In Situ Stimulation of Thiocyanate Biodegradation through Phosphate Amendment in Gold Mine Tailings Water

Elucidating the intricacies of the H2S signaling pathway in gasotransmitters: Highlighting the regulation of plant thiocyanate detoxification pathways

In recent decades, there has been increasing interest in elucidating the role of sulfur-containing compounds in plant metabolism, particularly emphasizing their function as signaling molecules. Among these, thiocyanate (SCN-), a compound imbued with sulfur and nitrogen, has emerged as a significant environmental contaminant frequently detected in irrigation water. This compound is known for its potential to adversely impact plant growth and agricultural yield. Although adopting exogenous SCN- as a nitrogen source in plant cells has been the subject of thorough investigation, the fate of sulfur resulting from the assimilation of exogenous SCN- has not been fully explored. There is burgeoning curiosity in probing the fate of SCN- within plant systems, especially considering the possible generation of the gaseous signaling molecule, hydrogen sulfide (H2S) during the metabolism of SCN-. Notably, the endogenous synthesis of H2S occurs predominantly within chloroplasts, the cytosol, and mitochondria. In contrast, the production of H2S following the assimilation of exogenous SCN- is explicitly confined to chloroplasts and mitochondria. This phenomenon indicates complex interplay and communication among various subcellular organelles, influencing signal transduction and other vital physiological processes. This review, augmented by a small-scale experimental study, endeavors to provide insights into the functional characteristics of H2S signaling in plants subjected to SCN--stress. Furthermore, a comparative analysis of the occurrence and trajectory of endogenous H2S and H2S derived from SCN--assimilation within plant organisms was performed, providing a focused lens for a comprehensive examination of the multifaceted roles of H2S in rice plants. By delving into these dimensions, our objective is to enhance the understanding of the regulatory mechanisms employed by the gasotransmitter H2S in plant adaptations and responses to SCN--stress, yielding invaluable insights into strategies for plant resilience and adaptive capabilities.

Gaseous signaling molecule H2S as a multitasking signal molecule in ROS metabolism of Oryza sativa under thiocyanate (SCN-) pollution

The induction of disruption in the electronic transport chain by thiocyanate (SCN-) leads to an excessive generation of reactive oxygen species (ROS) within rice (Oryza sativa). Hydrogen sulfide (H2S) assumes a crucial role as a gaseous signaling molecule, holding significant potential in alleviating SCN--related stress. Nevertheless, there remains a dearth of understanding regarding the intricate interplay between H2S and ROS in Oryza sativa amidst SCN- pollution. In this investigation, a hydroponics-based experiment was meticulously devised to explore how H2S-mediated modifications influence the genetic feedback network governing ROS metabolism within the subcellular organelles of Oryza sativa when exposed to varying effective concentrations (EC20: 24 mg SCN/L; EC50: 96 mg SCN/L; EC75: 300 mg SCN/L) of SCN-. The findings unveiled the enhanced capacity of Oryza sativa to uptake SCN- under H2S + SCN- treatments in comparison to SCN- treatments alone. Notably, the relative growth rate (RGR) of seedlings subjected to H2S + SCN- exhibited a superior performance when contrasted with seedlings exposed solely to SCN-. Furthermore, the application of exogenous H2S yielded a significant reduction in ROS levels within Oryza sativa tissues during SCN- exposure. To elucidate the intricacies of gene regulation governing ROS metabolism at the mRNA level, the 52 targeted genes were categorized into four distinct types, namely: initial regulatory ROS generation genes (ROS-I), direct ROS scavenging genes (ROS-II), indirect ROS scavenging genes (ROS-III), and lipid oxidation genes (ROS-IV). On the whole, exogenous H2S exhibited the capacity to activate the majority of ROS-I ∼ ROS-IV genes within both Oryza sativa tissues at the EC20 concentration of SCN-. However, genetic positive/negative feedback networks emphasized the pivotal role of ROS-II genes in governing ROS metabolism within Oryza sativa. Notably, these genes were predominantly activated within the cytoplasm, chloroplasts, mitochondria, peroxisomes, and the cell wall.

Carbon and hydrogen isotope fractionation of phthalates during photocatalysis reactions in aqueous solution containing Fe(III) complexes or iron minerals

The hydrogen and carbon isotope fractionation factor (ε2H, ε13C) of dimethyl-, diethyl‑ and dibutyl phthalic acid ester during photosensitized degradation by artificial sunlight with Fe(III) ions and iron minerals (hematite, goethite and magnetite) in aqueous solution were examined by compound-specific isotope analysis (CSIA) in order to analyze the degradation mechanism. Hematite does not catalyze photosensitized degradation of phthalates. The correlation of 2H and 13C isotope fractionation (Λ = Δδ2H/Δδ13C) of phthalates with increasing chain length (dimethyl-; diethyl‑; and dibutyl-) were compared with values of the ∙OH radical model reaction with the aromatic ring as well as acidic and alkaline hydrolysis. The Λ values of die photosensitized reaction of diethyl phthalate with goethite (-5.1 ± 1.8) and magnetite (-18.9 ± 3.9) show a large difference compared to Fe(III) solutions (4.7 ± 0.9 to 4.8 ± 1.0) suggesting specific reaction mechanisms. The fractionation factors determined here have potential to characterize the degradation of phthalates catalyzed by photo-induced reaction of Fe(III), goethite and magnetite in natural system and in remediation approaches.

Long-term degradation characteristics of cyanide in closed monofills and its effects on the environment and human health: Evidence from nine landfill sites in northen China

Cyanide residues weighing many millions of tons are disposed of in cyanide residue monofills (CRMs) worldwide. The degradation characteristics of cyanide in the anoxic environments of closed landfills may have been overestimated, leading to an underestimation of the long-term risk of cyanide residue landfills. To study the effect, a total of 387 cyanide residue samples were collected for analysis from nine closed CRMs in northen China that have been closed for more than 10 years. The study shows that the probability of achieving the target cyanide concentration (5 mg/L) in the nine sites was only 2.9%. And there is no significant reduction in the overall concentrations compared to the pre-closure period. The effectiveness of the CRM containment barrier needs to be maintained for at least 220 years to allow cyanide concentrations to degrade to harmless levels. Nine CRMs sites, except for CRMs A and B, had a low short-term risk, but in the long term exposure concentrations can exceed the groundwater Class III water quality limit by a factor of 1.64-30, posing a risk of groundwater contamination. This study reveals the risk of cyanide residue degradation in CRMs and its long-term evolution, providing theoretical support for site management and risk control.

Metabolic engineering of Oryza sativa for complete biodegradation of thiocyanate

Industrial thiocyanate (SCN^-) waste streams from gold mining and coal coking have caused serious environmental pollution worldwide. Phytoremediation is an efficient technology in treating hazardous wastes from the environment. However, the phytoremediation efficiency of thiocyanate is very low due to the fact that plants lack thiocyanate degradation enzymes. In this study, the thiocyanate hydrolase module was assembled correctly in rice seedlings and showed thiocyanate hydrolase activity. Rice seedlings engineered to express thiocyanate degrading activity were able to completely remove thiocyanate from coking wastewater. Our findings suggest that transforming the thiocyanate hydrolase module into plants is an efficient strategy for rapid phytoremediation of thiocyanate in the environment. Moreover, the rice seedlings expressing apoplastic or cytoplasmic targeted thiocyanate hydrolase module were constructed to compare the phytoremediation efficiency of secretory/intracellular recombinant thiocyanate hydrolase. The most obvious finding from this study is that the apoplastic expression system is more efficient than the cytoplasm expression system in the phytoremediation of thiocyanate. At last, this research also shows that the secreted thiocyanate hydrolase from engineered rice plants does not influence rhizosphere bacterial community composition.

Comparative effects of sodium hydrosulfide and proline on functional repair in rice chloroplast through the D1 protein and thioredoxin system under simulated thiocyanate pollution

Various environmental contaminants can find their way to enter plant cells and disturb and/or damage the essential components of PSII repair cycle in chloroplast, thereby resulting in dysfunction of chloroplast. In the current research, a microcosm hydroponic experiment was set up to evaluate the comparative effects of sodium hydrosulfide (NaHS)- and proline (Pro)-mediated functional repairing of chloroplast in rice plants under SCN^- stress. Our results displayed that when exposed to environmental realistic SCN^- concentrations (24-300 mg L^-1), foist significant (p < 0.05) gene-dose repercussion on the pathways of photosynthetic reactions and energy metabolism in rice shoots, and a downturn in the level of total soluble starch, sugar, and chlorophyll. Sodium hydrosulfide application effectively mitigated (p < 0.05) the toxic effects of SCN^- in rice seedlings by stimulating the processes of phosphorylation, dephosphorylation and new-synthesis of D1 protein in the PSII repair cycle, and increased the turnover of D1 protein to recover CO₂ assimilation. Evidently, Pro treatment mainly enhanced (p < 0.05) the expression of magnesium chelatase (MgCh) and ribulose-1,5-bisphosphate carboxylase (Rubisco) related genes under simulated SCN^- stress, suggesting that the targeted repairing site in chloroplast by Pro was different from NaHS. The outcome of the present research contributes to a better understanding at molecular level for repairing of chloroplast functional disorder by NaHS and Pro at different key nodes under SCN^- stress.

Phytoremediation of multiple persistent pollutants co-contaminated soil by HhSSB transformed plant

The high toxicity of persistent pollutants limits the phytoremediation of pollutants-contaminated soil. In this study, heterologous expressing Halorhodospira halophila single-stranded DNA binding protein gene (HhSSB) improves tolerance to 2,4,6-trinitrotoluene (TNT), 2,4,6-trichlorophenol (2,4,6-TCP), and thiocyanate (SCN^-) in A. thaliana and tall fescue (Festuca arundinacea). The HhSSB transformed Arabidopsis, and tall fescue also exhibited enhanced phytoremediation of TNT, 2,4,6-TCP, and SCN^- separately contaminated soil and co-contaminated soil compared to control plants. TNT assay was selected to explore the mechanism of how HhSSB enhances the phytoremediation of persistent pollutants. Our result indicates that HhSSB enhances the phytoremediation of TNT by enhancing the transformation of TNT in Arabidopsis. Moreover, transcriptomics and comet analysis revealed that HhSSB improves TNT tolerance through three pathways: strengthening the defense system, enhancing the ROS scavenging system, and reducing DNA damage. These results presented here would be particularly useful for further studies in the remediation of soil contaminated by organic and inorganic pollutants.

The effect of heavy metals on thiocyanate biodegradation by an autotrophic microbial consortium enriched from mine tailings

Abstract

Bioremediation systems represent an environmentally sustainable approach to degrading industrially generated thiocyanate (SCN⁻), with low energy demand and operational costs and high efficiency and substrate specificity. However, heavy metals present in mine tailings effluent may hamper process efficiency by poisoning thiocyanate-degrading microbial consortia. Here, we experimentally tested the tolerance of an autotrophic SCN⁻-degrading bacterial consortium enriched from gold mine tailings for Zn, Cu, Ni, Cr, and As. All of the selected metals inhibited SCN⁻ biodegradation to different extents, depending on concentration. At pH of 7.8 and 30 °C, complete inhibition of SCN⁻ biodegradation by Zn, Cu, Ni, and Cr occurred at 20, 5, 10, and 6 mg L⁻¹, respectively. Lower concentrations of these metals decreased the rate of SCN⁻ biodegradation, with relatively long lag times. Interestingly, the microbial consortium tolerated As even at 500 mg L⁻¹, although both the rate and extent of SCN⁻ biodegradation were affected. Potentially, the observed As tolerance could be explained by the origin of our microbial consortium in tailings derived from As-enriched gold ore (arsenopyrite). This study highlights the importance of considering metal co-contamination in bioreactor design and operation for SCN⁻ bioremediation at mine sites.

Key points

• Both the efficiency and rate of SCN⁻biodegradation were inhibited by heavy metals, to different degrees depending on type and concentration of metal.

• The autotrophic microbial consortium was capable of tolerating high concentrations of As, potential having adapted to higher As levels derived from the tailings source.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-020-10983-4.

Characterization of Mining-Related Aromatic Contaminants in Active and Abandoned Metal(loid) Tailings Ponds

This study reports on the compositional diversity of organic compounds in metal(loid)-bearing tailings samples from both active and abandoned tailings ponds. Tailings samples were qualitatively analyzed by comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-TOFMS). In addition, the priority PAHs (16), PAEs (6), and phenols (2) were quantitatively analyzed using gas chromatography-mass spectrometry (GC-MS). We attribute the presence of some of aromatic organics in studied tailings ponds to particular sources. Mineral floatation reagents are likely the major sources of small-ring aromatics in tailings ponds, and products from metallurgical processing and burning of fossil fuels in the mining area or further afield are also possible contributors and might be the main source of large-ring aromatics. We found that tailings ponds abandoned for decades can still have organics concentrations at levels of concern. Large-ring aromatics are generally more toxic than other contaminants, and these were more abundant in abandoned tailings ponds. This suggests that these large-ring organics do not readily decompose or biodegrade into less toxic byproducts, as do volatiles and many other organic compounds. Our aromatic contaminants database provides an important starting point for researchers to investigate and compare similar contaminants that might be also present in other tailings ponds and emphasizes the necessity of considering their transformations over time.

Genome-resolved metagenomics of an autotrophic thiocyanate-remediating microbial bioreactor consortium

Industrial thiocyanate (SCN^-) waste streams from gold mining and coal coking have polluted environments worldwide. Modern SCN^- bioremediation involves use of complex engineered heterotrophic microbiomes; little attention has been given to the ability of a simple environmental autotrophic microbiome to biodegrade SCN^-. Here we present results from a bioreactor experiment inoculated with SCN^- -loaded mine tailings, incubated autotrophically, and subjected to a range of environmentally relevant conditions. Genome-resolved metagenomics revealed that SCN^- hydrolase-encoding, sulphur-oxidizing autotrophic bacteria mediated SCN^- degradation. These microbes supported metabolically-dependent non-SCN^--degrading sulphur-oxidizing autotrophs and non-sulphur oxidizing heterotrophs, and "niche" microbiomes developed spatially (planktonic versus sessile) and temporally (across changing environmental parameters). Bioreactor microbiome structures changed significantly with increasing temperature, shifting from Thiobacilli to a novel SCN^- hydrolase-encoding gammaproteobacteria. Transformation of carbonyl sulphide (COS), a key intermediate in global biogeochemical sulphur cycling, was mediated by plasmid-hosted CS₂ and COS hydrolase genes associated with Thiobacillus, revealing a potential for horizontal transfer of this function. Our work shows that simple native autotrophic microbiomes from mine tailings can be employed for SCN^- bioremediation, thus improving the recycling of ore processing waters and reducing the hydrological footprint of mining.

A new family of periplasmic-binding proteins that sense arsenic oxyanions

Arsenic contamination of drinking water affects more than 140 million people worldwide. While toxic to humans, inorganic forms of arsenic (arsenite and arsenate), can be used as energy sources for microbial respiration. AioX and its orthologues (ArxX and ArrX) represent the first members of a new sub-family of periplasmic-binding proteins that serve as the first component of a signal transduction system, that’s role is to positively regulate expression of arsenic metabolism enzymes. As determined by X-ray crystallography for AioX, arsenite binding only requires subtle conformational changes in protein structure, providing insights into protein-ligand interactions. The binding pocket of all orthologues is conserved but this alone is not sufficient for oxyanion selectivity, with proteins selectively binding either arsenite or arsenate. Phylogenetic evidence, clearly demonstrates that the regulatory proteins evolved together early in prokaryotic evolution and had a separate origin from the metabolic enzymes whose expression they regulate.

Kinetics of Decomposition of Thiocyanate in Natural Aquatic Systems

Rates of thiocyanate degradation were measured in waters and sediments of marine and limnic systems under various redox conditions, oxic, anoxic (nonsulfidic, nonferruginous, nonmanganous), ferruginous, sulfidic, and manganous, for up to 200-day period at micromolar concentrations of thiocyanate. The decomposition rates in natural aquatic systems were found to be controlled by microbial processes under both oxic and anoxic conditions. The Michaelis-Menten model was applied for description of the decomposition kinetics. The decomposition rate in the sediments was found to be higher than in the water samples. Under oxic conditions, thiocyanate degradation was faster than under anaerobic conditions. In the presence of hydrogen sulfide, the decomposition rate increased compared to anoxic nonsulfidic conditions, whereas in the presence of iron(II) or manganese(II), the rate decreased. Depending on environmental conditions, half-lives of thiocyanate in sediments and water columns were in the ranges of hours to few dozens of days, and from days to years, respectively. Application of kinetic parameters presented in this research allows estimation of rates of thiocyanate cycling and its concentrations in the Archean ocean.

Thiocyanate biodegradation: harnessing microbial metabolism for mine remediation

Thiocyanate (SCN–) forms in the reaction between cyanide (CN–) and reduced sulfur species, e.g. in gold ore processing and coal-coking wastewater streams, where it is present at millimolar (mM) concentrations1. Thiocyanate is also present naturally at nM to µM concentrations in uncontaminated aquatic environments2. Although less toxic than its precursor CN–, SCN– can harm plants and animals at higher concentrations3, and thus needs to be removed from wastewater streams prior to disposal or reuse. Fortunately, SCN– can be biodegraded by microorganisms as a supply of reduced sulfur and nitrogen for energy sources, in addition to nutrients for growth4. Research into how we can best harness the ability of microbes to degrade SCN– may offer newer, more cost-effective and environmentally sustainable treatment solutions5. By studying biodegradation pathways of SCN– in laboratory and field treatment bioreactor systems, we can also gain fundamental insights into connections across the natural biogeochemical cycles of carbon, sulfur and nitrogen6.

The geomicrobiology of mining environments

As the global population increases, so does the demand for minerals and energy resources. Demand for some of the major global commodities is currently growing at rates of: copper – 1.6% p.a.1; iron ore: 1.4% p.a.2; aluminium – 5% p.a.3; rare earth elements – 7% p.a.4, driven not only by population growth in China, India, and Africa, but also by increasing urbanisation and industrialisation globally. Technological advances in renewable energy production and storage, construction materials, transport, and computing could see demand for some of these resources spike by 2600% over the next 25 years under the most extreme demand scenarios5. Coupled with declining ore grades, this demand means that the global extent of mining environments is set to increase dramatically. Land disturbance attributed to mining was estimated to be 400 000 km2 in 20076, with projected rates of increase of 10 000 km2 per year7. This will increase the worldwide extent of mining environments from around 500 000 km2 at present to 1 330 000 km2 by 2100, larger than the combined land area of New South Wales and Victoria (1 050 000 km2), making them a globally important habitat for the hardiest of microbial life. The extreme geochemical and physical conditions prevalent in mining environments present great opportunities for discovery of novel microbial species and functions, as well as exciting challenges for microbiologists to apply their understanding to solve complex remediation problems.