Crystalline Phase Regulates Microbial Methylation Potential of Mercury Bound to MoS2 Nanosheets: Implications for Safe Design of Mercury Removal Materials.

Transition-metal dichalcogenides (TMDs) have shown great promise as selective and high-capacity sorbents for Hg(II) removal from water. Yet, their design should consider safe disposal of spent materials, particularly the subsequent formation of methylmercury (MeHg), a highly potent and bioaccumulative neurotoxin. Here, we show that microbial methylation of mercury bound to MoS2 nanosheets (a representative TMD material) is significant under anoxic conditions commonly encountered in landfills. Notably, the methylation potential is highly dependent on the phase compositions of MoS2. MeHg production was higher for 1T MoS2, as mercury bound to this phase primarily exists as surface complexes that are available for ligand exchange. In comparison, mercury on 2H MoS2 occurs largely in the form of precipitates, particularly monovalent mercury minerals (e.g., Hg2MoO4 and Hg2SO4) that are minimally bioavailable. Thus, even though 1T MoS2 is more effective in Hg(II) removal from aqueous solution due to its higher adsorption affinity and reductive ability, it poses a higher risk of MeHg formation after landfill disposal. These findings highlight the critical role of nanoscale surfaces in enriching heavy metals and subsequently regulating their bioavailability and risks and shed light on the safe design of heavy metal sorbent materials through surface structural modulation.

Sulfur-Intercalated Layered Double Hydroxides Minimize Microbial Mercury Methylation: Implications for In Situ Remediation of Mercury-Contaminated Sites.

Conventional approaches for in situ remediation of mercury (Hg)-contaminated soils and sediments rely mostly on precipitation or adsorption. However, this can generate Hg-rich surfaces that facilitate microbial production of methylmercury (MeHg), a potent, bioaccumulative neurotoxin. Herein, we prove the concept that the risk of mercury methylation can be effectively minimized by adding sulfur-intercalated layered double hydroxide (S-LDH) to Hg-contaminated soils. Hg bound to S-LDH has minimal methylation potential when incubated with model methylating bacteria Pseudodesulfovibrio mercurii ND132 and Geobacter sulfurreducens PCA. With a combination of spectroscopic and microscopic evidence, as well as theoretical calculations, we confirm that dissolved Hg(II) tends to enter the interlayers of S-LDH to bind to the sulfur groups intercalated within, leading to the formation of nanoscale metacinnabar (ß-HgS). This not only physically blocks the contact of methylating microorganisms but also inhibits secondary release of bound mercury in the presence of strong binding ligands in porewater. This study highlights the promising concept of in situ risk reduction of heavy metal contamination by inducing precipitation within (nano)confined domains, achieving a sustainable outcome of enhanced removal and reduced bioaccessibility for pollutants that may otherwise be bioavailable in the form of nanoprecipitates.

An Examination of Students' Perspectives of Medical English Course Quality in Guangdong Medical Universities.

Phenomenon: In China, medical English courses are critical to medical education, equipping Chinese students with the linguistic tools necessary for international medical practice and collaboration. However, a disconnect persists between the pedagogical approaches of medical practitioners and language educators, leading to a curriculum that emphasizes grammatical accuracy over practical communication skills. This misalignment results in student disengagement and falls short of addressing the real-world demands of the medical profession. With the growing importance of English proficiency in the global health sector, the need for significant improvements in medical English education is evident. This study delves into the underlying causes of student demotivation and aims to reconcile educational delivery with the evolving expectations of the medical field. Insights gained from this research will inform targeted interventions, promising to enhance medical English courses and support improved educational experiences for Chinese medical undergraduates. Approach: This cross-sectional quantitative study surveyed 3,046 second-year medical students from four medical universities in Guangdong Province, China, leveraging means-analysis and Expectancy-Disconfirmation Theory (EDT) as its foundation. The research was conducted at the end of the 2022-2023 academic year, utilizing a questionnaire to assess students' perceptions of their medical English courses. Importance-Performance Analysis (IPA) was the primary analytical tool to discern discrepancies between students' expectations and experiences. Findings: The IPA revealed that course content, classroom environment, and instructor effectiveness were pivotal factors influencing the perceived quality of the medical English courses. Students expressed a need for practical and relevant course material, with current content and textbooks falling short of preparing them for future medical communication demands. Additionally, while learning technologies were acknowledged, there was a discernible preference against their excessive application, suggesting a misalignment between student satisfaction and learning outcomes. Insights: This study highlights the need for innovative staffing models, refined qualifications for part-time instructors, development of collaborative and practical teaching materials, and focused training for medical English instructors. It also emphasizes the judicious integration of e-learning to enhance the learning experience. These insights aim to improve instruction quality by informing potential pedagogical adjustments and resource allocations in medical English education.

Effects of Extracellular Polymeric Substances on the Formation and Methylation of Mercury Sulfide Nanoparticles.

Growing evidence has suggested that microbial biofilms are potential environmental "hotspots" for the production and accumulation of a bioaccumulative neurotoxin, methylmercury. Here, we demonstrate that extracellular polymeric substances (EPS), the main components of biofilm matrices, significantly interfere with mercury sulfide precipitation and lead to the formation of nanoparticulate metacinnabar available for microbial methylation, a natural process predominantly responsible for the environmental occurrence of methylmercury. EPS derived from mercury methylating bacteria, particularly Desulfovibrio desulfuricans ND132, substantially increase the methylation potential of nanoparticulate mercury. This is likely due to the abundant aromatic biomolecules in EPS that strongly interact with mercury sulfide via inner-sphere complexation and consequently enhance the short-range structural disorder while mitigating the aggregation of nanoparticulate mercury. The EPS-elevated bioavailability of nanoparticulate mercury to D. desulfuricans ND132 is not induced by dissolution of these nanoparticles in aqueous phase, and may be dictated by cell-nanoparticle interfacial reactions. Our discovery is the first step of mechanistically understanding methylmercury production in biofilms. These new mechanistic insights will help incorporate microbial EPS and particulate-phase mercury into mercury methylation models, and may facilitate the assessment of biogeochemical cycling of other nutrient or toxic elements driven by EPS-producing microorganisms that are prevalent in nature.

Understanding mercury methylation in the changing environment: Recent advances in assessing microbial methylators and mercury bioavailability.

Methylmercury (MeHg) is a neurotoxin, mainly derived from microbial mercury methylation in natural aquatic environments, and poses threats to human health. Polar regions and paddy soils are potential hotspots of mercury methylation and represent environmental settings that are susceptible to natural and anthropogenic perturbations. The effects of changing environmental conditions on the methylating microorganisms and mercury speciation due to global climate change and farming practices aimed for sustainable agriculture were discussed for polar regions and paddy soils, respectively. To better understand and predict microbial mercury methylation in the changing environment, we synthesized current understanding of how to effectively identify active mercury methylators and assess the bioavailability of different mercury species for methylation. The application of biomarkers based on the hgcAB genes have demonstrated the occurrence of potential mercury methylators, such as sulfate-reducing bacteria, iron-reducing bacteria, methanogen and syntrophs, in a diverse variety of microbial habitats. Advanced techniques, such as enriched stable isotope tracers, whole-cell biosensor and diffusive gradient thin film (DGT) have shown great promises in quantitatively assessing mercury availability to microbial methylators. Improved understanding of the complex structure of microbial communities consisting mercury methylators and non-methylators, chemical speciation of inorganic mercury under geochemically relevant conditions, and the pathway of cellular mercury uptake will undoubtedly facilitate accurate assessment and prediction of in situ microbial mercury methylation.