Metal resistant bacteria on gold particles: Implications of how anthropogenic contaminants could affect natural gold biogeochemical cycling

Microbial adaptations and biogeochemical cycling of uranium in polymetallic tailings

Microorganisms inhabiting uranium (U)-rich environments have specific physiological and biochemical coping mechanisms to deal with U toxicity, and thereby play a crucial role in the U biogeochemical cycling as well as associated heavy metals. We investigated the diversity and functional capabilities of indigenous bacterial communities inhabiting historic U- and Rare-Earth-Elements-rich polymetallic tailings from the Mount Painter Inlier, Northern Flinders Ranges, South Australia. Bacterial diversity profiling identified Actinobacteria as the predominant phylum in all samples. GeoChip analyses revealed the presence of diverse functional genes associated with biogenic element cycling, metal homeostasis/resistance, stress response, and secondary metabolism. The high abundance of metal-resistance and stress-tolerance genes indicates the adaptation of bacterial communities to the "harsh" environmental (metal-rich and semi-arid) conditions of the Northern Flinders Ranges. Additionally, a viable bacterial consortium was enriched from polymetallic tailings. Laboratory experiments demonstrated that the consortium scrubbed uranyl from solution by precipitating a uranyl phosphate biomineral (chernikovite), thus contributing to U biogeochemical cycling. These specialised microbial communities reflect the high specificity of the mineralogy/geochemistry, and biogeography of these U-rich settings. This study provides the fundamental knowledge to develop future applications in securing long-term stability of polymetallic mine waste, and for reprocessing this "waste" to further extract critical minerals.

Theoretical study of the adsorption capacity of potentially toxic Cd2+, Pb2+, and Hg2+ ions in hemicellulose matrices

Hemicellulose is widely available in nature, is a sustainable resource and has a wide range of applications. Among them, adsorption stands out for the removal of potentially toxic ions. Thus, in the study, the adsorption of Cd2+, Pb2+ and Hg2+ ions in two hemicellulose matrices were elucidated through computational simulations using density functional theory. Molecular electrostatic potential and frontier molecular orbitals demonstrated whether the interactions could happen. Four interaction complexes were highlighted due to the interaction energy criteria, ΔEBind, ΔH and ΔG < 0.00 kcal mol-1, that is: Hm1… Pb (1); Hm2… Pb (3); Hm2…Cd (4) and Hm2…Hg (4) and the results show that they occur through physisorption. In structural parameter studies, interaction distances smaller than 3000 Å were identified, which ranged from 2.253 Å to 2.972 Å. From the analysis of the topological parameters of QTAIM, it was possible to characterize the intensities of the interactions, as well as their nature, which were partially covalent or electrostatic in nature. Finally, based on the theoretical results, it can be affirmed that the hemicellulose can interact with Cd2+, Pb2+ and Hg2+ ions, evidencing that this study can support further experimental essays to remove contaminants from effluents.

From biomolecules to biogeochemistry: Exploring the interaction of an indigenous bacterium with gold

Specialised microbial communities colonise the surface of gold particles in soils/sediments, and catalyse gold dissolution and re-precipitation, thereby contributing to the environmental mobility and toxicity of this 'inert' precious metal. We assessed the proteomic and physiological response of Serratia proteamaculans, the first metabolically active bacterium enriched and isolated directly from natural gold particles, when exposed to toxic levels of soluble Au3+ (10 μM). The results were compared to a metal-free blank, and to cultures exposed to similarly toxic levels of soluble Cu2+ (0.1 mM); Cu was chosen for comparison because it is closely associated with Au in nature due to similar geochemical properties. A total of 273 proteins were detected from the cells that experienced the oxidative effects of soluble Au, of which 139 (51%) were upregulated with either sole expression (31%) or had synthesis levels greater than the Au-free control (20%). The majority (54%) of upregulated proteins were functionally different from up-regulated proteins in the bacteria-copper treatment. These proteins were related to broad functions involving metabolism and biogenesis, followed by cellular process and signalling, indicating significant specificity for Au. This proteomic study revealed that the bacterium upregulates the synthesis of various proteins related to oxidative stress response (e.g., Monothiol-Glutaredoxin, Thiol Peroxidase, etc.) and cellular damage repair, which leads to the formation of metallic gold nanoparticles less toxic than ionic gold. Therefore, indigenous bacteria may mediate the toxicity of Au through two different yet simultaneous processes: i) repairing cellular components by replenishing damaged proteins and ii) neutralising reactive oxygen species (ROS) by up-regulating the synthesis of antioxidants. By connecting the fields of molecular bacteriology and environmental biogeochemistry, this study is the first step towards the development of biotechnologies based on indigenous bacteria applied to gold bio-recovery and bioremediation of contaminated environments.

Heavy metals in influent and effluent from 146 drinking water treatment plants across China: Occurrence, explanatory factors, probabilistic health risk, and removal efficiency

Heavy metals (HMs) in drinking water have drawn worldwide attention due to their risks to public health; however, a systematic assessment of the occurrence of HMs in drinking water treatment plants (DWTPs) at a large geographical scale across China and the removal efficiency, human health risks, and the correlation with environmental factors have yet to be established. Therefore, this study characterised the occurrence patterns of nine conventional dissolved HMs in the influent and effluent water samples from 146 typical DWTPs in seven major river basins across China (which consist of the Yangtze River, the Yellow River, the Songhua River, the Pearl River, the Huaihe River, the Liaohe River and the Haihe River) for the first time and removal efficiency, probabilistic health risks, and the correlation with water quality. According to the findings, a total of eight HMs (beryllium (Be), antimony (Sb), barium (Ba), molybdenum (Mo), nickel (Ni), vanadium (V), cobalt (Co) and titanium (Ti)) were detected, with detection frequencies in influent and effluent water ranging from 2.90 (Mo) to 99.30% (Ba) and 1.40 (Ti) to 97.90% (Ba), respectively. The average concentration range was 0.41 (Be)- 77.36 (Sb) μg/L. Among them, Sb (exceeding standard rate 8%), Ba (2.89%), Ni (21.43%), and V (1.33%) were exceeded the national standard (GB5749-2022). By combining Spearman's results and redundancy analysis, our results revealed a close correlation among pH, turbidity (TURB), potassium permanganate index (COD_Mn), and total nitrogen (TN) along with the concentration and composition of HMs. In addition, the concentration of HMs in finished water was strongly affected by the concentration of HMs in raw water, as evidenced by the fact that HMs in surface water poses a risk to the quality of finished water. Metal concentration was the primary factor in assessing the health risk of a single metal, and the carcinogenic risk of Ba, Mo, Ni, and Sb should be paid attention to. In DWTPs, the removal efficiencies of various HMs also vary greatly, with an average removal rate ranging from 16.30% to 95.64%. In summary, our findings provide insights into the water quality and health risks caused by HMs in drinking water.

A genomic perspective of metal-resistant bacteria from gold particles: Possible survival mechanisms during gold biogeochemical cycling

A bacterial consortium was enriched from gold particles that ‘experienced’ ca. 80 years of biotransformation within waste-rock piles (Australia). This bacterial consortium was exposed to 10 µM AuCl3 to obtain Au-tolerant bacteria. From these isolates, Serratia sp. and Stenotrophomonas sp. were the most Au-tolerant and reduced soluble Au as pure gold nanoparticles, indicating that passive mineralisation is a mechanism for mediating the toxic effect of soluble Au produced during particle dissolution. Genome-wide analysis demonstrated that these isolates also possessed various genes that could provide cellular defence enabling survival under heavy-metal stressed condition by mediating the toxicity of heavy metals through active efflux/reduction. Diverse metal-resistant genes or genes clusters (cop, cus, czc, zntand ars) were detected, which could confer resistance to soluble Au. Comparative genome analysis revealed that the majority of detected heavy-metal resistant genes were similar (i.e. orthologous) to those genes of Cupriavidus metallidurans CH34. The detection of heavy-metal resistance, nutrient cycling and biofilm formation genes (pgaABCD, bsmAandhmpS) may have indirect yet important roles when dealing with soluble Au during particle dissolution. In conclusion, the physiological and genomic results suggest that bacteria living on gold particles would likely use various genes to ensure survival during Au-biogeochemical cycling.