Gold phytomining. A review of the relevance of this technology to mineral extraction in the 21st century

Controlling In Planta Gold Nanoparticle Synthesis and Size for Catalysis

Gold nanoparticles (Au-NPs) are used as catalysts for a diverse range of industrial applications. Currently, Au-NPs are synthesized chemically, but studies have shown that plants fed Au deposit, this element naturally as NPs within their tissues. The resulting plant material can be used to make biomass-derived catalysts. In vitro studies have shown that the addition of specific, short (∼10 amino acid) peptide/s to solutions can be used to control the NP size and shape, factors that can be used to optimize catalysts for different processes. Introducing these peptides into the model plant species, Arabidopsis thaliana (Arabidopsis), allows us to regulate the diameter of nanoparticles within the plant itself, consequently influencing the catalytic performance in the resulting pyrolyzed biomass. Furthermore, we show that overexpressing the copper and gold COPPER TRANSPORTER 2 (COPT2) in Arabidopsis increases the uptake of these metals. Adding value to the Au-rich biomass offers the potential to make plant-based remediation and stabilization of mine wastes financially feasible. Thus, this study represents a significant step toward engineering plants for the sustainable recovery of finite and valuable elements from our environment.

Biochemical, Catabolic, and PGP Activity of Microbial Communities and Bacterial Strains from the Root Zone of Baccharis linearis in a Mediterranean Mine Tailing

The management of mine tailings (MT) is commonly workload heavy, intrusive, and expensive. Phytostabilization offers a promising approach for MT management; however, it poses challenges due to the unfavorable physicochemical properties of these wastes. Nevertheless, native microorganisms capable of supporting plant growth and development could enhance the efficacy of phytostabilization. This study assesses the biological activity of microbial communities from the root zone of Baccharis linearis, which is naturally present in MT, in order to evaluate their biotechnological potential for phytostabilization. The root zone and bulk samples were collected from B. linearis plants located within a MT in the Mediterranean zone of Chile. Enzyme activities related to the cycling of C, N, and P were assessed. The community-level physiological profile was evaluated using the MicroRespTM system. Bacterial plant growth-promoting (PGP) traits and colony forming units (CFU) were evaluated through qualitative and microbiological methods, respectively. CFU, enzyme activities, and CLPP were higher in the root zone compared with the bulk samples. Five bacterial strains from the root zone exhibited PGP traits such as P solubilization and N acquisition, among others. The presence of microbial communities in the root zone of B. linearis with PGP traits suggests their potential to enhance the ecological management of MT through phytostabilization programs.

Phytoremediation as an Effective Remedy for Removing Trace Elements from Ecosystems

The pollution of soil by trace elements is a global problem. Conventional methods of soil remediation are often inapplicable, so it is necessary to search intensively for innovative and environment-friendly techniques for cleaning up ecosystems, such as phytoremediation. Basic research methods, their strengths and weaknesses, and the effects of microorganisms on metallophytes and plant endophytes resistant to trace elements (TEs) were summarised and described in this manuscript. Prospectively, bio-combined phytoremediation with microorganisms appears to be an ideal, economically viable and environmentally sound solution. The novelty of the work is the description of the potential of "green roofs" to contribute to the capture and accumulation of many metal-bearing and suspended dust and other toxic compounds resulting from anthropopressure. Attention was drawn to the great potential of using phytoremediation on less contaminated soils located along traffic routes and urban parks and green spaces. It also focused on the supportive treatments for phytoremediation using genetic engineering, sorbents, phytohormones, microbiota, microalgae or nanoparticles and highlighted the important role of energy crops in phytoremediation. Perceptions of phytoremediation on different continents are also presented, and new international perspectives are presented. Further development of phytoremediation requires much more funding and increased interdisciplinary research in this direction.

Shoot-root signal circuit: Phytoremediation of heavy metal contaminated soil

High concentrations of heavy metals in the environment will cause serious harm to ecosystems and human health. It is urgent to develop effective methods to control soil heavy metal pollution. Phytoremediation has advantages and potential for soil heavy metal pollution control. However, the current hyperaccumulators have the disadvantages of poor environmental adaptability, single enrichment species and small biomass. Based on the concept of modularity, synthetic biology makes it possible to design a wide range of organisms. In this paper, a comprehensive strategy of "microbial biosensor detection - phytoremediation - heavy metal recovery" for soil heavy metal pollution control was proposed, and the required steps were modified by using synthetic biology methods. This paper summarizes the new experimental methods that promote the discovery of synthetic biological elements and the construction of circuits, and combs the methods of producing transgenic plants to facilitate the transformation of constructed synthetic biological vectors. Finally, the problems that should be paid more attention to in the remediation of soil heavy metal pollution based on synthetic biology were discussed.

Phytomining of noble metals - A review

Phytomining of noble metals (NMs) offers a promising possibility of metal extraction at sites where traditional mining activities or recovering NMs from low-grade minerals are not competitive. In addition to conventional mining, producing NMs from secondary resources strengthening the circular economy has been paid worldwide attention. The review presented in this paper links three scientific areas as the essential elements to form the phytomining chain of NMs. The accumulation of NMs in plants is the first step, referred as the phytoextraction process. This is followed by heightening the concentration of NMs via the enrichment stage. Eventually, although less well understood, extraction methods of NMs from biomass solid remains as well as from diverse secondary sources particularly incineration ashes are discussed that assist to visualize the potential pathways in phytomining.

Growth-Promoting Gold Nanoparticles Decrease Stress Responses in Arabidopsis Seedlings

The global economic success of man-made nanoscale materials has led to a higher production rate and diversification of emission sources in the environment. For these reasons, novel nanosafety approaches to assess the environmental impact of engineered nanomaterials are required. While studying the potential toxicity of metal nanoparticles (NPs), we realized that gold nanoparticles (AuNPs) have a growth-promoting rather than a stress-inducing effect. In this study we established stable short- and long-term exposition systems for testing plant responses to NPs. Exposure of plants to moderate concentrations of AuNPs resulted in enhanced growth of the plants with longer primary roots, more and longer lateral roots and increased rosette diameter, and reduced oxidative stress responses elicited by the immune-stimulatory PAMP flg22. Our data did not reveal any detrimental effects of AuNPs on plants but clearly showed positive effects on growth, presumably by their protective influence on oxidative stress responses. Differential transcriptomics and proteomics analyses revealed that oxidative stress responses are downregulated whereas growth-promoting genes/proteins are upregulated. These omics datasets after AuNP exposure can now be exploited to study the underlying molecular mechanisms of AuNP-induced growth-promotion.

Visualizing Hotspots and Future Trends in Phytomining Research Through Scientometrics

Phytomining has attracted widespread attention as a technique for harvesting “bio-ore.” This technology has potential applications in the metal and minerals industry for low-grade metal and mineral mining as well as metal recycling from polluted soil. The hotspots and future trends of this technology deserve in-depth exploration. This paper presents a systematic review of the phytomining research area through the scientometrics method based on the citation data collected from the Web of Science Core Collection (WoSCC). The results show that the earliest phytomining-related research was published in 1997. Between 1997 and 2019, 232 publications were published in 109 journals. Plant and Soil, the International Journal of Phytoremediation, and the Journal of Geochemical Exploration were the top three most prolific journals and accounted for 18.1% of these publications. Guillaume Echevarria, J.L. Morel, and Antony Van der Ent were the top three most prolific authors, and their work accounted for 40.1% of these publications. The cluster results of document co-citation analysis revealed that the hotspots in phytomining research area mainly includes “nickel accumulation,” “heavy metal uptake,” “mining site,” “heavy metal,” “hyperaccumulation yield,” “growth effect,” and “alternative method.” Keyword burst detection results find that the hot topics have changed over time from “phytomining” to “agromining”; from “contaminated soil” to “serpentine soil”; and from “mechanism” to “phytomining process” and “commercial phytoextraction.” This study describes the intellectual landscape of research and provides future research directions for phytomining research so that researchers can identify future research topics and partners.

Localization of mercury and gold in cassava (Manihot esculenta Crantz)

The potential of cassava (Manihot esculenta Crantz.) for simultaneous Hg and Au phytoextraction was explored by investigating Hg and Au localization in cassava roots through Micro-Proton Induced X-Ray Emission, High-Resolution Transmission Electron Microscopy (HR-TEM) and X-Ray Diffractometry (XRD). The effect of Hg and Au in the cyanogenic glucoside linamarin distribution was also investigated using Matrix Assisted Laser Desorption Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (MALDI-FT-ICR-MS) imaging. Hg was located mainly in the root vascular bundle of plants grown in 50 or 100 μmol L^-1 Hg solutions. Au was localized in the epidermis and cortex or in the epidermis and endodermis for 50 and 100 μmol L^-1 Au solutions, respectively. For 50 μmol L^-1 solutions of both Hg and Au, the two metals were co-localized in the epidermis. When the Hg concentrations were increased to 100 μmol L^-1, Au was still localized to a considerable extent in the epidermis while Hg was located in all root parts. HR-TEM and XRD revealed that Au nanoparticles were formed in cassava roots. MALDI-FT-ICR-MS imaging showed linamarin distribution in the roots of control and plants and metal-exposed plants thus suggesting that linamarin might be involved in Hg and Au uptake and distribution.

Microbe-mediated sustainable bio-recovery of gold from low-grade precious solid waste: A microbiological overview

In an era of electronics, recovering the precious metal such as gold from ever increasing piles of electronic-wastes and metal-ion infested soil has become one of the prime concerns for researchers worldwide. Biological mining is an attractive, economical and non-hazardous to recover gold from the low-grade auriferous ore containing waste or soil. This review represents the recent major biological gold retrieval methods used to bio-mine gold. The biomining methods discussed in this review include, bioleaching, bio-oxidation, bio-precipitation, bio-flotation, bio-flocculation, bio-sorption, bio-reduction, bio-electrometallurgical technologies and bioaccumulation. The mechanism of gold biorecovery by microbes is explained in detail to explore its intracellular mechanistic, which help it withstand high concentrations of gold without causing any fatal consequences. Major challenges and future opportunities associated with each method and how they will dictate the fate of gold bio-metallurgy from metal wastes or metal infested soil bioremediation in the coming future are also discussed. With the help of concurrent advancements in high-throughput technologies, the gold bio-exploratory methods will speed up our ways to ensure maximum gold retrieval out of such low-grade ores containing sources, while keeping the gold mining clean and more sustainable.

Antagonic and plant growth-promoting effects of bacteria isolated from mine tailings at El Fraile, Mexico

Mine tailings contain high concentrations of heavy metals such as As, Pb, Cu, Mn, and Fe, which are detrimental to the health of humans and the environment. In tailings at the El Fraile mine in Guerrero, Mexico, some plant species are apparently tolerant of heavy metals and can be found growing in the tailings. These plants could be associating with heavy metal-tolerant bacteria that promote plant growth and improve biomass production, and these bacteria could be a useful alternative for bacteria-assisted phytoremediation. The objective of this study was to isolate bacteria detected in the mine tailings at El Fraile-Taxco, focusing on those in the soil from the rhizosphere, the inner tissue of the root, leachate, and water, which have the potential to promote plant growth. The ability of the isolated bacteria to promote plant growth was evaluated in vitro. Of the 151 morphotypes isolated, 51% fix nitrogen, 12% dissolve phosphates, and 12%, 39.7%, and 48.3% produce indole acetic acid, gibberellins, and siderophores, respectively. In addition, 66.7% were observed to produce lytic enzymes, such as proteases, celluloses, lipases, esterases, and amylases, which exhibited activity against Fusarium, Aspergillus, and Colletotrichum. The use of 16S rRNA analysis led to the identification of the bacterial genera Chryseobacterium, Bacillus, Pseudomonas, Mycobacterium, Staphylococcus, Curtobacterium, Enterobacter, Agrobacterium, Ochrobactrum, Serratia, Stenotrophomonas, and Acinetobacter. The bacteria isolated from the rhizosphere exhibited the greatest ability to fix nitrogen and produced indole acetic acid, gibberellins, siderophore, and lytic enzymes. In addition, the isolates collected from the soil samples demonstrated ability to solubilize phosphate.

A review on global metal accumulators-mechanism, enhancement, commercial application, and research trend

The biosphere is polluted with metals due to burning of fossil fuels, pesticides, fertilizers, and mining. The metals interfere with soil conservations such as contaminating aqueous waste streams and groundwater, and the evidence of this has been recorded since 1900. Heavy metals also impact human health; therefore, the emancipation of the environment from these environmental pollutants is critical. Traditionally, techniques to remove these metals include soil washing, removal, and excavation. Metal-accumulating plants could be utilized to remove these metal pollutants which would be an alternative option that would simultaneously benefit commercially and at the same time clean the environment from these pollutants. Commercial application of pollutant metals includes biofortification, phytomining, phytoremediation, and intercropping. This review discusses about the metal-accumulating plants, mechanism of metal accumulation, enhancement of metal accumulation, potential commercial applications, research trends, and research progress to enhance the metal accumulation, benefits, and limitations of metal accumulators. The review identified that the metal accumulator plants only survive in low or medium polluted environments with heavy metals. Also, more research is required about metal accumulators in terms of genetics, breeding potential, agronomics, and the disease spectrum. Moreover, metal accumulators' ability to uptake metals need to be optimized by enhancing metal transportation, transformation, tolerance to toxicity, and volatilization in the plant. This review would benefit the industries and environment management authorities as it provides up-to-date research information about the metal accumulators, limitation of the technology, and what could be done to improve the metal enhancement in the future.

Uptake and Fractionation of Thallium by Brassica juncea in a Geogenic Thallium-Amended Substrate

This study shows thallium (Tl) concentrations in Brassica juncea (Indian mustard) tissue are more than an order of magnitude higher (3830 μg/kg) than that of the substrate (100 μg/kg) and are strongly influenced by the underlying mineralogy; i.e., Tl bioaccessibility depends on the mineral structure: K-feldspar > Mn nodule > hendricksite mica. The majority of Tl for all substrates is contained in edible parts of the plant, i.e., leaves (41% of total Tl, on average) ≥ flower stems (34%) > seed pods (11%) ≈ stems (10%) > flowers (3%). We also show that Tl isotope fractionation induced by B. juncea is substantial, at nearly 10 ε²⁰⁵Tl units, and generates systematic plant-specific patterns. Progressive plant growth strongly fractionates Tl isotopes, discriminating against ²⁰⁵Tl as the plant matures. Thus, ²⁰⁵Tl values are systematically higher in the early formed stem (ε²⁰⁵Tl_avg = +2.5) than in plant elements formed later (ε²⁰⁵Tl_avg = -2.5 to +0.1), which demonstrates the large degree of translocation and the associated effects during plant growth. This study establishes the potential of Tl isotopes as a new tool for understanding heavy metal (re)distribution during anthropogenic and geologic processes and the utility of such information in environmental and health-related planning and in phytomining or bioprospecting.

Induced accumulation of Au, Ag and Cu in Brassica napus grown in a mine tailings with the inoculation of Aspergillus niger and the application of two chemical compounds

This study evaluated the ability of Brassica napus for extracting gold (Au), silver (Ag) and copper (Cu) from a mine tailings, with the inoculation of two Aspergillus niger strains, and the application of ammonium thiocyanate (NH₄SCN) or ammonium thiosulfate [(NH₄)₂S₂O₃]. After seven weeks of growth inoculated or non-inoculated plants were applied with 1 or 2 g kg^-1 of either NH₄SCN or (NH₄)₂S₂O₃, respectively. Eight days after the application of the chemical compounds, plants were harvested for determining the total dry biomass, and the content of Au, Ag, and Cu in plant organs. Application of (NH₄)₂S₂O₃ or NH₄SCN resulted in enhanced Au-accumulation in stems (447% and 507%, respectively), while either (NH₄)₂S₂O₃+Aspergillus, or NH₄SCN increased the Au-accumulation in roots (198.5% and 404%, respectively) when compared to the control. Treatments with (NH₄)₂S₂O₃ or (NH₄)₂S₂O₃+Aspergillus significantly increased (P ≤ 0.001) the accumulation of Ag in leaves (677% and 1376%, respectively), while NH₄SCN + Aspergillus, and (NH₄)₂S₂O₃ enhanced the accumulation in stems (7153% and 6717.5%). The Ag-accumulation in roots was stimulated by NH₄SCN+ Aspergillus, and (NH₄)₂S₂O₃+ Aspergillus (132.5% and 178%, respectively), when compared to the control. The combination of NH₄SCN+Aspergillus significantly enhanced the Cu-accumulation in leaves (228%); whereas NH₄SCN+ Aspergillus, or (NH₄)₂S₂O₃+ Aspergillus resulted in greater accumulation of Cu in stems (1233.5% and 1580%, respectively) than the control. Results suggest that either NH₄SCN or (NH₄)₂S₂O₃ (with or without Aspergillus) improved the accumulation of Au and Ag by B. napus. Accumulation of Au and Ag in plant organs overpassed the hyperaccumulation criterion (> 1 mg kg^-1 of plant biomass); whereas Cu-accumulation in stems and roots also overpassed such criterion (> 1000 mg kg^-1) by applying either NH₄SCN or (NH₄)₂S₂O₃ + A. niger.

Monitoring of Au(iii) species in plants using a selective fluorescent probe

A colorimetric and ratiometric probe with a push–pull chromophore dicyanoisophorone system, AuP, has been developed for the detection of Au(iii) species with highly sensitive and selective response to real-water samples and living tissues of Arabidopsis thaliana.

COPT2, a plasma membrane located copper transporter, is involved in the uptake of Au in Arabidopsis

The mechanism of gold nanoparticle formation and genes involved in such processes, especially Au transport in plants are not understood. Previous reports pointed to the probable role of COPT2 in Au transport based on the transcript accumulation of COPT2 under Au exposure. Here, we provide evidence revealing the additional role of COPT2 for Au mobilization in yeast and Arabidopsis. The COPT2 transcripts significantly accumulated in the root of Arabidopsis under Au exposure. The expression of COPT2 restores Cu uptake ability in ctr1Δctr3Δ mutants and leads to Au sensitivity in yeast, which is comparable to Cu in growth kinetics experiments. The metal measurement data showed that the Au level was increased in COPT2, expressing yeast cells compared to vector transformed control. The copt2 mutant of Arabidopsis displayed a similar growth pattern to that of Col-0 under Au treatment. However, a notable phenotypic difference was noticed in three-week-old plants treated with and without Au. Consistent with yeast, Au uptake was reduced in the copt2 mutant of Arabidopsis. Together, these results clearly reveal the Au uptake capability of COPT2 in yeast and Arabidopsis. This is the first report showing the potential role of any transporter towards uptake and accumulation of Au in plants.

Toward Financially Viable Phytoextraction and Production of Plant-Based Palladium Catalysts

Although a promising technique, phytoextraction has yet to see significant commercialization. Major limitations include metal uptake rates and subsequent processing costs. However, it has been shown that liquid-culture-grown Arabidopsis can take up and store palladium as nanoparticles. The processed plant biomass has catalytic activity comparable to that of commercially available catalysts, creating a product of higher value than extracted bulk metal. We demonstrate that the minimum level of palladium in Arabidopsis dried tissues for catalytic activity comparable to commercially available 3% palladium-on-carbon catalysts was achieved from dried plant biomass containing between 12 and 18 g·kg^-1 Pd. To advance this technology, species suitable for in-the-field application: mustard, miscanthus, and 16 willow species and cultivars, were tested. These species were able to grow, and take up, palladium from both synthetic and mine-sourced tailings. Although levels of palladium accumulation in field-suitable species are below that required for commercially available 3% palladium-on-carbon catalysts, this study both sets the target, and is a step toward, the development of field-suitable species that concentrate catalytically active levels of palladium. Life cycle assessment on the phytomining approaches described here indicates that the use of plants to accumulate palladium for industrial applications has the potential to decrease the overall environmental impacts associated with extracting palladium using present-day mining processes.

Waste or substrate for metal hyperaccumulating plants - The potential of phytomining on waste incineration bottom ash

Phytomining could represent an innovative low-cost technology for the selective recovery of valuable trace elements from secondary resources. In this context the potential of phytomining from waste incineration bottom ash was tested in a pot experiment. Fresh bottom ash was acidified, leached to reduce salinity and amended with organic material to obtain a suitable substrate for plant growth. Two hyperaccumulator species, Alyssum serpyllifolium subsp. lusitanicum and Sedum plumbizincicola as well as three metal tolerant species, Brassica napus, B. juncea and Nicotiana tabacum were tested for their phytomining potential on the pre-treated and amended bottom ashes from municipal solid waste and hazardous waste incineration. The hyperaccumulators had severe difficulties to establish on the bottom ash and to produce sufficient biomass, likely due to salinity and Cu toxicity. Nevertheless, concentrations of Ni in A. serpyllifolium and Zn in S. plumbizincicola were high, but total metal removal was limited by the low biomass production and was clearly less than on metalliferous soils. The Brassica species proved to be more tolerant to salinity and high Cu concentrations and produced considerably higher biomass, but total metal removal was limited by rather low shoot concentrations. The observed limitations of the phytomining process along with currently low market prices of Ni and Zn suggest that further optimisation of the process is required in order to make phytomining economically feasible on the tested waste incineration bottom ashes.

Engineered Gold Nanoparticles and Plant Adaptation Potential

Use of metal nanoparticles in biological system has recently been recognised although little is known about their possible effects on plant growth and development. Nanoparticles accumulation, translocation, growth response and stress modulation in plant system is not well understood. Plants exposed to gold and gold nanoparticles have been demonstrated to exhibit both positive and negative effects. Their growth and yield vary from species to species. Cytoxicity of engineered gold nanoparticles depends on the concentration, particle size and shape. They exhibit increase in vegetative growth and yield of fruit/seed at lower concentration and decrease them at higher concentration. Studies have shown that the gold nanoparticles exposure has improved free radical scavenging potential and antioxidant enzymatic activities and alter micro RNAs expression that regulate different morphological, physiological and metabolic processes in plants. These modulations lead to improved plant growth and yields. Prior to the use of gold nanoparticles, it has been suggested that its cost may be calculated to see if it is economically feasible.

Arbuscular mycorrhizal fungi in phytoremediation of contaminated areas by trace elements: mechanisms and major benefits of their applications

In recent decades, the concentration of trace elements has increased in soil and water, mainly by industrialization and urbanization. Recovery of contaminated areas is generally complex. In that respect, microorganisms can be of vital importance by making significant contributions towards the establishment of plants and the stabilization of impacted areas. Among the available strategies for environmental recovery, bioremediation and phytoremediation outstand. Arbuscular mycorrhizal fungi (AMF) are considered the most important type of mycorrhizae for phytoremediation. AMF have broad occurrence in contaminated soils, and evidences suggest they improve plant tolerance to excess of certain trace elements. In this review, the use of AMF in phytoremediation and mechanisms involved in their trace element tolerance are discussed. Additionally, we present some techniques used to study the retention of trace elements by AMF, as well as a summary of studies showing major benefits of AMF for phytoremediation.

Mining in New Caledonia: environmental stakes and restoration opportunities

New Caledonia is a widely recognised marine and terrestrial biodiversity hot spot. However, this unique environment is under increasing anthropogenic pressure. Major threats are related to land cover change and include fire, urban sprawling and mining. Resulting habitat loss and fragmentation end up in serious erosion of the local biodiversity. Mining is of particular concern due to its economic significance for the island. Open cast mines were exploited there since 1873, and scraping out soil to access ores wipes out flora. Resulting perturbations on water flows and dramatic soil erosion lead to metal-rich sediment transport downstream into rivers and the lagoon. Conflicting environmental and economic aspects of mining are discussed in this paper. However, mining practices are also improving, and where impacts are inescapable ecological restoration is now considered. Past and ongoing experiences in the restoration of New Caledonian terrestrial ecosystems are presented and discussed here. Economic use of the local floristic diversity could also promote conservation and restoration, while providing alternative incomes. In this regard, Ecocatalysis, an innovative approach to make use of metal hyperaccumulating plants, is of particular interest.

Investigating the toxicity, uptake, nanoparticle formation and genetic response of plants to gold

We have studied the physiological and genetic responses of Arabidopsis thaliana L. (Arabidopsis) to gold. The root lengths of Arabidopsis seedlings grown on nutrient agar plates containing 100 mg/L gold were reduced by 75%. Oxidized gold was subsequently found in roots and shoots of these plants, but gold nanoparticles (reduced gold) were only observed in the root tissues. We used a microarray-based study to monitor the expression of candidate genes involved in metal uptake and transport in Arabidopsis upon gold exposure. There was up-regulation of genes involved in plant stress response such as glutathione transferases, cytochromes P450, glucosyl transferases and peroxidases. In parallel, our data show the significant down-regulation of a discreet number of genes encoding proteins involved in the transport of copper, cadmium, iron and nickel ions, along with aquaporins, which bind to gold. We used Medicago sativa L. (alfalfa) to study nanoparticle uptake from hydroponic culture using ionic gold as a non-nanoparticle control and concluded that nanoparticles between 5 and 100 nm in diameter are not directly accumulated by plants. Gold nanoparticles were only observed in plants exposed to ionic gold in solution. Together, we believe our results imply that gold is taken up by the plant predominantly as an ionic form, and that plants respond to gold exposure by up-regulating genes for plant stress and down-regulating specific metal transporters to reduce gold uptake.

Hyperaccumulation by Plants

Society has long recognised that certain plant species can infer the existence of minerals in the underlying soil but only in the later years of the 20th century was the ability of some plants to accumulate heavy metals reliably quantified. The term hyperaccumulation was introduced to describe such plants. Of the many metals that can be hyperaccumulated, two of the more interesting are nickel and gold. Nickel is naturally hyperaccumulated by around 450 plant species, while plants can be forced to accumulate gold if the metal is made soluble in the soil (induced hyperaccumulation). The phytoextraction of metal from low‐grade ore, waste rock or contaminated soil represents a remediation technique, or in some cases an economically viable option for metal recovery. In some scenarios, the recovery of gold from a crop of plants can provide revenue for the remediation of more toxic metals from contaminated soil. In another, nickel farming may be an alternative livelihood for communities growing food on poor‐yielding metalliferous soils. As society progresses through the 21st century, the sustainability of the metals extractive industry must be improved. Hyperaccumulation will never replace conventional mining but it is a physiological trait that can be used for sustainable development. There are opportunities for ‘green technologies’ to support phytoextraction by better biomass processing and the realisation of specific applications for metal accumulated by plants.