Bioremedial potential of microbial mechanisms of metal mobilization and immobilization

Repurposing and reusing aquaculture wastes through a biosecure microfloc technology

Intensive aquaculture waste management is a significant challenge in the aquaculture industry, often contributing to environmental issues. Intensive aquaculture techniques demand new strategies and alternatives aimed at achieving sustainability. Repurposing and reusing wastes through innovative technologies can mitigate their negative impact. Biofloc technology (BFT) or bio-colloidal technology is based on the concept of aquaculture waste utilization by heterotrophic microbial biomass and presents a biosecure and sustainable solution. The dynamics of BFT are shaped by ecological interactions like commensalism, competition, and predation, forming a trophic micro-network consisting of rotifers, ciliates, heterotrophic bacteria, and microalgae. Metagenomic studies showed dominance of microbial communities within the biofloc, such as Cyanobacteria, Nitrosomonas, Proteobacteria, Bacteroidetes, Pseudomonadota, Rhodobacteraceae and Bacillus species that play a crucial role in the mineralization and bioremediation of waste. These microbes also help to break down hazardous toxic compounds into non-toxic, beneficial nutrients, which are subsequently utilized as food by fish and shellfish. Also, recycling waste reduces pollution, improves water quality, and enhances the efficiency of aquaculture system. With increasing incidences of microbial diseases and growing expenses for energy, biosecurity with BFT seems, by all means a sustainable production method for aquaculture. The incorporation of biosecure biofloc technology into aquaculture practices enhances environmental sustainability while optimizing resource use, creating more eco-friendly and cost-effective systems. This review highlights key aspects such as the microbial dynamics, role of metagenomics in identifying the bacterial communities, bioremediation of aquaculture waste, biosecurity concerns, and the biocontrol of pathogenic microbes across various biofloc systems.

Neobacillus driksii sp. nov. isolated from a Mars 2020 spacecraft assembly facility and genomic potential for lasso peptide production in Neobacillus

During microbial surveillance of the Mars 2020 spacecraft assembly facility, two novel bacterial strains, potentially capable of producing lasso peptides, were identified. Characterization using a polyphasic taxonomic approach, whole-genome sequencing and phylogenomic analyses revealed a close genetic relationship among two strains from Mars 2020 cleanroom floors (179-C4-2-HS, 179-J1A1-HS), one strain from the Agave plant (AT2.8), and another strain from wheat-associated soil (V4I25). All four strains exhibited high 16S rRNA gene sequence similarity (>99.2%) and low average nucleotide identity (ANI) with Neobacillus niacini NBRC 15566T, delineating new phylogenetic branches within the genus. Detailed molecular analyses, including gyrB (90.2%), ANI (86.4%), average amino acid identity (87.8%) phylogenies, digital DNA-DNA hybridization (32.6%), and percentage of conserved proteins (77.7%) indicated significant divergence from N. niacini NBRC 15566T. Consequently, these strains have been designated Neobacillus driksii sp. nov., with the type strain 179-C4-2-HST (DSM 115941T = NRRL B-65665T). N. driksii grew at 4°C to 45°C, pH range of 6.0 to 9.5, and 0.5% to 5% NaCl. The major cellular fatty acids are iso-C15:0 and anteiso-C15:0. The dominant polar lipids include diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, and an unidentified aminolipid. Metagenomic analysis within NASA cleanrooms revealed that N. driksii is scarce (17 out of 236 samples). Genes encoding the biosynthesis pathway for lasso peptides were identified in all N. driksii strains and are not commonly found in other Neobacillus species, except in 7 out of 26 recognized species. This study highlights the unique metabolic capabilities of N. driksii, underscoring their potential in antimicrobial research and biotechnology.

IMPORTANCE

The microbial surveillance of the Mars 2020 assembly cleanroom led to the isolation of novel N. driksii with potential applications in cleanroom environments, such as hospitals, pharmaceuticals, semiconductors, and aeronautical industries. N. driksii genomes were found to possess genes responsible for producing lasso peptides, which are crucial for antimicrobial defense, communication, and enzyme inhibition. Isolation of N. driksii from cleanrooms, Agave plants, and dryland wheat soils, suggested niche-specific ecology and resilience under various environmentally challenging conditions. The discovery of potent antimicrobial agents from novel N. driksii underscores the importance of genome mining and the isolation of rare microorganisms. Bioactive gene clusters potentially producing nicotianamine-like siderophores were found in N. driksii genomes. These siderophores can be used for bioremediation to remove heavy metals from contaminated environments, promote plant growth by aiding iron uptake in agriculture, and treat iron overload conditions in medical applications.

Microbial assisted alleviation of nickel toxicity in plants: A review

Nickel (Ni) is required in trace amounts (less than 500 µg kg-1) in plants to regulate metabolic processes, the immune system, and to act as an enzymatic catalytic cofactor. Conversely, when nickel is present in high concentration, it is considered as a toxic substance. Excessive human nickel exposure occurs through ingestion, inhalation, and skin contact, ultimately leading to respiratory, cardiovascular, and chronic kidney diseases. Due to anthropogenic activities, the nickel concentrations in various environmental scenarios have progressively risen to levels as high as 26,000 ppm in soil and 0.2 mg L-1 in water; surpassing the established safety threshold limits of 100 ppm for soil and 0.005 ppm for surface water. Nickel is required by various plant species for facilitating biological processes; in the range of 0.01-5 µg g-1 (dry weight). When present in excess, nickel toxicity in plants (10-1000 mg kg-1 dry weight mass) causes many disrupted metabolic processes; leading to lower growth, altered development, hindered seed germination, chlorosis, and necrosis. To tackle any metal-linked pollution issues, various remediation approaches are employed to remove heavy metals (especially nickel) and metalloids including physicochemical, and biological methods. Based on literature, the physicochemical methods are not commonly used due to their costly nature and the potential for producing secondary pollutants. Interestingly, bioremediation is considered by many practitioners as an easy-to-handle, efficient, and cost-effective approach, encompassing techniques such as phytoremediation, bioleaching, bioreactors, green landforming, and bio-augmentation. Operationally, phytoremediation is widely utilized for cleaning up contaminated sites. To support the phytoremediative processes, numerous nickel hyperaccumulating plants have been identified; these species can absorb from their surroundings and store high concentrations of nickel (through various mechanisms) in their biomass, thereby helping to detoxify nickel-contaminated soils via phytoextraction. The microbe-assisted phytoremediation further optimizes the nickel detoxification processes by fostering beneficial interactions between microbes and the nickel-hyperaccumulators; promoting enhanced metal uptake, transformation, and sequestration. Microbe-assisted phytoremediation can be categorized into four subtypes: bacterial-assisted phytoremediation, cyanoremediation, mycorrhizal-assisted remediation, and rhizoremediation. These diverse approaches are likely to offer more effective and sustainable remediative strategy to ecologically restore the nickel-contaminated environments.

Profile of Bacterial Communities in Copper Mine Tailings Revealed through High-Throughput Sequencing

Mine-tailing dumps are one of the leading sources of environmental degradation, often with public health and ecological consequences. Due to the complex ecosystems generated, they are ideal sites for exploring the bacterial diversity of specially adapted microorganisms. We investigated the concentrations of trace metals in solid copper (Cu) mine tailings from the Ovejería Tailings Dam of the National Copper Corporation of Chile and used high-throughput sequencing techniques to determine the microbial community diversity of the tailings using 16S rRNA gene-based amplicon sequence analysis. The concentrations of the detected metals were highest in the following order: iron (Fe) > Cu > manganese (Mn) > molybdenum (Mo) > lead (Pb) > chromium (Cr) > cadmium (Cd). Furthermore, 16S rRNA gene-based sequence analysis identified 12 phyla, 18 classes, 43 orders, 82 families, and 154 genera at the three sampling points. The phylum Proteobacteria was the most dominant, followed by Chlamydiota, Bacteroidetes, Actinobacteria, and Firmicutes. Genera, such as Bradyrhizobium, Aquabacterium, Paracoccus, Caulobacter, Azospira, and Neochlamydia, showed high relative abundance. These genera are known to possess adaptation mechanisms in high concentrations of metals, such as Cd, Cu, and Pb, along with nitrogen-fixation capacity. In addition to their tolerance to various metals, some of these genera may represent pathogens of amoeba or humans, which contributes to the complexity and resilience of bacterial communities in the studied Cu mining tailings. This study highlights the unique microbial diversity in the Ovejería Tailings Dam, including the discovery of the genus Neochlamydia, reported for the first time for heavy metal resistance. This underscores the importance of characterizing mining sites, particularly in Chile, to uncover novel bacterial mechanisms for potential biotechnological applications.

Basidiomycetes to the rescue: Mycoremediation of metal-organics co-contaminated soils

The increasing need for metals leads to contaminated post-mining landscapes. At the same time, the contamination with organic, recalcitrant contamination increases. This poses a problem of reuse of large areas, often co-contaminated with both, metals, and organic pollutants. For the remediation of areas contaminated with multiple contaminants and combining many stress factors, technical solutions including groundwater treatment, where necessary, have been devised. However, this is applied to highly contaminated, small sites. The reuse of larger, co-contaminated landscapes remains a major challenge. Mycoremediation with fungi offers a good option for such areas. Fungi cope particularly well with heterogeneous conditions due to their adaptability and their large hyphal network. This chapter summarizes the advantages of basidiomycetes with a focus on wood rot fungi in terms of their ability to tolerate metals, radionuclides, and organic contaminants such as polycyclic aromatic hydrocarbons. It also shows how these fungi can reduce toxicity of contaminants to other organisms including plants to allow for restored land-use. The processes based on diverse molecular mechanisms are introduced and their use for mycoremediation is discussed.

The role and mechanism of Bacillus megaterium strain A14 in inhibiting the cadmium uptake by peanut plants in acidic red soil

AIMS

This study explores the potential of cadmium (Cd)-resistant bacteria, specifically Bacillus megaterium A14, to decrease Cd accumulation in peanuts, a crop susceptible to metal uptake from contaminated soils, by understanding the bacterium's impact on plant Cd absorption mechanisms.

METHODS AND RESULTS

Through pot experiments, we observed that A14 inoculation significantly increased peanut biomass under Cd stress conditions, primarily by immobilizing the metal and reducing its bioavailability. The bacterium effectively changed the distribution of Cd, with a notable 46.53% reduction in the exchangeable fraction, which in turn limited the expression of genes related to Cd transport in peanuts. Additionally, A14 enhanced the plant's antioxidant response, improving its tolerance to stress. Microbial analysis through 16S sequencing demonstrated that A14 inoculation altered the peanut rhizosphere, particularly by increasing populations of Firmicutes and Proteobacteria, which play crucial roles in soil remediation from heavy metals.

CONCLUSION

The A14 strain effectively counters Cd toxicity in peanuts, promoting growth through soil Cd sequestration, root barrier biofilm formation, antioxidant system enhancement, suppression of Cd transport genes, and facilitation of Cd-remediating microorganisms.

In vitro and in vivo screening of bacterial species from contaminated soil for heavy metal biotransformation activity

Heavy metals (HMs) are widely used in various industries. High concentrations of HMs can be severely toxic to plants, animals and humans. Microorganism-based bioremediation has shown significant potential in degrading and detoxifying specific HM contaminants. In this study, we cultivated a range of bacterial strains in liquid and solid nutrient medium containing different concentrations of different HMs to select and analyze bacteria capable of transforming HMs. The bacterial strains most resistant to selected HMs and exhibiting the ability to remove HMs from contaminated soils were identified. Then, the bacterial species capable of utilizing HMs in soil model experiments were selected, and their ability to transform HMs was evaluated. This study has also generated preliminary findings on the use of plants for further removal of HMs from soil after microbial bioremediation. Alcaligenes faecalis, Delftia tsuruhatensis and Stenotrophomonas sp. were selected for their ability to grow in and utilize HM ions at the maximum permissible concentration (MPC) and two times the MPC. Lysinibacillus fusiformis (local microflora) can be used as a universal biotransformation tool for many HM ions. Brevibacillus parabrevis has potential for the removal of lead ions, and Brevibacillus reuszeri and Bacillus safensis have potential for the removal of arsenic ions from the environment. The bacterial species have been selected for bioremediation to remove heavy metal ions from the environment.

Release of chromium from Cr(III)- and Ni(II)-substituted goethite in presence of organic acids: Role of pH in the formation of colloids and complexes

High levels of Cr(III) are hosted in Fe (oxyhydr)oxides in soils derived on (ultra)mafic rocks, which can pose potential risks to the environment. Organic acids can cause the solubilization of Fe (oxyhydr)oxides and the release of Cr(III). However, the release behaviors of Cr(III) from Fe (oxyhydr)oxides by organic acids and its main factors remain unclear. This study investigates the speciation of Cr released from Cr(III)-substituted goethite in the presence of citrate and oxalate and the effects of pH (3-7). Batch experiments showed that Fe(III) and Cr(III) dissolution were significantly enhanced by citrate and oxalate, and the extent of dissolution was negatively correlated with pH. When at relatively high pH (5-7), AF4-ICP-MS results revealed that large proportions of dissolved Fe (>58 %) and Cr (18 %-73 %) were presented in the form of Cr(III)-citrate colloids in the sizes of 1-125 nm and 125-350 nm. Further, FTIR and cryogenic XPS characterization demonstrated that the formation of·Cr(III)-citrate colloids was attributed to the adsorption and complexation of citrate on the substituted goethite surface. However, Cr was mainly released as soluble Cr(III)-organic complexes when presented at pH 3. While low pH inhibited the formation of Cr(III)-organic colloids, it promoted the release of Cr by facilitating the dissociation of surface Cr(III)-organic complexes. In addition, the incorporation of Ni(II) in Cr(III)-substituted goethite weakened the adsorption of organic acid by shortening the crystal size of goethite, thus significantly inhibiting the formation of Cr(III)-organic complexes and colloids. This study confirms the formation of Cr(III)-organic acid colloids and highlights the importance of pH on Cr release behavior, which is essential for evaluating Cr transport and fate in soils with high background values.

Review on heavy metal contaminants in freshwater fish in South India: current situation and future perspective

The primary natural resource we use in our daily lives for a variety of activities is freshwater for drinking and various developmental goals. Furthermore, the pace of human population increase worldwide is rising rapidly and has a great impact on the Earth's natural resources. Natural water quality has diminished owing to various anthropogenic activities. Water is crucial to the life cycle. On the other hand, chemical and agricultural industries pollute heavy metals. Acute and chronic diseases caused by heavy metals, such as slow metabolism and damage to the gills and epithelial layer of fish species, are divided into two categories. Pollutants can also harm liver tissues and result in ulceration as well as diseases such as fin rot, tail rot, and gill disease. The most prevalent heavy metals are As, Cr, Pb, and Hg, which are systemic toxicants that affect human health. These metals are categorized as carcinogens by the US Environmental Protection Agency and the worldwide agency for cancer research because they cause organ damage even at low exposure levels. The focus of the current study is to review various freshwater sources of heavy metal pollution.

Arbuscular Mycorrhizal Fungi as an Important Factor Enabling the Adaptation of Anthyllis vulneraria L. to Zn-Pb-Polluted Tailings

The old Zn-Pb-contaminated (calamine) tailings in southern Poland are spontaneously colonized by metal-tolerant Anthyllis vulneraria L. (Fabaceae), which can form simultaneously symbiotic association with nitrogen-fixing rhizobia and phosphorus-acquiring arbuscular mycorrhizal fungi (AMF). So far, fungal colonization and the AMF diversity of calamine-inhabiting legumes have been poorly studied. Thus, we determined AMF spore density in the substratum and the mycorrhizal status of nodulated A. vulneraria plants occurring on calamine tailings (M) and on a reference non-metallicolous (NM) site. The results indicate the presence of the Arum-type of arbuscular mycorrhiza in the roots of both Anthyllis ecotypes. Despite the presence of AM fungi in M plant roots, the dark septate endophyte (DSE) fungi (hyphae and microsclerotia) were occasionally also detected. Metal ions were accumulated mainly in the nodules and intraradical fungal structures rather than thick plant cell walls. Mycorrhization parameters (frequency of mycorrhization and intensity of root cortex colonization) for M plants were markedly higher and differed in a statistically significant manner from the parameters for NM plants. Heavy metal excess had no negative effect on the number of AMF spores, the amounts of glomalin-related soil proteins and AMF species composition. Molecular identification of AMF using PCR-DGGE analysis based on the 18S rDNA ribosomal gene by nested-PCR with primers AM1/NS31 and NS31-GC/Glo1 revealed similar genera/species of AMF in the roots of both Anthyllis ecotypes: Rhizophagus sp., R. fasciculatus, and R. iranicus. The results of this work indicate the presence of unique fungal symbionts, which may enhance A. vulneraria tolerance to heavy metal stress and plant adaptation to extreme conditions on calamine tailings.

Myco- and phyco-remediation of polychlorinated biphenyls in the environment: a review

Polychlorinated biphenyls (PCBs) are toxic organic compounds and pose serious threats to environment and public health. PCBs still exist in different environments such as air, water, soil, and sediments even on ban. This review summarizes the phyco- and myco-remediation technologies developed to detoxify the PCB-polluted sites. It was found that algae mostly use bioaccumulation to biodegradation strategies to reclaim the environment. As bio-accumulator, Ulva rigida C. Agardh has been best at 25 ng/g dry wt to remove PCBs. Evidently, Anabaena PD-1 is the only known PCB degrading alga and efficiently degrade Aroclor 1254 and dioxin-like PCBs up to 84.4% and 37.4% to 68.4%, respectively. The review suggested that factors such as choice of algal strains, response of microalgae, biomass, the rate of growth, and cost-effective cultivation conditions significantly influence the remediation of PCBs. Furthermore, the Anabaena sp. linA gene of Pseudomonas paucimobilis Holmes UT26 showed enhanced efficiency. Pleurotus ostreatus (Jacq.) P. Kumm is the most efficient PCB degrading fungus, degrading up to 98.4% and 99.6% of PCB in complex and mineral media, respectively. Combine metabolic activities of bacteria and yeast led to the higher detoxification of PCBs. Fungi-algae consortia would be a promising approach in remediation of PCBs. A critical analysis on potentials and limits of PCB treatment through fungal and algal biosystems have been reviewed, and thus, new insights have emerged for possible bioremediation, bioaccumulation, and biodegradation of PCBs.

Plant growth-promoting bacteria in phytoremediation of metal-polluted soils: Current knowledge and future directions

Soil metal contamination is a major concern due to the ever-rising number of areas afflicted worldwide and the detrimental effects of metals to the environment and human health. Due to their non-biodegradability and toxicity, it is paramount to prevent further metal contamination and remediate the thousands of contaminated sites across the planet. Yet, conventional reclamation based on physical and chemical methods is often expensive, impractical, and triggers secondary pollution issues. Hence, microbe-aided phytoremediation has been gaining significant traction due to its environment-friendly character, cost-effectiveness, and the breakthroughs achieved during the past few decades. Microorganisms are an essential part of natural ecosystems and play a crucial role in their restoration. Indeed, plant-microbe associations in metal-polluted soils are pivotal for plants to tolerate metal toxicity and thrive in these harsh environments. Therefore, improving the understanding of this intricate relationship is invaluable for boosting phytoremediation. In this review, we focus on the potential of plant growth promoting bacteria (PGPB) for enhancing phytoremediation of metal-polluted soils. We discuss the mechanisms employed by microbes to promote plant growth and assist the removal or immobilization of metals in soil, thereby enhancing phytoextraction and phytostabilization, respectively. Microbe-mediated metal removal and detoxification through processes entailing adsorption, chelation, transformation, and precipitation, to list but a few, are also critically examined. Moreover, this work covers the direct and indirect mechanisms used by PGPB to facilitate plant acquisition of nutrients like nitrogen and phosphorus, supply and regulate phytohormones, and exert control over antagonistic microorganisms. Lastly, we provide an outlook on the future directions of microbe-aided phytoremediation and phytomining. Clearly, to fully validate and comprehend the potential of PGPB-aided phytoremediation, a considerable shift from bench-scale to field research is necessary. What's more, it is envisaged that recent advancements in genetic engineering may soon help furthering the efficiency of microbe-assisted phytoremediation.

Advances in biological methods for the sequestration of heavy metals from water bodies: A review

Pollution is a major concern of the modern era as it affects all the principal aspects of the environment, especially the hydrosphere. Pollution with heavy metals has unequivocally threatened aquatic bodies and organisms as these metals are persistent, non-biodegradable, and toxic. Heavy metals tend to accumulate in the environment and eventually in humans, which makes their efficient removal a topic of paramount importance. Treatment of metal-contaminated water can be done both via chemical and biological methods. Where remediation through conventional methods is expensive and generates a large amount of sludge, biological methods are favoured over older and prevalent chemical purification processes because they are cheaper and environment friendly. The present review attempts to summarise effective methods for the remediation of water contaminated with heavy metals. We concluded that in biological techniques, bio-sorption is among the most employed and successful mechanisms because of its high efficacy and eco-friendly nature.

Insight Into the Molecular Mechanisms Underpinning the Mycoremediation of Multiple Metals by Proteomic Technique

We investigated the fungus Aspergillus fumigatus PD-18 responses when subjected to the multimetal combination (Total Cr, Cd2+, Cu2+, Ni2+, Pb2+, and Zn2+) in synthetic composite media. To understand how multimetal stress impacts fungal cells at the molecular level, the cellular response of A. fumigatus PD-18 to 30 mg/L multimetal stress (5 mg/L of each heavy metal) was determined by proteomics. The comparative fungal proteomics displayed the remarkable inherent intracellular and extracellular mechanism of metal resistance and tolerance potential of A. fumigatus PD-18. This study reported 2,238 proteins of which 434 proteins were exclusively expressed in multimetal extracts. The most predominant functional class expressed was for cellular processing and signaling. The type of proteins and the number of proteins that were upregulated due to various stress tolerance mechanisms were post-translational modification, protein turnover, and chaperones (42); translation, ribosomal structure, and biogenesis (60); and intracellular trafficking, secretion, and vesicular transport (18). In addition, free radical scavenging antioxidant proteins, such as superoxide dismutase, were upregulated upto 3.45-fold and transporter systems, such as protein transport (SEC31), upto 3.31-fold to combat the oxidative stress caused by the multiple metals. Also, protein–protein interaction network analysis revealed that cytochrome c oxidase and 60S ribosomal protein played key roles to detoxify the multimetal. To the best of our knowledge, this study of A. fumigatus PD-18 provides valuable insights toward the growing research in comprehending the metal microbe interactions in the presence of multimetal. This will facilitate in development of novel molecular markers for contaminant bioremediation.

Microbial Interventions in Bioremediation of Heavy Metal Contaminants in Agroecosystem

Soil naturally comprises heavy metals but due to the rapid industrialization and anthropogenic events such as uncontrolled use of agrochemicals their concentration is heightened up to a large extent across the world. Heavy metals are non-biodegradable and persistent in nature thereby disrupting the environment and causing huge health threats to humans. Exploiting microorganisms for the removal of heavy metal is a promising approach to combat these adverse consequences. The microbial remediation is very crucial to prevent the leaching of heavy metal or mobilization into the ecosystem, as well as to make heavy metal extraction simpler. In this scenario, technological breakthroughs in microbes-based heavy metals have pushed bioremediation as a promising alternative to standard approaches. So, to counteract the deleterious effects of these toxic metals, some microorganisms have evolved different mechanisms of detoxification. This review aims to scrutinize the routes that are responsible for the heavy metal(loid)s contamination of agricultural land, provides a vital assessment of microorganism bioremediation capability. We have summarized various processes of heavy metal bioremediation, such as biosorption, bioleaching, biomineralization, biotransformation, and intracellular accumulation, as well as the use of genetically modified microbes and immobilized microbial cells for heavy metal removal.

Biofloc Microbiome With Bioremediation and Health Benefits

The biofloc system has recently attracted great attention as a cost-effective, sustainable, and environmentally friendly technology and expected to contribute toward human food security (Zero Hunger SDG 2). It is also expected that this endeavor can be adopted widely because of its characteristics of zero water exchange and reduced artificial feeding features. In the biofloc system, the flocs which are generally formed by aggregation of heterotrophic microorganisms, serve as natural bioremediation candidates. These microbes effectively maintain water quality by utilizing the nutrient wastes, mostly originated from digested, unconsumed, and metabolic processes of feed. Additionally, the flocs are important sources of nutrients, mainly a protein source, and when these are consumed by aquaculture animals they improve the growth performance, immunity, and disease tolerance of host against pathogenic microbial infection. Here in this review, we focus on recent advances that could provide a mechanistic insight on how the microbial community developed in the biofloc system helps in the bioremediation process and enhances the overall health of the host. We have also tried to address the possible role of these microbial communities against growth and virulence of pathogenic microbes.

Vertical distribution of the toxic metal(loid)s chemical fraction and microbial community in waste heap at a nonferrous metal mining site

Over the past few decades, nonferrous mining has produced numerous waste rock and part of the waste that has not been properly treated was generally dumped at roadsides and hill slopes. However, the vertical distributions of toxic metal(loid)s and composition of microbial communities in waste heap and the under-laid pristine soil are rarely studied. In this work, the fraction-related distributions of toxic metal(loid)s were investigated at a waste heap profile and the indigenous microbial assemblages were also analyzed by Illumina sequencing of 16 s rRNA genes. Results showed that compared to the under-laid pristine soil, content of toxic metal(loid)s, especially Cd, As and Pb, in waste rock layer were higher. Most of As in subsoil existed as non-specifically sorbed and specifically-sorbed fractions, which could be ascribed to the migration from the upper layer. The mobility was significantly correlated with Eh, EC, clay content, CEC and the total content of metal(loid)s. Phyla Proteobacteria, Acidobacteria and Firmicutes dominated the microbial communities. The microbial community compositions at the genus level were similar, but their relative abundances were mainly influenced by pH, CEC, Eh, SOM, and bioavailability content of toxic metal(loid)s. Besides, microbial functions of elements (S, Fe, Mn and As) oxidation/reduction and metabolites (siderophore, biosurfactant, organic acid, phosphatase and urease) potentially were used for pollutants bioremediation.

A mini-review of heavy metal recycling technologies for municipal solid waste incineration fly ash

This mini-review article summarizes the available technologies for the recycling of heavy metals (HMs) in municipal solid waste incineration (MSWI) fly ash (FA). Recovery technologies included thermal separation (TS), chemical extraction (CE), bioleaching, and electrochemical processes. The reaction conditions of various methods, the efficiency of recovering HMs from MSWI FA and the difficulties and solutions in the process of technical development were studied. Evaluation of each process has also been done to determine the best HM recycling method and future challenges. Results showed that while bioleaching had minimal environmental impact, the process was time-consuming. TS and CE were the most mature technologies, but the former process was not cost-effective. Overall, it has the greatest economic potential to recover metals by CE with scrubber liquid produced by a wet air pollution control system. An electrochemical process or solvent extraction could then be applied to recover HMs from the enriched leachate. Ongoing development of TS and bioleaching technologies could reduce the treatment cost or time.

Removal of metals from aqueous solutions using dried Cladophora parriaudii of varying biochemical composition

Macroalgal biosorption has shown promise for the removal of metal ions from wastewaters, whose presence can pose a threat to the aquatic environment. There is a wealth of literature published on macroalgal biosorption, the common thread being that the biosorbent material was collected from the field, under undefined conditions. These studies offer little insight into the impact of prior cultivation or biomass production practices upon the biosorbent material, its adsorptive physico-chemical properties and its subsequent capacity for metal removal. The present study sought to investigate the influence of changes in macroalgal cultivation, specifically nutrient regime, upon biomass properties and the resultant adsorption performance. The macroalga Cladophora parriaudii was cultivated under six different nutrient regimes; 2:1 and 12:1 N:P molar ratios, with nitrogen supplied either as ammonium (NH₄⁺), nitrate (NO₃^-), or urea (CO(NH₂)₂). These nutrient regimes were designed to produce biomass of varying biochemical and cell surface profiles. After cultivation, the biomass was rinsed, dried, biochemically analysed and then used for the removal of four individual metals from solution. Metal removal varied considerably between treatments and across initial metal concentrations, with removal values of 46-85%, 9-80%, 8-71%, and 49-94% achieved for Al, Cu, Mn, and Pb, respectively, with initial metal concentrations varying between 0 and 150 mg L^-1. The observed variation in metal removal can only be attributed to differences in biochemistry and cell surface properties of the biosorbent induced by nutrient regime, as all other variables were constant. This study demonstrates that prior cultivation conditions influence the biochemistry of a biosorbent material, namely macroalgae Cladophora parriaudii, which has an impact upon metal removal. This aspect should be given due consideration for future biosorption research and when reviewing already published literature.

Molecular characterization of marine bacterial isolates of Visakhapatnam coast-efficacy in dye decolorization and bioremediation of cadmium

Background

Microbial community is one of the diversified communities of the marine environment. Studies have shown that microorganisms isolated from the marine environment are metabolically active and have adapted to life in the ocean. The marine microorganisms use various survival strategies to combat heavy metal stress and decolorization of various textile dyes, thus playing an important role in the bioremediation of cadmium and degradation of textile dyes. The present study deals with the isolation and 16S rRNA molecular characterization of M3 and M8 bacterial strains isolated from marine water samples collected from Visakhapatnam harbor. M3 and M8 isolates were also checked for their efficacy in the removal of cadmium and decolorization of various textile dyes from the environment.

Results

The water sample was subjected to tube dilution method to isolate bacterial strains, and ten different isolates were screened. The biochemical tests were performed for the isolates to prove their validity and 16S rRNA molecular sequencing and phylogenetic analysis for species identification. Out of interest, two bacterial strains, namely, M3 and M8 were subjected to 16S rRNA molecular sequencing and phylogenetic analysis and were identified as Bacillus subtilis and Pseudomonas resinovorans. The two bacterial strains showed promising dye degradation property when checked with nine different textile dyes of wavelength ranging from 400 to 600 nm and removal of cadmium from the growth medium.

Conclusion

The present study demonstrates the isolates M3 and M8 to be potential strains having dye decolorization and bioremediation of cadmium applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-021-00189-0.

Microbial Degradation of Citric Acid in Low Level Radioactive Waste Disposal: Impact on Biomineralization Reactions

Organic complexants are present in some radioactive wastes and can challenge waste disposal as they may enhance subsurface mobility of radionuclides and contaminant species via chelation. The principal sources of organic complexing agents in low level radioactive wastes (LLW) originate from chemical decontamination activities. Polycarboxylic organic decontaminants such as citric and oxalic acid are of interest as currently there is a paucity of data on their biodegradation at high pH and under disposal conditions. This work explores the biogeochemical fate of citric acid, a model decontaminant, under high pH anaerobic conditions relevant to disposal of LLW in cementitious disposal environments. Anaerobic microcosm experiments were set up, using a high pH adapted microbial inoculum from a well characterized environmental site, to explore biodegradation of citrate under representative repository conditions. Experiments were initiated at three different pH values (10, 11, and 12) and citrate was supplied as the electron donor and carbon source, under fermentative, nitrate-, Fe(III)- and sulfate- reducing conditions. Results showed that citrate was oxidized using nitrate or Fe(III) as the electron acceptor at > pH 11. Citrate was fully degraded and removed from solution in the nitrate reducing system at pH 10 and pH 11. Here, the microcosm pH decreased as protons were generated during citrate oxidation. In the Fe(III)-reducing systems, the citrate removal rate was slower than in the nitrate reducing systems. This was presumably as Fe(III)-reduction consumes fewer moles of citrate than nitrate reduction for the same molar concentrations of electron acceptor. The pH did not change significantly in the Fe(III)-reducing systems. Sulfate reduction only occurred in a single microcosm at pH 10. Here, citrate was fully removed from solution, alongside ingrowth of acetate and formate, likely fermentation products. The acetate and lactate were subsequently used as electron donors during sulfate-reduction and there was an associated decrease in solution pH. Interestingly, in the Fe(III) reducing experiments, Fe(II) ingrowth was observed at pH values recorded up to 11.7. Here, TEM analysis of the resultant solid Fe-phase indicated that nanocrystalline magnetite formed as an end product of Fe(III)-reduction under these extreme conditions. PCR-based high-throughput 16S rRNA gene sequencing revealed that bacteria capable of nitrate Fe(III) and sulfate reduction became enriched in the relevant, biologically active systems. In addition, some fermentative organisms were identified in the Fe(III)- and sulfate-reducing systems. The microbial communities present were consistent with expectations based on the geochemical data. These results are important to improve long-term environmental safety case development for cementitious LLW waste disposal.

Changes in clover rhizosphere microbial community and diazotrophs in mercury-contaminated soils

Little is known about the response of the soil microbiome (including bacteria in the rhizosphere of legumes such as clover) to mercury (Hg) despite the toxicity of Hg to soil microorganisms. Here, Hg-contaminated soils collected from Guizhou province, southwest China, were divided into three groups according to their Hg contents and were planted with clover. High-throughput sequencing of bacterial 16S rRNA and nitrogenase (nifH) genes and quantitative polymerase chain reaction (qPCR) were used to study the response of bacteria and diazotrophs to soil Hg stress and the effects of Hg on the abundance of functional genes in rhizosphere soils. High concentrations of soil Hg decreased bacterial community abundance and diversity and increased the abundance and diversity of nitrogen-fixing bacteria. LEfSe analysis indicates that Rhizobium was a biomarker at sites with high soil Hg contents and the co-occurrence network results indicate a positive relationship between the abundance of the dominant module (from the co-occurrence network analysis) of Rhizobiaceae and soil Hg concentration. Structural equation modeling (SEM) indicates that the Hg content in the clover shoots (Shoot_Hg) was negatively correlated with the abundance of the mercury reductase (merA) gene (r = -0.26, P < 0.05) and the organomercury lyase (merB) gene (r = -0.23, P < 0.05) in rhizosphere soils. Moreover, correlation analysis and SEM indicate that soil total nitrogen (TN), nitrate‑nitrogen (NO₃^-N), soil organic matter (SOM), and available molybdenum (Mo) contents were also important factors affecting the structure of the microbial community and the abundance of functional genes. The results provide a basis for further study of the mechanism(s) by which microorganisms may impart tolerance of clover to Hg in contaminated soils.

A review on phytoremediation of mercury contaminated soils

Mercury (Hg) and its compounds are one of the most dangerous environmental pollutants and Hg pollution exists in soils in different degrees over the world. Phytoremediation of Hg-contaminated soils has attracted increasing attention for the advantages of low investment, in-situ remediation, potential economic benefits and so on. Searching for the hyperaccumulator of Hg and its application in practice become a research hotspot. In this context, we review the current literatures that introduce various experimental plant species for accumulating Hg and aided techniques improving the phytoremediation of Hg-contaminated soils. Experimental plant species for accumulating Hg and accumulation or translocation factor of Hg are listed in detail. The translocation factor (TF) is greater than 1.0 for some plant species, however, the bioaccumulation factor (BAF) is greater than 1.0 for Axonopus compressus only. Plant species, soil properties, weather condition, and the bioavailability and heterogeneity of Hg in soils are the main factors affecting the phytoremediation of Hg-contaminated soils. Chemical accelerator kinds and promoting effect of chemical accelerators for accumulating and transferring Hg by various plant species are also discussed. Potassium iodide, compost, ammonium sulphate, ammonium thiosulfate, sodium sulfite, sodium thiosulfate, hydrochloric acid and sulfur fertilizer may be selected to promote the absorption of Hg by plants. The review introduces transgenic gene kinds and promoting effect of transgenic plants for accumulating and transferring Hg in detail. Some transgenic plants can accumulate more Hg than non-transgenic plants. The composition of rhizosphere microorganisms of remediation plants and the effect of rhizosphere microorganisms on the phytoremediation of Hg-contaminated soils are also introduced. Some rhizosphere microorganisms can increase the mobility of Hg in soils and are beneficial for the phytoremediation.

Removal of uranium by immobilized biomass of a tropical marine yeast Yarrowia lipolytica

A marine yeast, Yarrowia lipolytica isolated from an oil polluted sea water and shown earlier to sequester dissolved uranium (U) at pH 7.5, was utilized in the present study for developing an immobilized-cell process for U removal from aqueous solutions under batch and continuous flow through systems. In batch system, optimum biosorption conditions for U removal were assessed by investigating the effects of biomass dose, initial U concentration, contact time and pH of solution using Y. lipolytica cells immobilized in calcium alginate beads. Appreciable uranium-binding capabilities over a wide pH range (3-9) were observed with the alginate beads bearing yeast cells. Out of Langmuir and Freundlich models employed for describing the sorption equilibrium data under batch mode, uranyl adsorption followed Langmuir approach with satisfactory correlation coefficient higher than 0.9. Uranyl adsorption kinetics by Y. lipolytica entrapped in alginate beads was best described by the pseudo-second-order model. While the environmental scanning electron microscopy established the immobilization and the uniform distribution of Y. lipolytica cells in the alginate beads, the Energy Dispersive X-ray spectroscopy analysis confirmed the deposition of U in the beads following their exposure to uranyl solution. Fixed bed flow-through column comprising of Y. lipolytica biomass immobilized in polyacrylamide matrix displayed high efficacy for continuous removal of uranium at pH 7.5 up to five adsorption-desorption cycles. Adsorbed U by immobilized cells could be significantly desorbed using 0.1 N HCl. Overall, our results present the superior efficiency of immobilized Y. lipolytica biomass for U removal using batch and regenerative approaches.

Microbacterium sp. MRS-1, a potential bacterium for cobalt reduction and synthesis of less/non-toxic cobalt oxide nanoparticles (Co3O4)

Background

Detoxification of heavy metal pollutants in wastewater has become a serious problem to surrounding environment. This research was conducted to utilize a potential heavy metal-resistant bacterium for the remediation of cobalt metal and simultaneous synthesis of cobalt oxide nanoparticles in the form of powder for various industrial applications. Metal oxide nanoparticles have great applications in electrochemical devices such as supercapacitors, biosensors, and batteries.

Method

A heavy metal-resistant bacterium Microbacterium sp. MRS-1 isolated from electroplating industrial effluent reduced cobalt ions from an initial concentration of 200 mg/L to 26 mg/L were analyzed by atomic absorption spectroscopy. Instrumental analysis of bacterially synthesized Co3O4 has been characterized. Cytotoxicity of synthesized nanoparticles was assessed by MTT assay.

Results

Microbacterium sp. MRS-1 isolated from electroplating industrial effluent was found to be suitable for cobalt oxide nanoparticles as it showed tolerance towards high concentration of metal. The nutrient broth containing metal solution and Microbacterium sp. MRS-1 showed color change from light pink to dark pink indicated the formation of extracellular nanoparticles. It also converted soluble cobalt salts into less soluble cobalt oxide nanoparticles outside the cell which allows easy recovery of nanoparticles without the destruction of cells and simultaneous detoxification of toxic metal ions. Electron microscopic imaging verified that nanoparticles were predominantly surrounding the bacterial cells and SEM imaging revealed that the produced particles were in the range of 10–100 nm in size. XRD spectrum exhibited 2θ values were corresponding to cubic face-centered cobalt oxide (Co3O4) nanoparticles.

Conclusion

The present study investigated new prospective for eco-friendly detoxification of toxic heavy metal Co from metal-polluted sites and the production of cobalt oxide nanoparticles in powder form for clinical and other industrial applications.

Assessment of Aspergillus niger Strain's Suitability for Arsenate-Contaminated Water Treatment and Adsorbent Recycling via Bioextraction in a Laboratory-Scale Experiment

In this work, the viability of bioaccumulation and bioextraction processes for arsenic removal from contaminated waters, as well as the recycling of arsenate-treated amorphous ferric oxyhydroxide adsorbent (FeOOH) were evaluated using the common soil microscopic filamentous fungus Aspergillus niger. After treating the contaminated arsenate solution (100 mg As L-1) with FeOOH, the remaining solution was exposed to the growing fungus during a static 19-day cultivation period to further decrease the arsenic concentration. Our data indicated that although the FeOOH adsorbent is suitable for arsenate removal with up to 84% removal efficiency, the fungus was capable of accumulating only up to 13.2% of the remaining arsenic from the culture media. This shows that the fungus A. niger, although highly praised for its application in environmental biotechnology research, was insufficient for decreasing the arsenic contamination to an environmentally acceptable level. However, the bioextraction of arsenic from arsenate-treated FeOOH proved relatively effective for reuse of the adsorbent. Due to its production of acidic metabolites, which decreased pH below 2.7, the fungal strain was capable of removing of up to 98.2% of arsenic from the arsenate-treated FeOOH adsorbent.

Stabilization of potentially toxic elements contained in mine waste: A microbiological approach for the environmental management of mine tailings

Metals are key materials extensively employed in several industries to produce technological and daily-life products. The mining industry that produces such commodities generates Tons of waste that if not remediated can be transferred to the surrounding environment, thus representing a water, air, and soil pollution threat. In this work, we evaluated the feasibility of microbial sulfate reduction (SR) as a management strategy for this waste. Mine tailings were sampled from two abandoned mining sites located in Sonora (northwestern Mexico) and treated in anaerobic microcosms under SR conditions using anaerobic sludge as the inoculum at two different tailing:inoculum ratios (TIR). Major TIR's were found to be the triggering factor for the highest SR activities observed (73.6 ± 8.8 mg SO₄^2- L^-1 day^-1). This stimulation was linked to the dissolution of sulfate bearing minerals (anglesite, jarosite, and gypsum) which provided additional sulfate for microbial activity. However, under this condition, longer lag phases for SR were observed, which was potentially due to pH inhibition at early incubation stages (pH ~3.7). Despite this, all biologically SR performing treatments presented important sulfide precipitation which was associated to changes in the mineralogy of the mine tailings. Metals of environmental concern such as As, Cd, Co, Cr and, Pb were detected to have shifted from the aqueous extractable phase to the bound to Fe and Mn oxides and residual phases. This finding was in accordance with the non-detectable concentrations of these metals in the aqueous phase by the end of the biological treatment which proved the effectiveness of this approach. This study provides insights into the promising potential of anaerobic microbes for the environmental management of mine tailings.

Bioremediation of toxic heavy metals (THMs) contaminated sites: concepts, applications and challenges

Heavy metal contamination is a global issue, where the prevalent contaminants are arsenic (As), cadmium (Cd), chromium (Cr)(VI), mercury (Hg), and lead (Pb). More often, they are collectively known as "most problematic heavy metals" and "toxic heavy metals" (THMs). Their treatment through a variety of biological processes is one of the prime interests in remediation studies, where heavy metal-microbe interaction approaches receive high interest for their cost effective and ecofriendly solutions. In this review, we provide an up to date information on different microbial processes (bioremediation) for the removal of THMs. For the same, emphasis is put on oxidation-reduction, biomineralization, bioprecipitation, bioleaching, biosurfactant technology, biovolatilization, biosorption, bioaccumulation, and microbe-assisted phytoremediation with their selective advantages and disadvantages. Further, the literature briefly discusses about the various setups of cleaning processes of THMs in environment under ex situ and in situ applications. Lately, the study sheds light on the manipulation of microorganisms through genetic engineering and nanotechnology for their advanced treatment approaches.

Influence of metals and metalloids on the composition and fluorescence quenching of the extracellular polymeric substances produced by the polymorphic fungus Aureobasidium pullulans

Abstract

Aureobasidium pullulans is a ubiquitous and widely distributed fungus in the environment, and exhibits substantial tolerance against toxic metals. However, the interactions between metals and metalloids with the copious extracellular polymeric substances (EPS) produced by A. pullulans and possible relationships to tolerance are not well understood. In this study, it was found that mercury (Hg) and selenium (Se), as selenite, not only significantly inhibited growth of A. pullulans but also affected the composition of produced EPS. Lead (Pb) showed little influence on EPS yield or composition. The interactions of EPS from A. pullulans with the tested metals and metalloids depended on the specific element and their concentration. Fluorescence intensity measurements of the EPS showed that the presence of metal(loid)s stimulated the production of extracellular tryptophan-like and aromatic protein-like substances. Examination of fluorescence quenching and calculation of binding constants revealed that the fluorescence quenching process for Hg; arsenic (As), as arsenite; and Pb to EPS were mainly governed by static quenching which resulted in the formation of a stable non-fluorescent complexes between the EPS and metal(loid)s. Se showed no significant interaction with the EPS according to fluorescence quenching. These results provide further understanding of the interactions between metals and metalloids and EPS produced by fungi and their contribution to metal(loid) tolerance.

Key points

• Metal(loid)s enhanced production of tryptophan- and aromatic protein-like substances.

• Non-fluorescent complexes formed between the EPS and tested metal(loid)s.

• EPS complexation and binding of metal(loid)s was dependent on the tested element.

• Metal(loid)-induced changes in EPS composition contributed to metal(loid) tolerance.

Bacterial remediation of heavy metal polluted soil and effluent from paper mill industry

Bacterial remediation of heavy metal polluted soil and effluent from paper mill was investigated using standard analytical methods. The paper mill was visited for 6 months at interval of 30 days to collect soil and effluent samples for the analysis. The pH of soil was slightly alkaline while effluent was acidic. There was a significant increase (P < 0.05) in total organic carbon (TOC) of soil; and turbidity, biochemical oxygen demand (BOD), chemical oxygen demand (COD) and TOC of effluent when compared to control. Bacteria isolated from the samples were grouped into two and used to remediate eight heavy metals. The remediation experiment consists of three treatments; Treatment 1 (treated with proteobacteria), Treatment 2 (treated with non-proteobacteria) and Treatment 3 (without bacteria) (control experiment). Result of the remediation study showed that there was a significant decrease (P < 0.05) in Treatment 1 and Treatment 2 of all the heavy metals in soil and effluent samples from day 30-180 when compared to day 0. The rate of removal of heavy metals in soil was highest in Treatment 1 for chromium (Cr; 0.00846 day-1) and lowest in Treatment 1 for cadmium (Cd; 0.00403 day-1) while the rate of removal in effluent was highest in Treatment 1 for zinc (Zn; 0.01207 day-1) and lowest in Treatment 1 for Cd (0.00391 day-1). It was concluded that bacteria isolated from soil and effluent samples were capable of remediating the concentration of Pb, arsenic (As), Cr, Zn and nickel (Ni) heavy metals.

A new magnetized thermophilic bacteria to preconcentrate uranium and thorium from environmental samples through magnetic solid-phase extraction

A magnetic solid-phase extraction (MSPE) method was developed for simultaneous preconcentrations of U(VI) and Th(IV) before their measurements by inductively coupled plasma optical emission spectrometry (ICP-OES). The main idea of this biotechnological application depends on the use of bacteria, thermophilic Bacillus cereus SO-14, as a solid-phase biosorbent. It was immobilized to γ-Fe₂O₃ magnetic nanoparticles and used for MSPE. Characterization of the biosorbent was performed using the scanning electron microscope (SEM), the energy dispersive X-ray (EDX) and Fourier transform infrared (FT-IR) spectroscopy. Also, the the best conditions of experimental parameters were examined, and the reliability of the method developed was verified by applying the certified reference materials. Limit of detections (LODs) of the U(VI) and Th(IV) was calculated as 0.008 and 0.013 ng mL^-1 respectively. Relative standard deviations (RSDs) were found to be 1.6 and 2.4 %, respectively, for U(VI) and Th(IV). R² was also calculated as 0.9991. Preconcentration factors were achieved as 100 for both elements. It should be highlighted that LODs were critically improved and the sensitivity of ICP-OES was enhanced.

Significance of manganese resistant bacillus cereus strain WSE01 as a bioinoculant for promotion of plant growth and manganese accumulation in Myriophyllum verticillatum

Endophytic bacteria are generally helpful for plant growth and protection. Strain WSE01, which was identified as bacillus cereus, was isolated from the stem of Myriophyllum verticillatum and it displayed a high tolerance to Mn (1500 mg/L). The strain was found to be able to produce indole-3-acetic acid (IAA) and siderophores, fix the atmospheric nitrogen and dissolve potassium from insoluble K-bearing minerals. In hydroponic culture experiments, the inoculation of strain WSE01 significantly promoted the growth and increased the leaf enzyme activity in the inoculated plant M. verticillatum. Furthermore, the manganese content was increased by 36.4% in stems and by 54.7% in leaves of the inoculated plant under Mn stress at 400 mg/L, compared to the non-inoculated group. This study suggests that the strain WSE01 has the potential to be used as biocontrol and/or biofertilizing agents for application in macrophyte M. verticillatum and conduces to achieving more effective phytoremediation of metal-contaminated waters.

Remediation of mercury contaminated soil, water, and air: A review of emerging materials and innovative technologies

Mercury contamination in soil, water and air is associated with potential toxicity to humans and ecosystems. Industrial activities such as coal combustion have led to increased mercury (Hg) concentrations in different environmental media. This review critically evaluates recent developments in technological approaches for the remediation of Hg contaminated soil, water and air, with a focus on emerging materials and innovative technologies. Extensive research on various nanomaterials, such as carbon nanotubes (CNTs), nanosheets and magnetic nanocomposites, for mercury removal are investigated. This paper also examines other emerging materials and their characteristics, including graphene, biochar, metal organic frameworks (MOFs), covalent organic frameworks (COFs), layered double hydroxides (LDHs) as well as other materials such as clay minerals and manganese oxides. Based on approaches including adsorption/desorption, oxidation/reduction and stabilization/containment, the performances of innovative technologies with the aid of these materials were examined. In addition, technologies involving organisms, such as phytoremediation, algae-based mercury removal, microbial reduction and constructed wetlands, were also reviewed, and the role of organisms, especially microorganisms, in these techniques are illustrated.

Bioremediation Options for Heavy Metal Pollution

BACKGROUND

Rapid industrialization and anthropogenic activities such as the unmanaged use of agro-chemicals, fossil fuel burning and dumping of sewage sludge have caused soils and waterways to be severely contaminated with heavy metals. Heavy metals are non-biodegradable and persist in the environment. Hence, remediation is required to avoid heavy metal leaching or mobilization into environmental segments and to facilitate their extraction.

OBJECTIVES

The present work briefly outlines the environmental occurrence of heavy metals and strategies for using microorganisms for bioremediation processes as reported in the scientific literature.

METHODS

Databases were searched from different libraries, including Google Scholar, Medline and Scopus. Observations across studies were then compared with the standards for discharge of environmental pollutants.

DISCUSSION

Bioremediation employs microorganisms for removing heavy metals. Microorganisms have adopted different mechanisms for bioremediation. These mechanisms are unique in their specific requirements, advantages, and disadvantages, the success of which depends chiefly upon the kind of organisms and the contaminants involved in the process.

CONCLUSIONS

Heavy metal pollution creates environmental stress for human beings, plants, animals and other organisms. A complete understanding of the process and various alternatives for remediation at different steps is needed to ensure effective and economic processes.

COMPETING INTERESTS

The authors declare no competing financial interests.

Gold Recycling at Laboratory Scale: From Nanowaste to Nanospheres

The market for products based on nanotechnology, and with it the use of nanomaterials and the generation of nanowaste, increases day by day. Among the vast variety of nanomaterials available, gold nanoparticles (AuNPs) are among the most studied and applied in commercial products. This current situation requires both the development of recovery methods to reduce the amount of nanowaste produced, and new synthetic methods that allow the reuse of recovered gold for new nanomaterial production, keeping in mind both economical and ecological considerations. In this work, a methodology to recover gold from aqueous laboratory nanowaste and transform it into an aqueous HAuCl₄ solution was developed, using extremely simple procedures and readily available chemical reagents (NaCl, HCl, H₂ O₂ ) and allowing the recovery of more than 99 % of the original gold. The experiments were performed by using both simulated and real laboratory nanowastes, and practically the same results were obtained. Moreover, the subsequent use of the obtained aqueous HAuCl₄ solution from the recovered gold to produce spherical AuNPs through a seed-mediated approach was demonstrated. Thus, this work presents for the first time a complete recycling cycle from nanowaste to the reagent and back to the nanomaterial.

Bioaccumulation of potentially toxic elements by submerged plants and biofilms: A critical review

The accumulation of potentially toxic elements (PTEs) in aquatic ecosystems has become a global concern, as PTEs may exert a wide range of toxicological impacts on aquatic organisms. Submerged plants and the microorganisms attached to their surfaces, however, have displayed great potential as a means of coping with such pollution. Therefore, it is crucial to understand the transport pathways of PTEs across sediment and organisms as well as their accumulation mechanisms in the presence of submerged plants and their biofilms. The majority of previous studies have demonstrated that submerged plants and their biofilms are indicators of PTE pollution in the aquatic environment, yet relatively little is known about PTE accumulation in epiphytic biofilms. In this review, we describe the transport pathways of PTEs in the aquatic environment in order to offer remarkable insights into bioaccumulation mechanisms in submerged plants and their biofilms. Based on the literature cited in this review, the roles of epiphytic biofilms in bioaccumulation and as an indicator of ecosystem health are discussed.

Arsenic (As) Removal Using Talaromyces sp. KM-31 Isolated from As-Contaminated Mine Soil

: Bioremediation is an environmentally-benign and cost-effective approach to removing arsenic from contaminated areas. A fungal strain hyper-tolerant to arsenic was isolated from soil from a mine site and used for the removal of arsenic. The isolated fungus was identified as Talaromyces sp., and its growth rate, arsenic tolerance, and removal rates were investigated for As(III) and As(V). Arsenic tolerance tests revealed that the fungus was highly resistant to arsenic, tolerating concentrations up to 1000 mg/L. Robust mycelial growth was observed in potato dextrose broth containing either As(III) or As(V), and there was no difference in growth between that in arsenic-free medium and medium amended with up to 300 mg/L of either arsenic species. The isolate showed relatively low growth rates at As(V) concentrations >500 mg/L, and almost no growth at As(III) concentrations >300 mg/L. Both arsenic species were effectively removed from aqueous medium (>70%) in tests of the biosorption of arsenic onto mycelial biomass. Surface modification of the biomass with Fe(III) (hydr)oxides significantly enhanced arsenic removal efficiency. The findings indicate that this soil fungal strain has promise for use in bioremediation strategies to remove arsenic from highly contaminated aqueous systems.

Study of characteristics on metabolism of Penicillium chrysogenum F1 during bioleaching of heavy metals from contaminated soil

Penicillium chrysogenum F1 is very efficient in bioleaching heavy metals from the soil and is used for that purpose. We found that F1 can extract 19.8 mg Cd, Cu, Pb, and Zn from 2.5 g soil; the total heavy metals’ bioleaching ratio was 60.4%. In this study, the bioleaching mechanism was investigated by means of metabonomics; different metabolite ions were screened (relative standard deviation >30%) and analyzed using clustering, univariate and multivariate analysis. Statistical analyses via Volcano Plot, principal component analysis, and partial least square discriminant analysis models revealed a difference between Ctrl 7 (the controls cultured and sampled on day 7) and Ctrl 15 (the controls cultured and sampled on day 15). Samp 15 (the samples cultured with heavy-metal-contaminated soil) was significantly different from Ctrl 7 and Ctrl 15. Analysis of the different ions demonstrated that the glucose catabolism pathways of glycolysis and the tricarboxylic acid (TCA) cycle were enhanced, and glucose anabolism through the pentose phosphate pathway was inhibited during bioleaching. At the same time, the metabolism of glutathione was also downregulated. Therefore, the catabolism of glucose was unaffected by the addition of heavy-metal-contaminated soil, and increasing glucose is beneficial to catabolism. The extraction of metals is mainly attributed to the metabolites of the TCA cycle.

Successional trajectories of soil bacterial communities in mine tailings: The role of plant functional traits

Plant species identity is assumed to be a major driver of belowground microbial diversity and composition. However, diagnosing which plant functional traits are responsible for shaping microbial communities remains elusive. Primary succession on barren metalliferous mining substrates was selected as the framework to study above-belowground interactions, and plant functional traits that lead the successional trajectories of soil bacterial communities were identified. The impact of the plant functional group (i.e. trees, shrubs, dwarf shrubs, perennial grasses), a trait integrating the life span and morphological structure, on the bacterial primary succession was monitored. Bacterial diversity and composition was estimated along plant size gradients including over 90 scattered patches ranging from seedlings to mature multispecific patches. Soil bacterial diversity was affected by heavy metals levels and increased towards higher resource availability underneath mature patches, with stress-tolerant heterotrophs and phototrophs being replaced by competitive heterotrophs. The plant functional group modulated these general patterns and shrubs had the greatest impact belowground by inducing the largest increase in soil fertility. Functional traits related to leaf decomposability and root architecture further determined the composition and structure of bacterial communities. These results underline the importance of plant functional traits in the assembly of soil bacterial communities, and can help guiding restoration of degraded lands.

Microbially induced calcium carbonate precipitation: a widespread phenomenon in the biological world

Biodeposition of minerals is a widespread phenomenon in the biological world and is mediated by bacteria, fungi, protists, and plants. Calcium carbonate is one of those minerals that naturally precipitate as a by-product of microbial metabolic activities. Over recent years, microbially induced calcium carbonate precipitation (MICP) has been proposed as a potent solution to address many environmental and engineering issues. However, for being a viable alternative to conventional techniques as well as being financially and industrially competitive, various challenges need to be overcome. In this review, the detailed metabolic pathways, including ammonification of amino acids, dissimilatory reduction of nitrate, and urea degradation (ureolysis), along with the potent bacteria and the favorable conditions for precipitation of calcium carbonate, are explained. Moreover, this review highlights the potential environmental and engineering applications of MICP, including restoration of stones and concrete, improvement of soil properties, sand consolidation, bioremediation of contaminants, and carbon dioxide sequestration. The key research and development questions necessary for near future large-scale applications of this innovative technology are also discussed.

Sorption mechanism and distribution of cadmium by different microbial species

Bioremediation programs of cadmium (Cd) by microorganisms have being proposed, but the underlying mechanism of the remediation ion remains unexplored. Here, the sorption efficiency and subcellular fraction distribution of Cd in three selected microbial species were investigated. Our results showed that both species of the microorganisms and initial Cd concentrations strongly affected the Cd sorption capacity. In the three microbial species, the Cd removal efficiency increased with decreased Cd concentrations. Specifically, Hansenula anomala removed the highest Cd ions in low concentration of 0.05 mg L^-1; while in medium concentration of 0.5 mg L^-1 and high concentration of 5 mg L^-1, Bacillus subtilis removed the highest Cd ions. The subcellular fractionation allocation showed that Cd was mainly allocated on cell wall (mantle and inner wall) in Pseudomonas stutzeri and B. subtilis, while cell cytomembrane accumulated similar amount of Cd compared to the cell wall of H. anomala at concentration of 0.5 mg L^-1. Meanwhile, the Cd distributions on cell subcellular fractionation of the three species changed along the contact times, suggesting varied migration models during the biosorption process. Moreover, the functional groups involved in biosorption differed among the species based on Fourier Transform Infrared (FTIR) analysis. Our results have important implications for developing and improving Cd remediation by microorganisms, which is a low-cost and environmentally friendly bioremediation strategy of Cd pollution in environments.

Enhanced oxytetracycline removal coupling with increased power generation using a self-sustained photo-bioelectrochemical fuel cell

Photo-bioelectrochemical fuel cell (PBFC) represents a promising technology for enhancing removal of antibiotic pollutants while simultaneously sustainable transformation of organic wastes and solar energy into electricity. In this study, simultaneous antibiotic removal and bioelectricity generation were investigated in a PBFC with daily light/dark cycle using oxytetracycline (OTC) as a model compound of antibiotic. The specific OTC removal rate increased by 61% at an external resistance of 50 Ω compared to that in the open-circuit control, which was attributed to bioelectrochemically enhanced co-metabolic degradation in the presence of the bioanode. The OTC removal was obviously accelerated during illumination of cathode in contrast with a dark cathode due to the higher driving force for anodic bioelectrochemical reaction by using photosynthetic oxygen as cathodic electron acceptor during illumination than that using nitrate in dark. The bioelectrocatalytic activity of anodic biofilm was continuously enhanced even at an initial OTC concentration of up to 50 mg L^-1. The degradation products of OTC can function as mediators to facilitate the electron transfer from bacteria to the anode, resulting in 1.2, 1.76 and 1.8 fold increase in maximum power output when 10, 30 and 50 mg L^-1 OTC was fed to the bioanode, compared to the OTC-free bioanode, respectively. The OTC feeding selective enriched OTC-tolerant bacterial community capable of degrading complex organic compounds and producing electricity. The occurrence of ARGs during bioelectrochemical degradation of OTC was affected more greatly by the succession of the anodic bacterial community than the initial OTC concentration.

Integration of nanoscale zero-valent iron and functional anaerobic bacteria for groundwater remediation: A review

The technology of integrating nanoscale zero-valent iron (nZVI) and functional anaerobic bacteria has broad prospects for groundwater remediation. This review focuses on the interactions between nZVI and three kinds of functional anaerobic bacteria: organohalide-respiring bacteria (OHRB), sulfate reducing bacteria (SRB) and iron reducing bacteria (IRB), which are commonly used in the anaerobic bioremediation. The coupling effects of nZVI and the functional bacteria on the contaminant removal in the integrated system are summarized. Generally, nZVI could create a suitable living condition for the growth and activity of anaerobic bacteria. OHRB and SRB could synergistically degrade organic halides and remove heavy metals with nZVI, and IRB could reactive the passivated nZVI by reducing the iron (hydr)oxides on the surface of nZVI. Moreover, the roles of these anaerobic bacteria in contaminant removal coupling with nZVI and the degradation mechanisms are illustrated. In addition, this review also discusses the main factors influencing the removal efficiency of contaminants in the integrated treatment system, including nZVI species and dosage, inorganic ions, organic matters, pH, type of pollutants, temperature, and carbon/energy sources, etc. Among these factors, the nZVI species and dosage play a fundamental role due to the potential cytotoxicity of nZVI, which might exert a negative impact on the performance of this integrated system. Lastly, the future research needs are proposed to better understand this integrated technology and effectively apply it in groundwater remediation.

Pseudomonas species isolated from tobacco seed promote root growth and reduce lead contents in Nicotiana tobacum K326

Endophytic bacteria are generally helpful for plant growth and protection. We isolated from tobacco seeds three Pseudomonas strains (K03, Y04, and N05) that could produce siderophores, indole-3-acetic acid, and 1-aminocyclopropane-1-carboxylate deaminase, fix nitrogen, dissolve phosphorus and potassium, and tolerate heavy metals. In pot experiments, the three isolated strains significantly promoted root growth and increased the root enzyme activity in Nicotiana tobacum K326. Furthermore, bacterial inoculations increased the proportion of residual lead (Pb) by 8.36%-51.63% and decreased the total Pb content by 3.28%-6.38% in the contaminated soil during tobacco planting, compared with uninoculated soils. An effective decrease in Pb content was also found in tobacco leaves with bacterial inoculations. K03 inoculation decreased the Pb content in the upper leaves by 49.80%, and Y04 inoculation had the best effect, decreasing the Pb content in the middle leaves by 70.12%. Additionally, soil pH and root activity had significant effects on transformation and translocation of Pb. The study suggested that in response to Pb pollution in soil, a reasonable application of endophytes (e.g., Pseudomonas) might be a promising approach in promoting tobacco growth and reducing Pb content in tobacco, while simultaneously enhancing Pb stabilization in soils.