Heavy Metal Resistance Mechanisms Research Articles

Copper Resistance Mechanism and Copper Response Genes in Corynebacterium crenatum.

Heavy metal resistance mechanisms and heavy metal response genes are crucial for microbial utilization in heavy metal remediation. Here, Corynebacterium crenatum was proven to possess good tolerance in resistance to copper. Then, the transcriptomic responses to copper stress were investigated, and the vital pathways and genes involved in copper resistance of C. crenatum were determined. Based on transcriptome analysis results, a total of nine significantly upregulated DEGs related to metal ion transport were selected for further study. Among them, GY20_RS0100790 and GY20_RS0110535 belong to transcription factors, and GY20_RS0110270, GY20_RS0100790, and GY20_RS0110545 belong to copper-binding peptides. The two transcription factors were studied for the function of regulatory gene expression. The three copper-binding peptides were displayed on the C. crenatum surface for a copper adsorption test. Furthermore, the nine related metal ion transport genes were deleted to investigate the effect on growth in copper stress. This investigation provided the basis for utilizing C. crenatum in copper bioremediation.

Omics technology draws a comprehensive heavy metal resistance strategy in bacteria.

The rapid industrial revolution significantly increased heavy metal pollution, becoming a major global environmental concern. This pollution is considered as one of the most harmful and toxic threats to all environmental components (air, soil, water, animals, and plants until reaching to human). Therefore, scientists try to find a promising and eco-friendly technique to solve this problem i.e., bacterial bioremediation. Various heavy metal resistance mechanisms were reported. Omics technologies can significantly improve our understanding of heavy metal resistant bacteria and their communities. They are a potent tool for investigating the adaptation processes of microbes in severe conditions. These omics methods provide unique benefits for investigating metabolic alterations, microbial diversity, and mechanisms of resistance of individual strains or communities to harsh conditions. Starting with genome sequencing which provides us with complete and comprehensive insight into the resistance mechanism of heavy metal resistant bacteria. Moreover, genome sequencing facilitates the opportunities to identify specific metal resistance genes, operons, and regulatory elements in the genomes of individual bacteria, understand the genetic mechanisms and variations responsible for heavy metal resistance within and between bacterial species in addition to the transcriptome, proteome that obtain the real expressed genes. Moreover, at the community level, metagenome, meta transcriptome and meta proteome participate in understanding the microbial interactive network potentially novel metabolic pathways, enzymes and gene species can all be found using these methods. This review presents the state of the art and anticipated developments in the use of omics technologies in the investigation of microbes used for heavy metal bioremediation.

Resistance mechanisms of cereal plants and rhizosphere soil microbial communities to chromium stress.

Agricultural soils contaminated with heavy metals poison crops and disturb the normal functioning of rhizosphere microbial communities. Different crops and rhizosphere microbial communities exhibit different heavy metal resistance mechanisms. Here, indoor pot studies were used to assess the mechanisms of grain and soil rhizosphere microbial communities on chromium (Cr) stress. Millet grain variety 'Jingu 21' (Setaria italica) and soil samples were collected prior to control (CK), 6 hours after (Cr_6h), and 6 days following (Cr_6d) Cr stress. Transcriptomic analysis, high-throughput sequencing and quantitative polymerase chain reaction (qPCR) were used for sample determination and data analysis. Cr stress inhibited the expression of genes related to cell division, and photosynthesis in grain plants while stimulating the expression of genes related to DNA replication and repair, in addition to plant defense systems resist Cr stress. In response to chromium stress, rhizosphere soil bacterial and fungal community compositions and diversity changed significantly (p<0.05). Both bacterial and fungal co-occurrence networks primarily comprised positively correlated edges that would serve to increase community stability. However, bacterial community networks were larger than fungal community networks and were more tightly connected and less modular than fungal networks. The abundances of C/N functional genes exhibited increasing trends with increased Cr exposure. Overall, these results suggest that Cr stress primarily prevented cereal seedlings from completing photosynthesis, cell division, and proliferation while simultaneously triggering plant defense mechanisms to resist the toxic effects of Cr. Soil bacterial and fungal populations exhibited diverse response traits, community-assembly mechanisms, and increased expression of functional genes related to carbon and nitrogen cycling, all of which are likely related to microbial survival during Cr stress. This study provides new insights into resistance mechanisms, microbial community structures, and mechanisms of C/N functional genes responses in cereal plants to heavy metal contaminated agricultural soils. Portions of this text were previously published as part of a preprint (https://www.researchsquare.com/article/rs-2891904/v1).

The copper resistance mechanism in a newly isolated Pseudoxanthomonas spadix ZSY-33.

Resolving the heavy metal resistance mechanisms of microbes is crucial for understanding the bioremediation of the ecological environment. In this study, a multiple heavy metal resistance bacterium, Pseudoxanthomonas spadix ZSY-33 was isolated and characterized. The copper resistance mechanism was revealed by analysis of the physiological traits, copper distribution, and genomic and transcriptomic data of strain ZSY-33 cultured with different concentrations of copper. The growth inhibition assay in basic medium showed that the growth of strain ZSY-33 was inhibited in the presence of 0.5 mM copper. The production of extracellular polymeric substances increased at a lower concentration of copper and decreased at a higher concentration of copper. Integrative analysis of genomic and transcriptomic, the copper resistance mechanism in strain ZSY-33 was elucidated. At a lower concentration of copper, the Cus and Cop systems were responsible for the homeostasis of intracellular copper. As the concentration of copper increased, multiple metabolism pathways, including the metabolism of sulfur, amino acids, and pro-energy were cooperated with the Cus and Cop systems to deal with copper stress. These results indicated a flexible copper resistance mechanism in strain ZSY-33, which may acquire from the long-term interaction with the living environment.

Cloning, expression, and bioinformatics analysis of heavy metal resistance-related genes fd-I and fd-II from Acidithiobacillus ferrooxidans.

Five heavy metals were introduced into the bacterial heavy metal resistance tests. The results showed that apparent inhibition effects of Cd2+ and Cu2+ on the growth of Acidithiobacillus ferrooxidans BYSW1 occurred at high concentrations (>0.04 mol l-1). Significant differences (P<0.001) were both noticed in the expression of two ferredoxin-encoding genes (fd-I and fd-II) related to heavy metal resistance in the presence of Cd2+ and Cu2+ . When exposed to 0.06 mol l-1 Cd2+, the relative expression levels of fd-I and fd-II were about 11 and 13 times as much as those of the control, respectively. Similarly, exposure to 0.04 mol l-1 Cu2+ caused approximate 8 and 4 times higher than those of the control, respectively. These two genes were cloned and expressed in Escherichia coli, and the structures, functions of two corresponding target proteins, i.e. Ferredoxin-I (Fd-I) and Ferredoxin-II (Fd-II), were predicted. The recombinant cells inserted by fd-I or fd-II were more resistant to Cd2+ and Cu2+ compared with wild-type cells. This study was the first investigation regarding the contribution of fd-I and fd-II to enhancing heavy metal resistance of this bioleaching bacterium, and laid a foundation for further elucidation of heavy metal resistance mechanisms caused by Fd.

Investigating Phragmites australis response to copper exposure using physiologic, Fourier Transform Infrared and metabolomic approaches.

Phragmites australis (Cav.) Trin. ex Steud is a landscape plant with resistance to heavy metals that has significance in phytoremediation. However, little is known about the metabolomic background of the heavy metal resistance mechanisms of Phragmites . We studied copper stress on Phragmites and monitored physiological indicators such as malondialdehyde (MDA) and electrolyte leakage (EL). In addition, Fourier Transform Infrared (FTIR) was used to study the related chemical composition in the roots, stems, and leaves under copper stress. Furthermore, LC-MS technology was used to analyse the plants metabolic profile. Results showed that increased copper concentration in Phragmites led to the accumulation of MDA and EL. FTIR spectrum detected the presence of O-H and C=O stretching. O-H stretching was related to the presence of flavonoids, while C=O stretching reflected the presence of protein amide I. The latter was related to the change of amino acid composition. Both flavonoids and amino acids are regarded as contributors to the antioxidant of Phragmites under copper stress. Metabolomics analysis revealed that arginine and ayarin were accumulated and Phragmites leaves responded to copper stress with changes in the pool size of arginine and ayarin. It is speculated that they could improve resistance. Arginine is accumulated through two pathways: the citrulline decomposition and conversion pathway; and the circular pathway composed of ornithine, citrulline, l -argininosuccinate and arginine. Ayarin is synthesised through the quercetin methylation pathway. This study elucidates the antioxidant mechanisms for enhancing its resistance to heavy metal stress, thus improving of phytoremediation efficiency.

Revealing taxon-specific heavy metal-resistance mechanisms in denitrifying phosphorus removal sludge using genome-centric metaproteomics

BackgroundDenitrifying phosphorus removal sludge (DPRS) is widely adopted for nitrogen and phosphorus removal in wastewater treatment but faces threats from heavy metals. However, a lack of understanding of the taxon-specific heavy metal-resistance mechanisms hinders the targeted optimization of DPRS’s robustness in nutrient removal.ResultsWe obtained 403 high- or medium-quality metagenome-assembled genomes from DPRS treated by elevating cadmium, nickel, and chromium pressure. Then, the proteomic responses of individual taxa under heavy metal pressures were characterized, with an emphasis on functions involving heavy metal resistance and maintenance of nutrient metabolism. When oxygen availability was constrained by high-concentration heavy metals, comammox Nitrospira overproduced highly oxygen-affinitive hemoglobin and electron-transporting cytochrome c-like proteins, underpinning its ability to enhance oxygen acquisition and utilization. In contrast, Nitrosomonas overexpressed ammonia monooxygenase and nitrite reductase to facilitate the partial nitrification and denitrification process for maintaining nitrogen removal. Comparisons between phosphorus-accumulating organisms (PAOs) demonstrated different heavy metal-resistance mechanisms adopted by Dechloromonas and Candidatus Accumulibacter, despite their high genomic similarities. In particular, Dechloromonas outcompeted the canonical PAO Candidatus Accumulibacter in synthesizing polyphosphate, a potential public good for heavy metal detoxification. The superiority of Dechloromonas in energy utilization, radical elimination, and damaged cell component repair also contributed to its dominance under heavy metal pressures. Moreover, the enrichment analysis revealed that functions involved in extracellular polymeric substance formation, siderophore activity, and heavy metal efflux were significantly overexpressed due to the related activities of specific taxa.ConclusionsOur study demonstrates that heavy metal-resistance mechanisms within a multipartite community are highly heterogeneous between different taxa. These findings provide a fundamental understanding of how the heterogeneity of individual microorganisms contributes to the metabolic versatility and robustness of microbiomes inhabiting dynamic environments, which is vital for manipulating the adaptation of microbial assemblages under adverse environmental stimuli.CwPGueRoJa1DHw3799rWT1Video abstract

Microbial bioremediation of heavy metals

Heavy metal pollution is one of the most serious environmental problems, due to metal ions persistence, bioavailability, and toxicity. There are many conventional physical and chemical techniques traditionally used for environmental clean-up. Due to several drawbacks regarding these methods, the use of living organisms, or bioremediation, is becoming more prevalent. Biotechnological application of microorganisms is already successfully implemented and is in constant development, with many microbial strains successfully removing heavy metals. This paper provides an overview of the main heavy metal characteristics and describes the interactions with microorganisms. Key heavy metal resistance mechanisms in microorganisms are described, as well as the main principles and types of heavy metal bioremediation methods, with details on successful pilot scale bioreactor studies. Special attention should be given to indigenous bacteria isolated from the polluted environments since such species are already adapted to contamination and possess resistance mechanisms. Utilization of bacterial biofilms or consortia could be advantageous due to higher resistance and a combination of several metabolic pathways, and thus, the possibility to remove several heavy metals simultaneously. Novel technologies covered in this review, such as nanotechnology, genetic engineering, and metagenomics, are being introduced to the field of bioremediation in order to improve the process. To conclude, bioremediation is a potentially powerful solution for cleaning the environment.

Responses of Phragmites australis to copper stress: A combined analysis of plant morphology, physiology and proteomics.

Few relevant research attempts have been made to determine heavy metal resistance mechanisms of rhizomatous perennial plants. Thus, it is pertinent to investigate the physiological and biochemical changes in Phragmites australis under metal-stressed conditions to facilitate the development of strategies to enhance copper (Cu) tolerance. We measured parameters related to plant growth and development, metal translocation and physiological responses of P. australis subjected to Cu stress. In addition, the differentially expressed proteins (DEP) were evaluated using the isobaric tag for relative and absolute quantification (iTRAQ) system. A large amount of copper accumulates in the roots of P.australis, but the growth parameters were not sensitive to Cu. However, the high concentration of Cu reduced the content of chlorophyll a and chlorophyll b, and the expression of important photosynthesis proteins PsbD, PsbO and PsaA were all down-regulated, so photosynthesis was inhibited. In contrast, the content of ascorbic acid and proline both increased with the increase of copper stress. P.australis fixed a large amount of Cu in its roots, limiting the migration of Cu to other parts of the plant. Moreover, Cu stress can affect photosynthesis by inhibiting the activity of PSI, PSII and LHCII. In addition, P.australis synthesizes ascorbic acid through the D-mannose/L-galactose pathway, and synthesizes proline through the ornithine pathway. Ascorbic acid and proline can increase Cu tolerance and protect photosynthesis. These results provide a theoretical basis for understanding the tolerance and repair mechanisms of plants in response to heavy metal pollution.

Whole genome sequence analysis of Cupriavidus campinensis S14E4C, a heavy metal resistant bacterium

Cupriavidus sp. are model organisms for heavy metal(loid) resistance and aromatic compound’s degradation studies and these characteristics make them a perfect candidate for biotechnological purposes. Bacterial strain S14E4C (identified as Cupriavidus campinensis) was isolated from a playground by enrichment method in a 0.25 mM containing medium. The analysis revealed that this bacterium is able to tolerate high concentrations of heavy metal(loid)s: Cd up to 19.5 mM, Pb to 9 mM, Hg to 5.5 mM and As to 2 mM in heavy metal(loid) salt containing nutrient medium. The whole genome data and analysis of the type strain of C. campinensis CCUG:44526T have not been available so far, thus here we present the genome sequencing results of strain S14E4C of the same species. Analysis was carried out to identify possible mechanisms for the heavy metal resistance and to map the genetic data of C. campinensis. The annotation pipelines revealed that the total genome of strain S14E4C is 6,375,175 bp length with a GC content of 66.3% and contains 2 plasmids with 295,460 bp (GC content 59.9%) and 50,483 bp (GC content 63%). In total 4460 coding sequences were assigned to known functions and 1508 to hypothetical proteins. Analysis proved that strain S14E4C is having gene clusters such as czc, mer, cus, chr, ars to encode various heavy metal resistance mechanisms that play an important role to survive in extreme environments.

Multidrug-Resistant Salmonella enterica Isolated from Food Animal and Foodstuff May Also Be Less Susceptible to Heavy Metals.

Salmonella enterica is a foodborne pathogen showing increasing multidrug resistance (MDR). We characterized the antimicrobial resistance (AMR) genotype using microarrays in a panel of 105 nontyphoidal S. enterica isolated from food animals and foodstuff. Nineteen isolates were chosen on the basis of their MDR and virulence for determination of heavy metal susceptibilities and screened by polymerase chain reaction for heavy metal resistance genes. Whole-genome sequencing (WGS) was performed on three isolates carrying clinically important AMR genes and the cdtB toxin gene to detect other heavy metal resistance mechanisms, and conjugation assays were performed to evaluate transfer of AMR/toxin genes with heavy metal resistance genes. AMR genotyping results showed isolates harbored between 1 and 12 mobile AMR genes, with 58% being classified as MDR. The tested subset of isolates showed reduced susceptibility to zinc (78%), copper (68%), silver (63%), arsenic (47%), and tellurite (26%); phenotypes that could be attributed to zitB (n = 32%), pcoA/pcoD (n = 32%), tcrB (n = 16%), arsB (n = 16%), silA/silE (n = 42%), and terF (n = 26%) genes. WGS confirmed the presence of other heavy metal resistance genes such as copA, cusA, and czcD. Isolates often harbored multiple heavy metal resistance genes. Two strains (Sal25 and Sal368) were able to conjugate with Escherichia coli J53 at a relatively high frequency (∼10-4 colony-forming units per recipient). Transformants selected in the presence of copper harbored either an IncHI2 (J53/Sal25 transconjugant) or IncF (J53/Sal368 transconjugant) plasmid with decreased susceptibilities to tellurite, zinc, copper, cobalt, arsenic, lead, mercury, and silver. blaCTX-M-1 and mcr-1 genes were also transferred to one transconjugant, and tet(M) and blaTEM-1 genes to the other. This work shows the presence of a diversity of AMR genes in this zoonotic pathogen, and suggests that heavy metals may contribute to selection of clinically important ones through the food chain, such as the plasmid-mediated colistin resistance gene mcr-1.

Inorganic polyphosphates and heavy metal resistance in microorganisms.

The mechanisms of heavy metal resistance in microbial cells involve multiple pathways. They include the formation of complexes with specific proteins and other compounds, the excretion from the cells via plasma membrane transporters in case of procaryotes, and the compartmentalization of toxic ions in vacuoles, cell wall and other organelles in case of eukaryotes. The relationship between heavy metal tolerance and inorganic polyphosphate metabolism was demonstrated both in prokaryotic and eukaryotic microorganisms. Polyphosphates, being polyanions, are involved in detoxification of heavy metals through complex formation and compartmentalization. The bacteria and fungi cultivated in the presence of some heavy metal cations contain the enhanced levels of polyphosphate. In bacteria, polyphosphate sequesters heavy metals; some of metal cations stimulate an exopolyphosphatase activity, which releases phosphate from polyphosphates, and MeHPO4- ions are then transported out of the cells. In fungi, the overcoming of heavy metal stresses is associated with the accumulation of polyphosphates in cytoplasmic inclusions, vacuoles and cell wall and the formation of cation/polyphosphate complexes. The effects of knockout mutations and overexpression of the genes encoding polyphosphate-metabolizing enzymes on heavy metal resistance are discussed.

Pb2+ Effects on Growth, Lipids, and Protein and DNA Profiles of the Thermophilic Bacterium Thermus Thermophilus.

Extremophiles are organisms able to thrive in extreme environmental conditions and some of them show the ability to survive high doses of heavy metals thanks to defensive mechanisms provided by primary and secondary metabolic products, i.e., extremolytes, lipids, and extremozymes. This is why there is a growing scientific and industrial interest in the use of thermophilic bacteria in a host of tasks, from the environmental detoxification of heavy metal to industrial activities, such as bio-machining and bio-metallurgy. In this work Thermus thermophilus was challenged against increasing Pb2+ concentrations spanning from 0 to 300 ppm in order to ascertain the sensitiveness of this bacteria to the Pb environmental pollution and to give an insight on its heavy metal resistance mechanisms. Analysis of growth parameters, enzyme activities, protein profiles, and lipid membrane modifications were carried out. In addition, genotyping analysis of bacteria grown in the presence of Pb2+, using random amplified polymorphic DNA-PCR and DNA melting evaluation, were also performed. A better knowledge of the response of thermophilic bacteria to the different pollutants, as heavy metals, is necessary for optimizing their use in remediation or decontamination processes.

Streptococcus agalactiae from pregnant women: antibiotic and heavy-metal resistance mechanisms and molecular typing.

We investigated the antibiotic and heavy-metal resistance mechanisms, virulence genes and clonal relationships of macrolide- and/or lincosamide-resistant (M+/-LR) Streptococcus agalactiae (group B Streptococcus, GBS) isolates from pregnant women in La Rioja in Northern Spain, a region with a significant immigrant population. In total 375 GBS isolates were recovered during 2011. About three-quarters of isolates were from European nationals and the remainder distributed among 23 other nationalities. Seventy-five (20%) were classified as M+/-LR strains and 28 (37%) of these were resistant to ⩾3 classes of antibiotics. Capsular serotypes III (29·3%), V (21·3%) and II (12%) were the most frequent. A wide variety of antibiotic resistance genes were detected in M+/-LR strains; notably, 5·3% harboured the lsa(C) gene associated with cross-resistance, and tet(W) was identified in a single strain. We report, for the first time, the detection of cadmium and copper resistance encoded by tcrB + cadA + cadC genes in 20 M+/-LR strains, which raises the possibility of co-selection of antibiotic and heavy-metal resistance disseminated through mobile genetic elements. The M+/-LR strains were highly diverse by DNA macrorestriction profiles (65 patterns) and 16 multilocus sequence types (STs) distributed among six clonal complexes; the most frequent were ST1, ST19, and ST12, and two strains were novel (ST586 and ST601). In conclusion, a wide diversity of genetic lineages of macrolide, lincosamide and heavy-metal- resistant GBS strains was observed in an ethnically diverse maternal population.

Diversity and Distribution of Heavy Metal-Resistant Bacteria in Polluted Sediments of the Araça Bay, São Sebastião (SP), and the Relationship Between Heavy Metals and Organic Matter Concentrations.

Heavy metals influence the population size, diversity, and metabolic activity of bacteria. In turn, bacteria can develop heavy metal resistance mechanisms, and this can be used in bioremediation of contaminated areas. The purpose of the present study was to understand how heavy metals concentration influence on diversity and distribution of heavy metal-resistant bacteria in Araça Bay, São Sebastião, on the São Paulo coast of Brazil. The hypothesis is that activities that contribute for heavy metal disposal and the increase of metals concentrations in environment can influence in density, diversity, and distribution of heavy metal-resistant bacteria. Only 12% of the isolated bacteria were sensitive to all of the metals tested. We observed that the highest percentage of resistant strains were in areas closest to the São Sebastião channel, where port activity occurs and have bigger heavy metals concentrations. Bacterial isolated were most resistant to Cr, followed by Zn, Cd, and Cu. Few strains resisted to Cd levels greater than 200mgL(-1). In respect to Cr, 36% of the strains were able to grow in the presence of as much as 3200mgL(-1). Few strains were able to grow at concentrations of Zn and Cu as high as 1600mgL(-1), and none grew at the highest concentration of 3200mgL(-1). Bacillus sp. was most frequently isolated and may be the dominant genus in heavy metal-polluted areas. Staphylococcus sp., Planococcus maritimus, and Vibrio aginolyticus were also isolated, suggesting their potential in bioremediation of contaminated sites.

Genome sequencing reveals mechanisms for heavy metal resistance and polycyclic aromatic hydrocarbon degradation in Delftia lacustris strain LZ-C

Strain LZ-C, isolated from a petrochemical wastewater discharge site, was found to be resistant to heavy metals and to degrade various aromatic compounds, including naphenol, naphthalene, 2-methylnaphthalene and toluene. Data obtained from 16S rRNA gene sequencing showed that this strain was closely related to Delftia lacustris. The 5,889,360bp genome of strain LZ-C was assembled into 239 contigs and 197 scaffolds containing 5855 predicted open reading frames (ORFs). Among these predicted ORFs, 464 were different from the type strain of Delftia. The minimal inhibitory concentrations were 4mM, 30µM, 2mM and 1mM for Cr(VI), Hg(II), Cd(II) and Pb(II), respectively. Both genome sequencing and quantitative real-time PCR data revealed that genes related to Chr, Czc and Mer family genes play important roles in heavy metal resistance in strain LZ-C. In addition, the Na(+)/H(+) antiporter NhaA is important for adaptation to high salinity resistance (2.5M NaCl). The complete pathways of benzene and benzoate degradation were identified through KEGG analysis. Interestingly, strain LZ-C also degrades naphthalene but lacks the key naphthalene degradation gene NahA. Thus, we propose that strain LZ-C exhibits a novel protein with a function similar to NahA. This study is the first to reveal the mechanisms of heavy metal resistance and salinity tolerance in D. lacustris and to identify a potential 2-methylnaphthalene degradation protein in this strain. Through whole-genome sequencing analysis, strain LZ-C might be a good candidate for the bioremediation of heavy metals and polycyclic aromatic hydrocarbons.

Exogenous malic and acetic acids reduce cadmium phytotoxicity and enhance cadmium accumulation in roots of sunflower plants

There is increasing evidence showing that low molecular weight organic acids (LMWOA) are involved in heavy metal resistance mechanisms in plants. The aim of this study was to investigate the effects of exogenous malic (MA) or acetic (AA) acids on the toxicity and accumulation of cadmium (Cd) in sunflower (Helianthus annuus L.). For this purpose, plants were grown in hydroponics under controlled conditions. Single Cd stress (5 μM Cd for 14 days) induced strong phytotoxic effects, as indicated by a decrease in all growth parameters, concentration of photosynthetic pigments, and root activity, as well as a high level of hydrogen peroxide (H2O2) accumulation. Exogenous MA or AA (250 or 500 μM) applied to the Cd-containing medium enhanced the accumulation of Cd by the roots and limited Cd translocation to the shoots. Moreover, the MA or AA applied more or less reduced Cd phytotoxicity by increasing the growth parameters, photosynthetic pigment concentrations, decreasing accumulation of H2O2, and improving the root activity. Of the studied organic acids, MA was much more efficient in mitigation of Cd toxicity than AA, probably by its antioxidant effects, which were stronger than those of AA. Plant response to Cd involved decreased production of endogenous LMWOA, probably as a consequence of severe Cd toxicity. The addition of MA or AA to the medium increased endogenous accumulation of LMWOA, especially in the roots, which could be beneficial for plant metabolism. These results imply that especially MA may be involved in the processes of Cd uptake, translocation, and tolerance in plants.

Analysis of Hydrocarbon-Contaminated Groundwater Metagenomes as Revealed by High-Throughput Sequencing

The tendency for chlorinated aliphatics and aromatic hydrocarbons to accumulate in environments such as groundwater and sediments poses a serious environmental threat. In this study, the metabolic capacity of hydrocarbon (aromatics and chlorinated aliphatics)-contaminated groundwater in the KwaZulu-Natal province of South Africa has been elucidated for the first time by analysis of pyrosequencing data. The taxonomic data revealed that the metagenomes were dominated by the phylum Proteobacteria (mainly Betaproteobacteria). In addition, Flavobacteriales, Sphingobacteria, Burkholderiales, and Rhodocyclales were the predominant orders present in the individual metagenomes. These orders included microorganisms (Flavobacteria, Dechloromonas aromatica RCB, and Azoarcus) involved in the degradation of aromatic compounds and various other hydrocarbons that were present in the groundwater. Although the metabolic reconstruction of the metagenome represented composite cell networks, the information obtained was sufficient to address questions regarding the metabolic potential of the microbial communities and to correlate the data to the contamination profile of the groundwater. Genes involved in the degradation of benzene and benzoate, heavy metal-resistance mechanisms appeared to provide a survival strategy used by the microbial communities. Analysis of the pyrosequencing-derived data revealed that the metagenomes represent complex microbial communities that have adapted to the geochemical conditions of the groundwater as evidenced by the presence of key enzymes/genes conferring resistance to specific contaminants. Thus, pyrosequencing analysis of the metagenomes provided insights into the microbial activities in hydrocarbon-contaminated habitats.

Cadmium‐induced oxalate secretion from root apex is associated with cadmium exclusion and resistance in Lycopersicon esulentum

The mechanisms of heavy metal resistance in plants can be classified into internal tolerance and exclusion mechanisms, but exclusion of heavy metals with the help of organic acids secretion has not been well documented. Here we demonstrated the contribution of oxalate secretion to cadmium (Cd) exclusion and resistance in tomato. Different Cd resistance between two tomato cultivars was evaluated by relative root elongation (RRE) and Cd accumulation. Cultivar 'Micro-Tom' showed better growth and lower Cd content in roots than 'Hezuo903' at different Cd concentrations not only in short-term hydroponic experiment but also in long-term hydroponic and soil experiments, indicating that the genotypic difference in Cd resistance is related to the exclusion of Cd from roots. 'Micro-Tom' had greater ability to secrete oxalate, suggesting that oxalate secretion might contribute to Cd resistance. Cd-induced secretion of oxalate was localized to root apex at which the majority of Cd accumulated. Phenylglyoxal, an anion-channel inhibitor, effectively blocked Cd-induced oxalate secretion and aggravated Cd toxicity while exogenous oxalate supply ameliorated Cd toxicity efficiently. These results indicated that the oxalate secreted from the root apex helps to exclude Cd from entering tomato roots, thus contributes to Cd resistance in the Cd-resistant tomato cultivar.

Biomolecular Mechanisms of Microbial Mercury Resistance in the Environment

Microbes have naturally evolved to deal with heavy metal toxicity resulting in elaborate heavy metal resistance mechanisms. Bacterial mercury resistance is mediated by the mer operon. It encodes specific genes that facilitate removal of toxic mercury species from the cell. Here, we apply experimental biophysics and high performance computer simulation to investigate molecular mechanisms of bacterial resistance to mercury.