Facilitating New Chromium Reducing Microbes to Enhance Hexavalent Chromium Reduction by In Situ Sonoporation-Mediated Gene Transfer in Soils

Electron flow dynamics in sulfur-based autotrophic bioreduction of Cr(VI) mediated by inorganic carbon species: Insights for environmental remediation

The deployment of sulfur-based autotrophic bioremediation for in situ groundwater remediation faces hurdles due to electron competition among electron acceptors, impacting contaminant removal efficiency and causing pH instability. Notably, the sulfur-based bioreduction of Cr(VI) [Cr(VI)-SAR] exemplifies gaps in our comprehension of electron competition dynamics with inorganic carbon (IC), and its subsequent influence on pH. Herein, we established a Cr(VI)-SAR system interfaced with diverse IC species, providing definitive insights into electron transfer mechanisms through rigorous multi-biocycle analysis and thermodynamically consistent half-reaction calculations. Through quantification of electron transfer pathways, we derived reaction equations for Cr(VI) reduction in conjunction with various IC species. Furthermore, metagenomics were used to quantify functional enzymes and identify diverse electron transport patterns alongside IC fixation pathways. Notably, the enrichment of genes associated with electron shuttles and conductive pili expands the paradigm of extracellular electron transfer, while the Wood-Ljungdahl pathway streamlines microbial metabolic proliferation with reduced energy expenditure. Quantitative analysis of these functional genes offers a plausible mechanism underlying the observed shifts in electron competition between IC and Cr(VI). This research marks an advancement in the Cr(VI)-SAR foundational theory, with a particular focus on the dynamics of electron competition, contributing to a deeper understanding of this environmentally significant process.

Identification of key chromium resistance genes in Cellulomonas using transcriptomics

Cellulomonas fimi Clb-11 can reduce high toxic Cr(VI) to less toxic Cr(III), and transcriptomics was used to reveal the key Cr(VI) uptake and reduction genes of C. fimi Clb-11 in this study. The results showed that under 0.5 mM Cr(VI) stress, 654 genes were upregulated. Among the upregulated genes, phosphate transport protein encoding genes phoU and TC.PIT, and molybdate transport protein encoding genes modA, modB, and modC were involved in the cell uptake of Cr(VI). Cytochrome c subunits encoding genes qcrA and qcrC were involved in the intracellular reduction of Cr(VI), and cytochrome c oxidase subunits encoding genes coxB and coxC were involved in extracellular electron secretion. Additionally, several unreported genes were found to be upregulated in C. fimi Clb-11 under Cr(VI) stress. The upregulated manganese transport protein encoding gene mntH may also assist Cr(VI) uptake in C. fimi Clb-11. Proton pump subunit encoding genes atpA, atpB, atpE, atpF, and atpH, as well as sodium-hydrogen antiporter subunit encoding genes mnhA and mnhC were upregulated, it may be involved in the extracellular proton secretion to reduce Cr(VI). Iron complex transport system substrate-binding protein encoding gene ABC.FEV.S, vacuolar iron transporter encoding gene VIT, FMN reductase gene encoding gene ssuE, and quinone oxidoreductase encoding genes qor and qorB were upregulated to reduce Cr(VI) in the intracellular. Our study showed theoretical importance by revealing the Cr(VI) reduction mechanism using chrome-resistant microorganisms.

Microbial adaptation and genetic modifications for enhanced remediation in low-permeability soils

Low-permeability soils, characterized by fine texture and high clay content, pose significant challenges to traditional soil remediation techniques due to limited hydraulic conductivity, restricted nutrient flow, and reduced oxygen availability. These unique properties enable low-permeability soils to function as natural barriers in environmental protection; however, they also trap contaminants, making traditional remediation efforts challenging. This review synthesizes current knowledge on microbial adaptation and genetic engineering approaches that enhance the effectiveness of bioremediation in such environments. Key microbial adaptations, including anaerobic metabolism, extracellular enzyme production, and stress response mechanisms, allow individual microbes to adapt in low-permeability soils. Additionally, community-level strategies like microhabitat creation, biofilm formation, and functional redundancy further support microbial resilience. Advancements in genetic engineering now enable the modification of microbial traits-such as soil adhesion, nutrient utilization, and stress tolerance-to enhance bioremediation efficacy. Synthetic biology techniques further allow for the design of tailored microbial consortia that work cooperatively to degrade contaminants in complex soil matrices. This review highlights the integration of microbial and genetic engineering strategies, offering a comprehensive overview that informs current practices and guides future research in low-permeability soil remediation.

Heavy metal contamination assessment and source attribution in the Vicinity of an iron slag pile in Hechi, China: Integrating multi-medium analysis

Heavy metals, such as mercury, cadmium, and nickel, may contaminate human inhabited environments, with critical consequences for human health. This study examines the health impacts of heavy metal pollution from an iron slag pile in Hechi, China, by analyzing heavy metal contamination in water, sediment, soil, and crops. Here, the Nemerow pollution index (NI) indicated severe pollution at most sampling sites, the mean NI of groundwater, and surface water had reached 594.13 and 26.79, respectively. Bioaccumulation of mercury (Hg), cadmium (Cd), and nickel (Ni) was noted in crops, cucumbers showed comparatively lower risk levels. Logarithmic surface water-sediment partition coefficient calculations indicated that heavy metals such as chromium (Cr), ferrum (Fe), zinc (Zn), copper (Cu), Ni, arsenic (As), and lead (Pb) tend to accumulate in sediments. There was a high risk in groundwater (67.48-6590.54) and surface water (13.73-2500.85). Variably influenced by rainfall, these metals can be diluted and mobilized from surface water and sediments, thereby changing the contamination levels and ecological risks. Probabilistic health risk assessments indicated that health risks were higher in children than in adults, the mean total carcinogenic risk values of soil, groundwater, and surface water, were 6.79E-04, 4.20E-06, and 1.15E-6 for children, respectively. Moderate soil pollution is the main health hazard. A Positive Matrix Factorization model attributed over 60% of the pollution to slag stacking. Biotechnologies, solidification/stabilization techniques, field management, and institutional controls, driven by principles of green, low-carbon, and economic efficiency may mitigate. These findings contribute to the management of heavy metal pollution in iron slag pile areas.

Combined impact of antibiotics and Cr(VI) on antibiotic resistance, ARGs, and growth of Bacillussp. SH-1: A functionl analysis from gene to protease

The simultaneous selection of antibiotic resistance genes (ARGs) induced by heavy metals and antibiotics has emerged as a growing environmental problem. This study investigated the combined effects of chromium (Cr(VI)) and antibiotics on the ARGs of Bacillus cereus SH-1. As Cr(VI) concentration increased, it triggered reactive oxygen species oxidative stress in SH-1, increased antioxidant enzyme activity, enhanced plasmid conjugative transfer, and reduced the efficiency of Cr(VI) removal by SH-1. Antibiotic resistance varied with increasing tetracycline and amoxicillin minimum inhibitory concentrations (MICs), whereas azithromycin and chloramphenicol MICs decreased with Cr(VI) induction. The overexpression of eight genes of the HAE-1 family of efflux pumps was detected using metagenomics and proteomics. Co-contamination with Cr(VI) and antibiotics has led to the emergence and spread of antibiotic-resistant bacteria. Therefore, resistance gene contamination resulting from Cr(VI)-polluted environments cannot be overlooked.

Mechanisms and influential factors of soil chromium long-term stability by an accelerated aging system after chemical stabilization

Chemical stabilization is one of the most widely used remediation strategies for chromium (Cr)-contaminated soils by reducing Cr(VI) to Cr(III), and its performance is affected by human and natural processes in a prolonged period, challenging long-term Cr stability. In this work, we established a method for evaluating the long-term effectiveness of remediation of Cr-contaminated soils, and developed an accelerated aging system to simultaneously simulate acid rain leaching and freeze-thaw cycles. The mechanisms and influencing factors of long-term (50-year) change in soil Cr speciation were unravelled after stabilization with Metafix®. Chemical stabilization remarkably decreased the contents of Cr(VI)soil, Crtotal-leach and Cr(VI)leach, among which the removal rate of Cr(VI) in soil was up to 89.70 %, but it also aggravated soil Cr instability. During the accelerated aging process, Crtotal-leach change rates in chemically stabilized soil samples were 0.0462-0.0587 mg/(L·a), and soil Cr became instable after 20-year accelerated aging. The proportion of Cr bound to organic matter and residual Cr increased in soil, and exchangeable Cr decreased. Linear combination fitting results of XANES also showed that Cr(VI) and Cr3+ were transformed into OM-Cr(III), Fh-Cr(III) and CrFeO3 after restoration. During the accelerated aging process, acid rain leaching activated Cr(III) and dissolved Cr(VI), whereas freeze-thaw cycle mainly affected OM-Cr. Chemical stabilization, acid rain leaching and aging time were the major factors influencing the stability of soil Cr, and the freeze-thaw cycle promoted the influence of acid rain leaching. This study provided a new way to explore the long-term effectiveness and instability mechanisms at Cr-contaminated site after chemical stabilization.

Groundwater hydrochemistry, source identification and health assessment based on self-organizing map in an intensive mining area in Shanxi, China

The Changzhi Basin in Shanxi is renowned for its extensive mining activities. It's crucial to comprehend the spatial distribution and geochemical factors influencing its water quality to uphold water security and safeguard the ecosystem. However, the complexity inherent in hydrogeochemical data presents challenges for linear data analysis methods. This study utilizes a combined approach of self-organizing maps (SOM) and K-means clustering to investigate the hydrogeochemical sources of shallow groundwater in the Changzhi Basin and the associated human health risks. The results showed that the groundwater chemical characteristics were categorized into 48 neurons grouped into six clusters (C1-C6) representing different groundwater types with different contamination characteristics. C1, C3, and C5 represent uncontaminated or minimally contaminated groundwater (Ca-HCO3 type), while C2 signifies mixed-contaminated groundwater (HCO3-Ca type, Mixed Cl-Mg-Ca type, and CaSO4 type). C4 samples exhibit impacts from agricultural activities (Mixed Cl-Mg-Ca), and C6 reflects high Ca and NO3- groundwater. Anthropogenic activities, especially agriculture, have resulted in elevated NO3- levels in shallow groundwater. Notably, heightened non-carcinogenic risks linked to NO3-, Pb, F-, and Mn exposure through drinking water, particularly impacting children, warrant significant attention. This research contributes valuable insights into sustainable groundwater resource development, pollution mitigation strategies, and effective ecosystem protection within intensive mining regions like the Changzhi Basin. It serves as a vital reference for similar areas worldwide, offering guidance for groundwater management, pollution prevention, and control.