Ion Exchange Properties of the Root Cell Walls Isolated from the Halophyte Plants (Suaeda altissima L.) Grown Under Conditions of Different Salinity

Metal and metalloid resistances in plant species and genotypes/accessions are becoming increasingly better understood at the molecular and physiological level. Much of the recent focus into metal resistances has been on hyperaccumulators as these are excellent systems to study resistances due to their very abnormal metal(loid) physiology and because of their biotechnological potential. Advances into the mechanistic basis of metal(loid) resistances have been made through the investigation of metal(loid) transporters, the construction of mutants with altered metal(loid) transport and metabolism, a better understanding of the genetic basis of resistance and hyperaccumulation and investigations into the role of metal(loid) ion chelators. This review highlights these recent advances.     

Volatilisation of metals and metalloids by the microbial population of an alluvial soil

In order to assess the microbial contribution to the volatilisation of metal(loid)s by methylation and hydridisation in the environment, we focused on soils of different origin. Here, we describe the biogenic production of volatile metal(loid) species of an alluvial soil with rather low metal(loid) contamination. The production of volatile metal(loid) compounds was monitored in soil suspensions kept under anaerobic conditions over an incubation time of 3 months. In the headspace of the samples, we detected mainly hydrids and methylated derivatives of a broad variety of elements such as arsenic, antimony, bismuth, selenium, tellurium, mercury, tin and lead, with the volatile products of arsenic, antimony and selenium representing the highest portions. Classical cultivation-dependent procedures resulted in the isolation of a strictly anaerobic Gram-positive strain (ASI-1), which shows a high versatility in transforming metal(loid) ions to volatile derivatives. Strain ASI-1 is affiliated to the species Clostridium glycolicum due to its high 16S rDNA sequence similarity with members of that species. As shown by fluorescence in situ hybridisation, strain ASI-1 amounts to approximately 2% of the total microbial flora of the alluvial soil. Since the spectrum of volatile metal(loid) compounds produced by this strain is very similar to that obtained by the whole population regarding both the broad variety of metal(loid)s converted and the preference for volatilising arsenic, antimony and selenium, we suggest that this strain may represent a dominant member of the metal(loid) volatilisating population in this habitat.     

Interaction of heavy metals with the sulphur metabolism in angiosperms from an ecological point of view

The metabolism of sulphur in angiosperms is reviewed under the aspect of exposure to ecologically relevant concentrations of sulphur, heavy metals and metalloids. Because of the inconsistent use of the term 'metal tolerance', in this review the degree of tolerance to arsenic and heavy metals is divided into three categories: hypotolerance, basal tolerance and hypertolerance. The composition of nutrient solutions applied to physiological experiments let see that the well-known interactions of calcium, sulphate and zinc supply with uptake of heavy metals, especially cadmium are insufficiently considered. Expression of genes involved in reductive sulphate assimilation pathway and enzyme activities are stimulated by cadmium and partially by copper, but nearly not by other heavy metals. The synthesis of the sulphur-rich compounds glucosinolates, metallothioneins and phytochelatins is affected in a metal-specific way. Phytochelatin levels are low in all metal(loid)-hypertolerant plant species growing in the natural environment on metal(loid)-enriched soils. If laboratory experiments mimic the natural environments, especially high Zn/Cd ratios and good sulphur supply, and chemical analyses are extended to more mineral elements than the single metal(loid) under investigation, a better understanding of the impact of metal(loid)s on the sulphur metabolism can be achieved.     

Heavy Metal/Metalloid Indexing and Balances in Agricultural Soils: Methodological Approach for Research

Heavy metal(loid) accumulation in agricultural soils is a threat to the soil capacity, quality, and productivity. It also increases human exposure to heavy metal(loid)s via consumption of contaminated plant-based foods. The detrimental effects of soil contamination also deteriorate the environment of plants and animals. For sustainable agriculture, therefore, the soil must be protected from toxic levels of heavy metal(loid)s. Studies on heavy metal(loid) balances in agricultural soils are important in predicting future risks to sustainable production from agro-ecological zones and human exposure to heavy metal(loid)s. The latest and continuous indexing of the problem seems a prerequisite for sustainable agriculture. This review provides some background information and then summarizes key methodological approaches for studies on indexing and balance of heavy metal(loid)s in agricultural soils. In the end, important soil and health indices are explained that may be useful in understanding the extent of the problem. The provided information would contribute to sustainable heavy metal(loid) management in the agricultural soils, high crop production, better soil protection, and ultimately to human health.     

Metabolic potentials of members of the class Acidobacteriia in metal-contaminated soils revealed by metagenomic analysis

The relative abundance of Acidobacteriia correlated positively with the concentrations of arsenic (As), mercury (Hg), chromium (Cr), copper (Cu) and other metals, suggesting their adaptation of the metal-rich environments. Metagenomic binning reconstructed 29 high-quality metagenome-assembled genomes (MAGs) associated with Acidobacteriia, providing an opportunity to study their metabolic potentials. These MAGs contained genes to transform As, Hg and Cr through oxidation, reduction, efflux and demethylation, suggesting the potential of Acidobacteriia to transform such metal(loid)s. Additionally, genes associated with alleviation of acidic and metal stress were also detected in these MAGs. Acidobacteriia may have the capabilities to resist or transform metal(loid)s in acidic metal-contaminated sites. Moreover, these genes encoding metal transformation could be also identified in the Acidobacteriia-associated MAGs from five additional metal-contaminated sites across Southwest China, as well as Acidobacteriia-associated reference genomes from the NCBI database, suggesting that the capability of metal transformation may be widespread among Acidobacteriia members. This discovery provides an understanding of metabolic potentials of the Acidobacteriia in acidic metal-rich sites.     

Anaerobic Respiration on Tellurate and Other Metalloids in Bacteria from Hydrothermal Vent Fields in the Eastern Pacific Ocean

This paper reports the discovery of anaerobic respiration on tellurate by bacteria isolated from deep ocean (1,543 to 1,791 m) hydrothermal vent worms. The first evidence for selenite- and vanadate-respiring bacteria from deep ocean hydrothermal vents is also presented. Enumeration of the anaerobic metal(loid)-resistant microbial community associated with hydrothermal vent animals indicates that a greater proportion of the bacterial community associated with certain vent fauna resists and reduces metal(loid)s anaerobically than aerobically, suggesting that anaerobic metal(loid) respiration might be an important process in bacteria that are symbiotic with vent fauna. Isolates from Axial Volcano and Explorer Ridge were tested for their ability to reduce tellurate, selenite, metavanadate, or orthovanadate in the absence of alternate electron acceptors. In the presence of metal(loid)s, strains showed an ability to grow and produce ATP, whereas in the absence of metal(loid)s, no growth or ATP production was observed. The protonophore carbonyl cyanide m-chlorophenylhydrazone depressed metal(loid) reduction. Anaerobic tellurate respiration will be a significant component in describing biogeochemical cycling of Te at hydrothermal vents.     

Anaerobic respiration on tellurate and other metalloids in bacteria from hydrothermal vent fields in the eastern Pacific Ocean

This paper reports the discovery of anaerobic respiration on tellurate by bacteria isolated from deep ocean (1,543 to 1,791 m) hydrothermal vent worms. The first evidence for selenite- and vanadate-respiring bacteria from deep ocean hydrothermal vents is also presented. Enumeration of the anaerobic metal(loid)-resistant microbial community associated with hydrothermal vent animals indicates that a greater proportion of the bacterial community associated with certain vent fauna resists and reduces metal(loid)s anaerobically than aerobically, suggesting that anaerobic metal(loid) respiration might be an important process in bacteria that are symbiotic with vent fauna. Isolates from Axial Volcano and Explorer Ridge were tested for their ability to reduce tellurate, selenite, metavanadate, or orthovanadate in the absence of alternate electron acceptors. In the presence of metal(loid)s, strains showed an ability to grow and produce ATP, whereas in the absence of metal(loid)s, no growth or ATP production was observed. The protonophore carbonyl cyanide m-chlorophenylhydrazone depressed metal(loid) reduction. Anaerobic tellurate respiration will be a significant component in describing biogeochemical cycling of Te at hydrothermal vents.     

Accumulation of Trace Metals by Mangrove Plants in Indian Sundarban Wetland: Prospects for Phytoremediation

The work investigates on the potential of ten mangrove species for absorption, accumulation and partitioning of trace metal(loid)s in individual plant tissues (leaves, bark and root/pneumatophore) at two study sites of Indian Sundarban Wetland. The metal(loid) concentration in host sediments and their geochemical characteristics were also considered. Mangrove sediments showed unique potential in many- fold increase for most metal(loid)s than plant tissues due to their inherent physicochemical properties. The ranges of concentration of trace metal(loid)s for As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn in plant tissue were 0.006–0.31, 0.02–2.97, 0.10–4.80, 0.13–6.49, 4.46–48.30, 9.2–938.1, 0.02–0.13, 9.8–1726, 11–5.41, 0.04–7.64, 3.81–52.20 μg g ^?1respectively. The bio- concentration factor (BCF) showed its maximum value (15.5) in Excoecaria agallocha for Cd, suggesting that it can be considered as a high-efficient plant for heavy metal bioaccumulation. Among all metals, Cd and Zn were highly bioaccumulated in E. agallocha (2.97 and 52.2 μg g ^?1 respectively. Our findings suggest that the species may be classified as efficient metal trap for Cd in aerial parts, as indicated by higher metal accumulation in the leaves combined with BCF and translocation factor (TF) values.     

Sources and Ecological Risk Assessment of Heavy Metal(loid)s in Agricultural Soils of Huzhou,China

To identify sources of heavy metal(loid) (HM) contamination in agricultural soils of Huzhou, surface soil samples were sampled from 89 different agricultural regions in 2012. Concentrations of heavy metal(loid)s, along with pH, total phosphorus (TP), total nitrogen (TN), and soil organic matter (SOM), were determined. Ecological risk was then assessed using a modified Hakanson ecological risk index, and the sources of contamination were determined using principal component analysis (PCA). Mean concentrations of heavy metal(loid)s were 10.26, 23.21, 83.75, 22.81, 0.25, 61.86, 33.03, and 0.15 mg kg^?1 for As, Cu, Zn, Ni, Cd, Cr, Pb, and Hg, respectively. Cu, Zn, Ni, Cr, Cd, Hg, and Pb were correlated positively with TP and there were obvious positive correlations among Cu, Zn, Ni, Cr, and Cd. Risk index (RI) values varied from 39 to 1246 with a mean value of 137. Enrichment of Pb, Zn, Cu, and especially Cd can be attributed to excessive use of nitrogen and phosphorus fertilizers containing heavy metals, as well to surface irrigation and natural soil formation. While the ecological risk of most agricultural soils in Huzhou is low, it is recommended that the use of phosphate and nitrogen fertilizers be restricted and production technology be improved to reduce the heavy metal(loid) concentrations. Results suggest that the Chinese environmental quality standard for soil should be revised to better address heavy metal(loid) contamination.     

Effect of Glomus mosseae on accumulation efficiency,hazard index and antioxidant defense mechanisms in tomato under metal(loid) Stress

In the present study, the phytoremedation potential along with growth, physiological and biochemical response of tomato (Solanum lycopersicum) was assessed under heavy metal(loid) (HM) and arbuscular mycorrhizal fungus (AMF) amendment. Effect of AMF on uptake and accumulation of metal(loid)s was assessed and accumulation characteristics were expressed in terms of bioabsorption coefficient (BAC), bioconcentration factor (BCF), translocation factor (TLF) and transfer factor (TF). Results showed that AMF-inoculated plants showed not only a better growth, chlorophyll content, strengthened non-enzymatic and enzymatic defense mechanism, but also accumulated higher concentration of metal(loid)s. The correlation between biochemical and physiological parameters was significant at 0.01 level. A significant difference (p ≤ 0.001) in antioxidant enzyme activity was found on increasing metal(loid) dose and application of AMF. The accumulation of Cd and Pb in edible part exceeded the chronic reference dose stated by USEPA. The target hazard quotient (THQ) was >1 for Cd and Pb, whereas <1 for As. The study shows that tomato has good potential as Cd and Pb phytoremediator, hence must not be consumed when grown on Cd or Pb contaminated sites.     

Genome sequence of the highly efficient arsenite-oxidizing bacterium Achromobacter arsenitoxydans SY8

We report the draft genome sequence of Achromobacter arsenitoxydans SY8, the first reported arsenite-oxidizing bacterium belonging to the genus Achromobacter and containing a genomic arsenic island, an intact type III secretion system, and multiple metal(loid) transporters. The genome may be helpful to explore the mechanisms intertwining metal(loid) resistance and pathogenicity.     

Patterns of metal distribution in hypersaline microbialites during early diagenesis: Implications for the fossil record

The use of metals as biosignatures in the fossil stromatolite record requires understanding of the processes controlling the initial metal(loid) incorporation and diagenetic preservation in living microbialites. Here, we report the distribution of metals and the organic fraction within the lithifying microbialite of the hypersaline Big Pond Lake (Bahamas). Using synchrotron‐based X‐ray microfluorescence, confocal, and biphoton microscopies at different scales (cm–μm) in combination with traditional geochemical analyses, we show that the initial cation sorption at the surface of an active microbialite is governed by passive binding to the organic matrix, resulting in a homogeneous metal distribution. During early diagenesis, the metabolic activity in deeper microbialite layers slows down and the distribution of the metals becomes progressively heterogeneous, resulting from remobilization and concentration as metal(loid)‐enriched sulfides, which are aligned with the lamination of the microbialite. In addition, we were able to identify globules containing significant Mn, Cu, Zn, and As enrichments potentially produced through microbial activity. The similarity of the metal(loid) distributions observed in the Big Pond microbialite to those observed in the Archean stromatolites of Tumbiana provides the foundation for a conceptual model of the evolution of the metal distribution through initial growth, early diagenesis, and fossilization of a microbialite, with a potential application to the fossil record.     

Microbial formation and transformation of organometallic and organometalloid compounds

Abstract: Microbial formation and transformation of organometallic and organometalloid compounds comprise significant components of biogeochemical cycles for the metals mercury, lead and tin and the metalloids arsenic, selenium, tellurium and germanium. Methylated derivatives of such elements can arise as a result of chemical and biological mechanisms and this frequently results in altered volatility, solubility, toxicity and mobility. The major microbial methylating agents are methylcobalamin (CH₃CoB₁₂), involved in the methylation of mercury, tin and lead, and S -adenosylmethionine (SAM), involved in the methylation of arsenic and selenium. Evidence for the methylation of other toxic metal(loid)s is sparse. Biomethylation may result in metal(loid) detoxification since methylated derivatives may be excreted readily from cells, are often volatile and may be less toxic, e.g. organoarsenicals. However, for mercury, low yields of methylated derivatives and the existence of more efficient resistance mechanisms, e.g. reduction of Hg²⁺ to Hg⁰, suggest a lower significance in detoxification. Bioalkylation has only been characterised in detail for arsenic. Microorganisms can accumulate organometal(loid)s, a phenomenon relevant to toxicant transfer to higher organisms. As well as bioaccumulation, many microorganisms are capable of the degradation and detoxification of organometal(loid) compounds by, e.g. demethylation and dealkylation. Several organometal(loid) transformations have potential for environmental bioremediation.     

Metal(loid)-Contaminated Soils as a Source of Culturable Heterotrophic Aerobic Bacteria for Remediation Applications

Heavy metal–contaminated soils are a serious environmental problem. Herein, the culturable heterotrophic bacterial community present on two metal(loid)-contaminated sites in the Northern Portugal was investigated. The bacterial counts ranged from 5.96 to 7.69 and 7.04 to 7.51 (log CFU g^?1 soil) in Sites 1 and 3, respectively. The bacterial population was predominantly composed of Firmicutes, Proteobacteria, and Actinobacteria on both sites. The most represented genera in Site 1 were Bacillus (41%) and Pseudomonas (27%), whereas Arthrobacter (21%) and Pseudomonas (13%) were the most represented genera in Site 3. Several bacterial isolates showed tolerance to high concentrations of metal(loid)s, suggesting that both contaminated sites are a valuable source of metal(loid)-tolerant bacteria, which may be further used in bioremediation and/or phytoremediation processes.     

Metal(loid)s and radionuclides cytotoxicity in Saccharomyces cerevisiae. Role of YCF1, glutathione and effect of buthionine sulfoximine

The presence of heavy metal(loid)s in soils and waters is an important issue with regards to human health. Taking into account speciation problems, in the first part of this report, we investigated under identical growth conditions, yeast tolerance to a set of 15 cytotoxic metal(loid)s and radionuclides. The yeast cadmium factor 1 (YCF1) is an ATP-Binding Cassette transporter mediating the glutathione detoxification of heavy metals. In the second part, metal(loid)s that could be handled by YCF1 and a possible re-localisation of the transporter after heavy metal exposure were evaluated. YCF1 and a C-terminal GFP fusion, YCF1-GFP, were overexpressed in wild-type and Deltaycf1 strains. Both forms were functional, conferring a tolerance to Cd, Sb, As, Pb, Hg but not to Ni, Zn, Cu, Ag, Se, Te, Cr, Sr, Tc, U. Confocal experiments demonstrated that during exposure to cytotoxic metals, the localisation of YCF1-GFP was restricted to the yeast vacuolar membrane. In the last part, the role of glutathione in this resistance mechanism to metal(loid)s was studied. In the presence of heavy metals, application of buthionine sulfoximine (BSO), a well-known inhibitor of gamma-glutamylcysteine synthetase, led to a decrease in the cytosolic pool of GSH and to a limitation of yeast growth. Surprisingly, BSO was able to phenocopy the deletion of gamma-glutamylcysteine synthetase after exposure to Cd but not to Sb or As. In the genetic context of gsh1 and gsh2 yeast mutants, the critical role of GSH for Cd, As, Sb and Hg tolerance was compared to that of wild-type and Deltaycf1.     

Health risk perspectives of metal(loid) exposure via consumption of striped venus clam (Chamelea gallina Linnaeus, 1758)

The metal(loid)s are mainly transported to the Black Sea via rivers and affect the coastal area where fisheries are intensive. Chamelea gallina (striped venus clam) is the most abundant mollusk species in the Black Sea and used as a bioindicator for monitoring of metal(loid) pollution. In this respect, the concentrations of As, Cd, Cr, Cu, Pb, and Zn in C. gallina collected from 16 different locations along the Black Sea coast (Turkey, Bulgaria, and Russia) were determined and evaluated for the potential human health risk. The results obtained were found to be below the maximum permissible limits indicated in the food safety guidelines. Similarly, the analyzed metal(loid)s did not pose a potential hazard to humans for consumption of the C. gallina, regarding provisional tolerable weekly intake described by the Joint FAO/WHO Expert Committee on Food Additives (JECFA). Moreover, target hazard quotient and hazard index were found to be lower than 1 for not only average consumer but also the consumer who consumed a portion (160 g) once a week. In contrast, consumption of C. gallina could cause health risks for Cd and As in case of increasing portion sizes.     

Nitric oxide-mediated regulation of oxidative stress in plants under metal stress: a review on molecular and biochemical aspects

Given their sessile nature, plants continuously face unfavorable conditions throughout their life cycle, including water scarcity, extreme temperatures and soil pollution. Among all, metal(loid)s are one of the main classes of contaminants worldwide, posing a serious threat to plant growth and development. When in excess, metals which include both essential and non-essential elements, quickly become phytotoxic, inducing the occurrence of oxidative stress. In this way, in order to ensure food production and safety, attempts to enhance plant tolerance to metal(loid)s are urgently needed. Nitric oxide (NO) is recognized as a signaling molecule, highly involved in multiple physiological events, like the response of plants to abiotic stress. Thus, substantial efforts have been made to assess NO potential in alleviating metal-induced oxidative stress in plants. In this review, an updated overview of NO-mediated protection against metal toxicity is provided. After carefully reviewing NO biosynthetic pathways, focus was given to the interaction between NO and the redox homeostasis followed by photosynthetic performance of plants under metal excess.     

Biological characterization of Bacillus flexus strain SSAI1 transforming highly toxic arsenite to less toxic arsenate mediated by periplasmic arsenite oxidase enzyme encoded by aioAB genes

Bacillus flexus strain SSAI1 isolated from agro-industry waste, Tuem, Goa, India displayed high arsenite resistance as minimal inhibitory concentration was 25 mM in mineral salts medium. This bacterial strain exposed to 10 mM arsenite demonstrated rapid arsenite oxidation and internalization of 7 mM arsenate within 24 h. The Fourier transformed infrared (FTIR) spectroscopy of cells exposed to arsenite revealed important functional groups on the cell surface interacting with arsenite. Furthermore, scanning electron microscopy combined with electron dispersive X-ray spectroscopy (SEM-EDAX) of cells exposed to arsenite revealed clumping of cells with no surface adsorption of arsenite. Transmission electron microscopy coupled with electron dispersive X-ray spectroscopic (TEM-EDAX) analysis of arsenite exposed cells clearly demonstrated ultra-structural changes and intracellular accumulation of arsenic. Whole-genome sequence analysis of this bacterial strain interestingly revealed the presence of large number of metal(loid) resistance genes, including aioAB genes encoding arsenite oxidase responsible for the oxidation of highly toxic arsenite to less toxic arsenate. Enzyme assay further confirmed that arsenite oxidase is a periplasmic enzyme. The genome of strain SSAI1 also carried glpF, aioS and aioE genes conferring resistance to arsenite. Therefore, multi-metal(loid) resistant arsenite oxidizing Bacillus flexus strain SSAI1 has potential to bioremediate arsenite contaminated environmental sites and is the first report of its kind.