel tests revealed that T-Cd and available zinc were the dominant factors affecting the functions of the microbial communities. Overall, the research indicated that in contaminated soils, the presence of multiple heavy metals/metalloids, and the soil properties synergistically shaped the structure and function of the microbial communities.