Arsenic-resistant bacteria solubilized arsenic in the growth media and increased growth of arsenic hyperaccumulator Pteris vittata L

Arsenic Detoxification Using Arsenic-Resistant Bacteria Bacillus cereus SP21 and Bacillus toyonensis SP23: A Laboratory-Scale Study

Groundwater is the most important source for drinking, cooking and other household purposes. Agriculture is also largely dependent on groundwater. In this study, arsenic was detoxified by microbial oxidation using arsenic-resistant bacteria (ARB) in synthetic water. Two bacterial strains of Bacillus cereus (SP21) and Bacillus toyonensis (SP23), which are resistant to arsenic (As3+), were tested well up to 100 ppm. Optimization of arsenic for oxidation using different carbon sources (glucose, cellulose and starch), of which 1.5% cellulose was the best concentration, considering different temperatures and pH values. Oxidation of arsenite (As3+ to As5+) through a lab-scale column filled with immobilized bacterial cells and a synthetic medium through a lab-scale bioreactor. Samples were taken at regular intervals and processed for the oxidation of arsenite to arsenate (846 nm). The reduction of arsenate to arsenite (869 nm) was determined according to the corresponding protocol using the molybdenum blue method with a UV spectrophotometer. The selected arsenic-tolerant bacterial strains Bacillus cereus SP21 and Bacillus toyonensis SP23 achieved maximum removal of arsenic by the immobilized bacterial cell inclusion method.

Alleviating arsenic stress affecting the growth of Vigna radiata through the application of Klebsiella strain ASBT-KP1 isolated from wastewater

Arsenic contamination of soil and water is a major environmental issue. Bioremediation through plant growth-promoting bacteria is viable, cost-effective, and sustainable. Along with arsenic removal, it also improves plant productivity under stressful conditions. A crucial aspect of such a strategy is the selection of bacterial inoculum. The described study demonstrates that the indigenous wastewater isolate, ASBT-KP1, could be a promising candidate. Identified as Klebsiella pneumoniae, ASBT-KP1 harbors genes associated with heavy metal and oxidative stress resistance, production of antimicrobial compounds and growth-promotion activity. The isolate efficiently accumulated 30 μg/g bacterial dry mass of arsenic. Tolerance toward arsenate and arsenite was 120 mM and 70 mM, respectively. Plant biomass content of Vigna radiata improved by 13% when grown in arsenic-free soil under laboratory conditions in the presence of the isolate. The increase became even more significant under the same conditions in the presence of arsenic, recording a 37% increase. The phylogenetic analysis assigned ASBT-KP1 to the clade of Klebsiella strains that promote plant growth. Similar results were also observed in Oryza sativa, employed to assess the ability of the strain to promote growth, in plants other than V. radiata. This study identifies a prospective candidate in ASBT-KP1 that could be employed as a plant growth-promoting rhizoinoculant in agricultural practices.

A comprehensive review of flue gas bio-mitigation: chemolithotrophic interactions with flue gas in bio-reactors as a sustainable possibility for technological advancements

Flue gas mitigation technologies aim to reduce the environmental impact of flue gas emissions, particularly from industrial processes and power plants. One approach to mitigate flue gas emissions involves bio-mitigation, which utilizes microorganisms to convert harmful gases into less harmful or inert substances. The review thus explores the bio-mitigation efficiency of chemolithotrophic interactions with flue gas and their potential application in bio-reactors. Chemolithotrophs are microorganisms that can derive energy from inorganic compounds, such as carbon dioxide (CO2), nitrogen oxides (NOx), and sulfur dioxide (SO2), present in the flue gas. These microorganisms utilize specialized enzymatic pathways to oxidize these compounds and produce energy. By harnessing the metabolic capabilities of chemolithotrophs, flue gas emissions can be transformed into value-added products. Bio-reactors provide controlled environments for the growth and activity of chemolithotrophic microorganisms. Depending on the specific application, these can be designed as suspended or immobilized reactor systems. The choice of bio-reactor configuration depends on process efficiency, scalability, and ease of operation. Factors influencing the bio-mitigation efficiency of chemolithotrophic interactions include the concentration and composition of the flue gas, operating conditions (such as temperature, pH, and nutrient availability), and reactor design. Chemolithotrophic interactions with flue gas in bio-reactors offer a potentially efficient approach to mitigating flue gas emissions. Continued research and development in this field are necessary to optimize reactor design, microbial consortia, and operating conditions. Advances in understanding the metabolism and physiology of chemolithotrophic microorganisms will contribute to developing robust and scalable bio-mitigation technologies for flue gas emissions.

Intercropping of Pteris vittata and maize on multimetal contaminated soil can achieve remediation and safe agricultural production

Soil contamination by multimetals is widespread. Hyperaccumulator-crop intercropping has been confirmed to be an effective method for arsenic (As)- or cadmium (Cd)-contaminated soil that can achieve soil cleanup and agricultural production. However, the influencing factors and response of hyperaccumulator-crop intercropping to multimetal-contaminated soil are still unclear. In this study, intercropping of the As hyperaccumulator Pteris vittata and maize was conducted on two typical types of multimetal-contaminated soil, namely, Soil A contaminated by As, Cd, and lead (Pb) and Soil B contaminated by As, Cd, and chromium (Cr). Intercropping reduced As, Cd, and Pb in the maize grains by 60 %, 66.7 %, and 20.4 %, respectively. The concentrations of As, Cd, Pb, and Cr in P. vittata increased by 314 %, 300 %, 447.3 %, and 232.6 %, respectively, relative to their concentrations in the monoculture plants. Two soils with different levels of contamination showed that higher heavy metal content might diminish the ability of intercropping to reduce soil heavy metal risk. No notable difference in soil microbial diversity was found between the intercropped and monocultured plants. The composition of microbial communities of intercropping groups were more similar to those of monoculture P. vittata on two different soils (Soils A and B). An imbalance between the amount of As taken up by the plants and the reduction in As in the soil was observed, and this imbalance may be related to watering, As leaching, and heterogeneity of soil As distribution. Reducing the risk resulting from As leaching and enhancing the efficiency of phytoextraction should be emphasized in remediation practices.

Vermi-converted Tea Industry Coal Ash efficiently substitutes chemical fertilization for growth and yield of cabbage (Brassica oleracea var. capitata) in an alluvial soil: A field-based study on soil quality, nutrient translocation, and metal-risk remediation

Although coal ashes (CA) can be converted into an eco-friendly product through vermicomposting, the utility of vermiconverted CA in agriculture still needs to be explored. Therefore, the feasibility of vermicomposted tea industry coal ash (VCA) as an alternative nutrient source for cabbage (Brassica oleracea, var. Capitata) production was evaluated through an on-field experiment in alluvial soil. Two types of vermicomposts were prepared using Eisenia fetida (VCAE) and Lampito mauritii (VCAL) and were applied in different combinations with chemical fertilizers. The results revealed a significant increase in nutrient availability (nitrogen, phosphorus, and potassium) in the soil treated with VCA, alongside a concurrent build-up of soil organic carbon stocks, activation of microbial growth, and enhanced soil enzyme activity. Additionally, VCA application substantially reduced toxic metals in the soil, thereby improving soil health and promoting the uptake of essential nutrients (nitrogen, phosphorus, potassium, iron, manganese, copper, and zinc) in cabbage. VCA application reduced the bioaccumulation of potentially toxic metals (chromium, lead, and cadmium) from coal ash, ensuring safer food production. Notably, a 25 % substitution of chemical fertilizers with VCA and farmyard manure (FYM) led to a two-fold increase in the growth and productivity of cabbage. The economic assessment also indicated that large-scale and sustainable recycling of toxic tea industry coal ash in agriculture is feasible. Hence, by integrating VCA-based nutrient management into agricultural practices, developing nations can take significant strides toward achieving circular economy objectives while addressing environmental challenges associated with CA disposal.

Isolation and characterization of rhizobacteria from lentil for arsenic resistance and plant growth promotion

Low-cost microbial remediation strategies serve as a viable and potent weapon for curbing the arsenic menace. In the present study, two arsenic-resistant bacteria were isolated from the contaminated lentil rhizosphere in Gangetic plain of eastern India. LAR-21 (Burkholderia cepacia, MW356875) and LAR-25 (Burkholderia cenocepacia, MW356894) could remove 87.6% and 85.9% of arsenite (10 mM) from the liquid culture medium in laboratory condition. They were highly resistant to arsenate and arsenite and also had a high arsenite oxidase activity. LAR-21 showed the highest level of minimum inhibitory concentration value of 390 mM for arsenate and 31 mM for arsenite. The same strain was found to show highest arsenite oxidase activity, i.e., 5.2 nM min-1 mg-1of protein. These two strains further possess potential plant growth-promoting characteristics like indole acetic acid production (5-15 mM IAA mL-1), 1-aminocyclopropane-1-carboxylate deaminase (8-21 nM α-keto butyrate mg protein-1 h-1), nitrogenase activity (3-8.99 nM ethylene mg cell protein-1 h-1), siderophore production (17-22.1 µM deferoxamine mesylate mL-1), phosphate solubilization (261-453 µg mL-1) under arsenic stress condition. The plant growth promotion of the strains was further validated by pot study of lentil by assessing their agronomic and growth-related traits, and potential to recover from arsenic stress (17.2-21.2% arsenic reduction in root and shoot, 16-19.2% in leaf and pod, and 15-23% reduction in seeds). The LAR-21 strain, thus, emerged as the most suited candidate for bioremediation and plant (lentil) growth promotion in arsenic polluted environment.

Improved efficiency of Sedum lineare (Crassulaceae) in remediation of arsenic-contaminated soil by phosphate-dissolving strain P-1 in association with phosphate rock

The selection of appropriate plants and growth strategies is a key factor in improving the efficiency and universal applicability of phytoremediation. Sedum lineare grows rapidly and tolerates multiple adversities. The effects of inoculation of Acinetobacter sp. phosphate solubilizing bacteria P-1 and application of phosphate rock (PR) as additives on the remediation efficiency of As-contaminated soil by S. lineare were investigated. Compared with the control, both the single treatment and the combination of inoculation with strain P-1 and application of PR improved the biomass by 30.7-395.5%, chlorophyll content by 48.1-134.8%, total protein content by 12.5-92.4% and total As accumulation by 45.1-177.5%, and reduced the As-induced oxidative damage. Inoculation with strain P-1 increased the activities of superoxide dismutases and catalases of S. lineare under As stress, decreased the accumulation of reactive oxygen species in plant tissues and promoted the accumulation of As in roots. In contrast, simultaneous application of PR decreased As concentration in S. lineare tissues, attenuated As-induced lipid peroxidation and improved As transport to shoots. In addition, the combined application showed the best performance in improving resistance and biomass, which significantly increased root length by 149.1%, shoot length by 33%, fresh weight by 395.5% and total arsenic accumulation by 159.2%, but decreased the malondialdehyde content by 89.1%. Our results indicate that the combined application of strain P-1 and PR with S. lineare is a promising bioremediation strategy to accelerate phytoremediation of As-contaminated soils.

Chemical transformations of arsenic in the rhizosphere-root interface of Pityrogramma calomelanos and Pteris vittata

Pityrogramma calomelanos and Pteris vittata are cosmopolitan fern species that are the strongest known arsenic (As) hyperaccumulators, with potential to be used in the remediation of arsenic-contaminated mine tailings. However, it is currently unknown what chemical processes lead to uptake of As in the roots. This information is critical to identify As-contaminated soils that can be phytoremediated, or to improve the phytoremediation process. Therefore, this study identified the in situ distribution of As in the root interface leading to uptake in P. calomelanos and P. vittata, using a combination of synchrotron micro-X-ray fluorescence spectroscopy and X-ray absorption near-edge structure imaging to reveal chemical transformations of arsenic in the rhizosphere-root interface of these ferns. The dominant form of As in soils was As(V), even in As(III)-dosed soils, and the major form in P. calomelanos roots was As(III), while it was As(V) in P. vittata roots. Arsenic was cycled from roots growing in As-rich soil to roots growing in control soil. This study combined novel analytical approaches to elucidate the As cycling in the rhizosphere and roots enabling insights for further application in phytotechnologies to remediated As-polluted soils.

Enterobacter sp. E1 increased arsenic uptake in Pteris vittata by promoting plant growth and dissolving Fe-bound arsenic

Microbes affect arsenic accumulation in the arsenic-hyperaccumulator Pteris vittata, but the associated molecular mechanism remains uncertain. Here, we investigated the effect of Enterobacter sp. E1 on arsenic accumulation by P. vittata. Strain E1 presented capacities of arsenate [As(V)] and Fe(III) reduction during cultivation. In the pot experiment with P. vittata, the biomass, arsenic content, and chlorophyll content of P. vittata significantly increased by 30.03%, 74.9%, and 112.1%, respectively. Strikingly, the water-soluble plus exchangeable arsenic (WE-As) significantly increased by 52.05%, while Fe-bound arsenic (Fe-As) decreased by 29.64% in the potted soil treated with strain E1. The possible role of activation of arsenic by strain E1 was subsequently investigated by exposing As(V)-absorbed ferrihydrite to the bacterial culture. Speciation analyses of As showed that strain E1 significantly increased soluble levels of As and Fe and that more As(V) was reduced to arsenite. Additionally, increased microbial diversity and soil enzymatic activities in soils indicated that strain E1 posed few ecological risks. These results indicate that strain E1 effectively increased As accumulation in P. vittata mainly by promoting plant growth and dissolving soil arsenic. Our findings suggest that As(V) and Fe(III)-reducer E1 could be used to enhance the phytoremediation of P. vittata in arsenic-contaminated soils.

Combined maize straw-biochar and oxalic acids induced a relay activity of abundant specific degraders for efficient phenanthrene degradation: Evidence based on the DNA-SIP technology

Biochar-oxalic acid composite application (BCOA) have shown to be efficient in the remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil, but the functional degraders and the mechanism of improving biodegradation remains unclear. In this study, with the help of stable isotope probing technology of phenanthrene (Phe), we determined that BCOA significantly improved Phe mineralization by 2.1 times, which was ascribed to the increased numbers and abundances of functional degraders. The BCOA increased contents of dissolved organic carbon and available nutrients and decreased pH values in soil, thus promoting the activity, diversity and close cooperation of the functional Phe-degraders, and stimulating their functions associated with Phe degradation. In addition, there is a relay activity among more and diverse functional Phe-degraders in the soil with BCOA. Specifically, Pullulanibacillus persistently participated in Phe-degradation in the soil with BCOA throughout the incubation period. Moreover, Pullulanibacillus, Blastococcus, Alsobacter, Ramlibacter, and Mizugakiibacter were proved to be potential Phe-degraders in soil for the first time. The specific Phe degraders and their relay and cooperation activity in soils as impacted by BCOA were first identified with DNA-stable isotope probing technology. Our findings provided a novel perspective to understand the efficient degradation of PAH in the BCOA treatments, revealed the potential of soil native microbes in the efficient bioremediation of PAH-contaminated natural soil, and provided a basis for the development of in-situ phytoremediation technologies to remediate PAH pollution in future.

Microbial community composition in the rhizosphere of Pteris vittata and its effects on arsenic phytoremediation under a natural arsenic contamination gradient

Arsenic contamination causes numerous health problems for humans and wildlife via bioaccumulation in the food chain. Phytoremediation of arsenic-contaminated soils with the model arsenic hyperaccumulator Pteris vittata provides a promising way to reduce the risk, in which the growth and arsenic absorption ability of plants and the biotransformation of soil arsenic may be greatly affected by rhizosphere microorganisms. However, the microbial community composition in the rhizosphere of P. vittata and its functional role in arsenic phytoremediation are still poorly understood. To bridge this knowledge gap, we carried out a field investigation and pot experiment to explore the composition and functional implications of microbial communities in the rhizosphere of four P. vittata populations with a natural arsenic contamination gradient. Arsenic pollution significantly reduced bacterial and fungal diversity in the rhizosphere of P. vittata (p < 0.05) and played an important role in shaping the microbial community structure. The suitability of soil microbes for the growth of P. vittata gradually decreased following increased soil arsenic levels, as indicated by the increased abundance of pathogenic fungi and parasitic bacteria and the decrease in symbiotic fungi. The analysis of arsenic-related functional gene abundance with AsChip revealed the gradual enrichment of the microbial genes involved in As(III) oxidation, As(V) reduction, and arsenic methylation and demethylation in the rhizosphere of P. vittata following increased arsenic levels (p < 0.05). The regulation of indigenous soil microbes through the field application of fungicide, but not bactericide, significantly reduced the remediation efficiency of P. vittata grown under an arsenic contamination gradient, indicating the important role of indigenous fungal groups in the remediation of arsenic-contaminated soil. This study has important implications for the functional role and application prospects of soil microorganisms in the phytoremediation of arsenic-polluted soil.

Leveraging arsenic resistant plant growth-promoting rhizobacteria for arsenic abatement in crops

Arsenic is a toxic metalloid categorized under class 1 carcinogen and is detrimental to both plants and animals. Agricultural land in several countries is contaminated with arsenic, resulting in its accumulation in food grains. Increasing global food demand has made it essential to explore neglected lands like arsenic-contaminated lands for crop production. This has posed a severe threat to both food safety and security. Exploration of arsenic-resistant plant growth-promoting rhizobacteria (PGPR) is an environment-friendly approach that holds promise for both plant growth promotion and arsenic amelioration in food grains. However, their real-time performance is dependent upon several biotic and abiotic factors. Therefore, a detailed analysis of associated mechanisms and constraints becomes inevitable to explore the full potential of available arsenic-resistant PGPR germplasm. Authors in this review have highlighted the role and constraints of arsenic-resistant PGPR in reducing the arsenic toxicity in food crops, besides providing the details of arsenic transport in food grains.

Impact of Soil Inoculation with Bacillus amyloliquefaciens FZB42 on the Phytoaccumulation of Germanium, Rare Earth Elements, and Potentially Toxic Elements

Bioaugmentation promises benefits for agricultural production as well as for remediation and phytomining approaches. Thus, this study investigated the effect of soil inoculation with the commercially available product RhizoVital^®42, which contains Bacillus amyloliquefaciens FZB42, on nutrient uptake and plant biomass production as well as on the phytoaccumulation of potentially toxic elements, germanium, and rare earth elements (REEs). Zea mays and Fagopyrum esculentum were selected as model plants, and after harvest, the element uptake was compared between plants grown on inoculated versus reference soil. The results indicate an enrichment of B. amyloliquefaciens in inoculated soils as well as no significant impact on the inherent bacterial community composition. For F. esculentum, inoculation increased the accumulation of most nutrients and As, Cu, Pb, Co, and REEs (significant for Ca, Cu, and Co with 40%, 2042%, and 383%, respectively), while it slightly decreased the uptake of Ge, Cr, and Fe. For Z. mays, soil inoculation decreased the accumulation of Cr, Pb, Co, Ge, and REEs (significant for Co with 57%) but showed an insignificant increased uptake of Cu, As, and nutrient elements. Summarily, the results suggest that bioaugmentation with B. amyloliquefaciens is safe and has the potential to enhance/reduce the phytoaccumulation of some elements and the effects of inoculation are plant specific.

Rhizosphere interactions between PAH-degrading bacteria and Pteris vittata L. on arsenic and phenanthrene dynamics and transformation

The pot experiment was conducted to monitor the dynamics in soil solution chemistry in order to determine the main rhizosphere processes determining As and PAH bioavailability when utilizing P. vittata and PAH-degrading bacteria to remediate co-contaminated soils. The result showed that P. vittata was capable of depleting soil solution As and increasing phenanthrene solubilization, and thus facilitating plant As uptake and phenanthrene dissipation. Bacterial inoculation enhanced soil phenanthrene dissipation and concurrently modified As bioavailability though increasing soil pH, facilitating Fe and Ca minerals solubilization, and accelerating organic matter decomposition. However, the main factors that determine As bioavailability in the rhizosphere considerably varied with plant genotypes. Upon bacterial inoculation, P and Fe strongly influenced As(V) availability and its uptake by the Guangxi accession, and DOC, Fe, and pH were the main parameters correlated with As(V) availability in the rhizosphere of the Hunan accession. Bacterial inoculation tended to stimulate As(V) reduction in the rhizosphere of P. vittata. Microbial-induced changes in Ca, S, and C cycling and pH were indicators of As(V) reduction. Although bacterial inoculation increased soil As and phenanthrene availability, striking differences in As and nutrients uptake and phenanthrene dissipation were observed between P. vittata genotypes. It is suggested that apart from the microbial transformation, plant genotypes and bacterial mediated plant nutrition are also the critical factors in controlling the fates of As and phenanthrene. Our results uncovered the interactions between P. vittata and PAH-degrading bacteria on rhizosphere properties and nutrients cycling regulating As and PAH availability and remediation efficiency.

Rehabilitation of mine soils by phytostabilization: Does soil inoculation with microbial consortia stimulate Agrostis growth and metal(loid) immobilization?

Metal(loid) soil pollution resulting from mining activities is an important issue that has negative effects on the environment (soil acidification, lack of vegetation, groundwater pollution) and human health (cancer, chronic diseases). In the context of a phytostabilization process for the bioremediation of a mine soil highly contaminated by arsenic (As) and lead (Pb), a pot experiment was set up to study the effect of plant sowing and microbial inoculation on soil properties, metal(loid) (im)mobilization in soil and accumulation in plant, and plant growth. For this, mine soil was sown with endemic metallicolous Agrostis seeds and/or inoculated with endogenous microbial consortia previously selected for their As and Pb tolerance. Agrostis was able to develop on the contaminated mine soil and immobilized metal(loid)s through metal(loid) accumulation in the roots. Its growth was improved by microbial consortium inoculation. Moreover, microbial consortium inoculation increased soil organic content and electrical conductivity, and led to an increase in soil microbial activities (linked to C and P cycles); however, it also induced a metal(loid) mobilization. In conclusion, microbial consortium inoculation stimulated the growth of endemic Agrostis plants and thus ameliorated the phytostabilization of a former mine soil highly polluted by As and Pb. This study is thus a good example of the benefits of coupling several approaches such as phytostabilization and bioaugmentation for the bioremediation of former mine contaminated sites.

Bioaugmentation with As-transforming bacteria improves arsenic availability and uptake by the hyperaccumulator plant Pteris vittata (L)

Inorganic arsenic (As) is a toxic and carcinogenic pollutant that has long-term impacts on environmental quality and human health. Pteris vittata plants hyperaccumulate As from soils. Soil bacteria are critical for As-uptake by P. vittata. We examined the use of taxonomically diverse soil bacteria to modulate As speciation in soil and their effect on As-uptake by P. vittata. Aqueous media inoculated with Pseudomonas putida MK800041, P. monteilii MK344656, P. plecoglossicida MK345459, Ochrobactrum intermedium MK346993 or Agrobacterium tumefaciens MK346997 resulted in the oxidation of 5-30% As(III) and a 49-79% reduction of As(V). Soil inoculated with P. monteilii increased extractable As(III) and As(V) from 0.5 and 0.09 in controls to 0.9 and 0.39 mg As kg^-1 soil dry weight, respectively. Moreover, and P. vittata plants inoculated with P. monteilii, P. plecoglossicida, O. intermedium strains, and A. tumefaciens strains MK344655, MK346994, MK346997, significantly increased As-uptake by 43, 32, 12, 18, 16, and 14%, respectively, compared to controls. The greatest As-accumulation (1.9 ± 0.04 g kg^-1 frond Dwt) and bioconcentration factor (16.3 ± 0.35) was achieved in plants inoculated with P. monteilii. Our findings indicate that the tested bacterial strains can increase As-availability in soils, thus enhancing As-accumulation by P. vittata. Novelty statement Pteris vittata, a well-known As-hyperaccumulator, has the remarkable ability to accumulate higher levels of As in their above-ground biomass. The As-tolerant bacteria-plant interactions play a significant role in bioremediation by mediating As-redox and controlling As-availability and uptake by P. vittata. Our studies indicated that most of the tested bacterial strains isolated from As-impacted soil significantly enhanced As-uptake by P. vittata. P. monteilii oxidized 20% of As(III) and reduced 50% of As(V), increased As-extraction from soils, and increased As-uptake by 43% greater compared with control. Therefore, these strains associated with P. vittata can be used in large-scale field applications to remediate As-contaminated soil.

Application of constructed wetlands in the PAH remediation of surface water: A review

Polycyclic aromatic hydrocarbons (PAHs) pose adverse risks to ecosystems and public health because of their carcinogenicity and mutagenicity. As such, the extensive occurrence of PAHs represents a worldwide concern that requires urgent solutions. Wastewater treatment plants are not, however, designed for PAH removal and often become sources of the PAHs entering surface waters. Among the technologies applied in PAH remediation, constructed wetlands (CWs) exhibit several cost-effective and eco-friendly advantages, yet a systematic examination of the application and success of CWs for PAH remediation is missing. This review discusses PAH occurrence, distribution, and seasonal patterns in surface waters during the last decade to provide baseline information for risk control and further treatment. Furthermore, based on the application of CWs in PAH remediation, progress in understanding and optimising PAH-removal mechanisms is discussed focussing on sediments, plants, and microorganisms. Wetland plant traits are key factors affecting the mechanisms of PAH removal in CWs, including adsorption, uptake, phytovolatilization, and biodegradation. The physico-chemical characteristics of PAHs, environmental conditions, wetland configuration, and operation parameters are also reviewed as important factors affecting PAH removal efficiency. Whilst significant progress has been made, several key problems need to be addressed to ensure the success of large-scale CW projects. These include improving performance in cold climates and addressing the toxic threshold effects of PAHs on wetland plants. Overall, this review provides future direction for research on PAH removal using CWs and their large-scale operation for the treatment of PAH-contaminated surface waters.

Investigation of arsenic-resistant, arsenite-oxidizing bacteria for plant growth promoting traits isolated from arsenic contaminated soils

The problem of arsenic (As) pollution being severe warrants opting for low-cost microbial remediation strategies. The present study of identifying suitable bacterial strains led to the isolation of eleven As-tolerant strains from the As-contaminated rhizosphere soils of West Bengal, India. They were found to oxidize/reduce 55-31.6% of 5 mM As(III) and 73-37.6% of 5 mM As(V) within 12 h. The four isolates (BcAl-1, JN 73, LAR-2, and AR-30) had a high level of As(III) oxidase activity along with a higher level of As(V) and As(III) resistance. The agar diffusion assay of the isolates further confirmed their ability to endure As stress. The presence of aoxB gene was observed in these four As(III) oxidizing isolates. Evaluation of plant growth-promoting characteristics revealed that BcAl-1 (Burkholderia cepacia), JN 73 (Burkholderia metallica), AR-30 (Burkholderia cenocepacia), and LAR-2 (Burkholderia sp.) had significant plant growth-promoting characteristics (PGP), including the ability to solubilize phosphate, siderophore production, indole acetic acid-like molecules production, ACC deaminase production, and nodule formation under As stressed condition. BcAl-1 and JN 73 emerged as the most promising traits in As removal as well as plant growth promotion.

Unraveling the role of plant growth-promoting rhizobacteria in the alleviation of arsenic phytotoxicity: A review

The toxic metalloid arsenic (As), is a major pollutant of soil and water, imposing severe health concerns on human lives. It enters the food chain mainly through As-contaminated crops. The uptake, translocation and accumulation of As in plant tissue are often controlled by certain soil-inhabiting microbial communities. Among them, indigenous, free-living As-resistant plant growth-promoting rhizobacteria (PGPR) plays a pivotal role in As-immobilization. Besides, the plant's inability to withstand As after a threshold level is actively managed by these PGPR increasing As-tolerance in host plants by a synergistic plant-microbe interaction. The dual functionality of As-resistant PGPR i.e., phytostimulation and minimization of As-induced phytotoxic damages are one of the main focal points of this review article. It is known that such PGPR having the functional arsenic-resistant genes (in ars operon) including As-transporters, As-transforming genes contributed to the As accumulation and detoxification/transformation respectively. Apart from assisting in nutrient acquisition and modulating phytohormone levels, As-resistant PGPR also influences the antioxidative defense system in plants by maneuvering multiple enzymatic and non-enzymatic antioxidants. Furthermore, they are effective in reducing membrane damage and electrolyte leakage in plant cells. As-induced photosynthetic damage is also found to be salvaged by As-resistant PGPR. Briefly, the eco-physiological, biochemical and molecular mechanisms of As-resistant PGPR are thus elaborated here with regard to the As-exposed crops.

Insights into conventional and recent technologies for arsenic bioremediation: A systematic review

Arsenic (As) bioremediation has been an economical and sustainable approach, being practiced widely under several As-contaminated environments. Bioremediation of As involves the use of bacteria, fungi, yeast, plants, and genetically modified organisms for detoxification/removal of As from the contaminated site. The understanding of multi-factorial biological components involved in these approaches is complex and more and more efforts are on their way to make As bioremediation economical and efficient. In this regard, we systematically reviewed the recent literature (n=200) from the last two decades regarding As bioremediation potential of conventional and recent technologies including genetically modified plants for phytoremediation and integrated approaches. Also, the responsible mechanisms behind different approaches have been identified. From the literature, it was found that As bioremediation through biosorption, bioaccumulation, phytoextraction, and volatilization involving As-resistant microbes has proved a very successful technology. However, there are various pathways of As tolerance of which the mechanisms have not been fully understood. Recently, phytosuction separation technology has been introduced and needs further exploration. Also, integrated approaches like phytobial, constructed wetlands using As-resistant bacteria with plant growth-promoting activities have not been extensively studied. It is speculated that the integrated bioremediation approaches with practical applicability and reliability would prove most promising for As remediation. Further technological advancements would help explore the identified research gaps in different approaches and lead us toward sustainability and perfection in As bioremediation.

Root microbiome assembly of As-hyperaccumulator Pteris vittata and its efficacy in arsenic requisition

The assemblage of root-associated microorganisms plays important roles in improving their capability to adapt to environmental stress. Metal(loid) hyperaccumulators exhibit disparate adaptive capability compared to that of non-hyperaccumulators when faced with elevated contents of metal(loid)s. However, knowledge of the assemblage of root microbes of hyperaccumulators and their ecological roles in plant growth is still scarce. The present study used Pteris vittata as a model plant to study the microbial assemblage and its beneficial role in plant growth. We demonstrated that the assemblage of microbes from the associated bulk soil to the root compartment was based on their lifestyles. We used metagenomic analysis and identified that the assembled microbes were primarily involved in root-microbe interactions in P. vittata root. Notably, we identified that the assembled root microbiome played an important role in As requisition, which promoted the fitness and growth of P. vittata. This study provides new insights into the root microbiome and potential valuable knowledge to understand how the root microbiome contributes to the fitness of its host.

Nanoscale zerovalent iron, carbon nanotubes and biochar facilitated the phytoremediation of cadmium contaminated sediments by changing cadmium fractions, sediments properties and bacterial community structure

Environment functional materials have been widely used, but whether their effects on the contaminated environment could facilitate phytoremediation is not yet well understood. In this study, starch stabilized nanoscale zerovalent iron (SN), multiwall carbon nanotubes (MW) and tea waste derived biochar (TB) were used to facilitate the phytoremediation of cadmium (Cd) contaminated sediments by Boehmeria nivea (L.) Gaudich. Results showed that 100 mg/kg SN, 500 mg/kg MW and 500 mg/kg TB facilitated phytoremediation, as evidenced by increasing Cd accumulation and/or promoting plant growth. These concentrations of materials increased the reducible fraction of Cd by 9-10% and decreased the oxidizable proportion of Cd by 48-52%, indicating the improvement of Cd bioavailability through converting the oxidizable Cd into reducible form. The activities of urease, phosphatase and catalase, which related to nutrient utilization and oxidative stress alleviation, increased by 20-24%, 25-26%, and 8-9% in the sediments treated with 500 mg/kg MW and 500 mg/kg TB, respectively. In addition, the 16S rRNA gene sequence results showed that these concentrations of materials changed the bacterial diversity. The abundance of Acidobacteria, Actinobacteria, Nitrospirae and Firmicutes were increased by some of the applied materials, which could promote plant growth, change Cd bioavailability and reduce Cd toxicity. These findings indicated that the applied environment functional materials could facilitate the phytoremediation of Cd contaminated environment by changing Cd fractions, sediments properties and bacterial community structure.

Unravelling the Role of Rhizospheric Plant-Microbe Synergy in Phytoremediation: A Genomic Perspective

Background

Accretion of organic and inorganic contaminants in soil interferes in the food chain, thereby posing a serious threat to the ecosystem and adversely affecting crop productivity and human life. Both endophytic and rhizospheric microbial communities are responsible for the biodegradation of toxic organic compounds and have the capability to enhance the uptake of heavy metals by plants via phytoremediation approaches. The diverse set of metabolic genes encoding for the production of biosurfactants and biofilms, specific enzymes for degrading plant polymers, modification of cell surface hydrophobicity and various detoxification pathways for the organic pollutants, plays a significant role in bacterial driven bioremediation. Various genetic engineering approaches have been demonstrated to modulate the activity of specific microbial species in order to enhance their detoxification potential. Certain rhizospheric bacterial communities are genetically modified to produce specific enzymes that play a role in degrading toxic pollutants. Few studies suggest that the overexpression of extracellular enzymes secreted by plant, fungi or rhizospheric microbes can improve the degradation of specific organic pollutants in the soil. Plants and microbes dwell synergistically, where microbes draw benefit by nutrient acquisition from root exudates whereas they assist in plant growth and survival by producing certain plant growth promoting metabolites, nitrogen fixation, phosphate solubilization, auxin production, siderophore production, and inhibition or suppression of plant pathogens. Thus, the plant-microbe interaction establishes the foundation of the soil nutrient cycle as well as decreases soil toxicity by the removal of harmful pollutants.

Conclusion

The perspective of integrating genetic approach with bioremediation is crucial to evaluate connexions among microbial communities, plant communities and ecosystem processes with a focus on improving phytoremediation of contaminated sites.

Arsenic accumulation and distribution in Pteris vittata fronds of different maturity: Impacts of soil As concentrations

Arsenic (As) hyperaccumulator Pteris vittata is efficient in As uptake, translocation and accumulation, but the impacts of soil As concentrations on As accumulation and distribution in P. vittata are still unclear. The impacts of soil As (7.3, 63 and 228 mg kg^-1) on plant growth and As accumulation in P. vittata after 6 months of growth were evaluated. Arsenic concentrations in the roots, midribs and pinna margin of P. vittata fronds of different maturity were determined by inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy coupled with an energy dispersive spectrometer (SEM-EDS). While moderate As level at As₆₃ didn't impact P. vittata growth, higher As at As₂₂₈ decreased plant biomass by 38%. Under As stress, more As was accumulated in the senescing fronds (47%) and mature fronds (11%) than the young fronds. In senescing fronds, As concentrations in pinna margin were 2.3 times that of the midribs, consistent with As-induced necrotic symptom. Arsenic distribution based on SEM-EDS analysis revealed good correlation between Si and As in the pinnae (r = 0.49). Our data showed that As accumulation in pinna margin caused necrosis and Si may have a potential role in As detoxification in P. vittata.

A multifunctional rhizobacterial strain with wide application in different ferns facilitates arsenic phytoremediation

Pteris vittata and Pteris multifida are widely studied As hyperaccumulators that absorb As mainly via roots. Hence, rhizobacteria exhibit promising potential in phytoextraction owing to their immense microbial diversity and interactions with plants. Pseudomonas vancouverensis strain m318 that contains aioA-like genes was screened from P. multifida's rhizosphere through the high As resistance (minimum inhibitory concentrations (MICs) against As(III): 16 mM; MICs against As(V): 320 mM), rapid As oxidation (98% oxidation by bacterial cultures (OD_600nm = 1) from 200 μL of 0.1 mM As(III) within 24 h), predominant secretion of IAA (12.45 mg L^-1) and siderophores (siderophore unit: 88%). Strain m318 showed significant chemotactic response and high colonization efficiency to P. vittata roots, which suggested its wide host affinity. Interestingly, inoculation with strain m318 enhanced the proportion of aioA-like genes in the rhizosphere. And in field trials, inoculation with strain m318 increased As accumulation in P. vittata by 48-146% and in P. multifida by 42-233%. Post-transplantation inoculations also increased As accumulation in both ferns. The abilities of the isolated multifunctional strain m318 and the increase in the rhizosphere microbial aioA-like genes are thus speculated to be involved in As transformation in the rhizospheres and roots of P. vittata and P. multifida.

Cupriavidus basilensis strain r507, a toxic arsenic phytoextraction facilitator, potentiates the arsenic accumulation by Pteris vittata

As a toxic and carcinogenic metalloid, arsenic has posed serious threat to human health. Phytoremediation has emerged as a promising approach to circumvent this problem. Arsenic uptake by Pteris vittata is largely determined by arsenic speciation and mainly occurs via roots; thus, rhizospheric microbial activities may play a key role in arsenic accumulation. The aim of this study was to investigate the potential of arsenic resistant rhizobacteria to enhance arsenic phytoextraction. A total of 49 cultivable rhizobacteria were isolated from the arsenic hyperaccumulating fern, Pteris vittata, and subjected to an initial analysis to identify potentially useful traits for arsenic phytoextraction, such as arsenic resistance and the presence of aioA(aroA)-like (arsenite oxidase-like) gene. Isolated strain r507, named as Cupriavidus basilensis strain r507, was a selected candidate for its outstanding arsenic tolerance, rapid arsenite oxidation ability, and strong colonization to P. vittata. Strain r507 was used in co-cultivation trials with P. vittata in the field for six months. Results showed that the inoculation with strain r507 potentiated As accumulation of P. vittata up to 171%. Molecular analysis confirmed that the inoculation increased the abundance of aioA-like genes in the rhizosphere, which might have facilitated arsenite oxidation and absorption. The findings of this study suggested the feasibility of co-cultivating hyperaccumulators with facilitator bacteria for practical arsenic phytoremediation.

The effect of plant growth-promoting rhizobacteria (PGPR) on arsenic accumulation and the growth of rice plants (Oryza sativa L.)

Large areas of paddy fields in southern China have been contaminated by arsenic (As), which causes serious problems such as the reduction of grain yield and high As concentrations in brown rice. To test the possibility of using plant growth-promoting rhizobacteria (PGPR) to promote rice growth and decrease As accumulation in brown rice in such paddy fields, three As-resistant PGPRs (strains S6, S7 and S10) were isolated and incorporated into two rice cultivars (Zhendao 14 and Jianyou G2) grown in different environments. The results showed that all PGPRs promoted rice growth (grain yield increased by 10.50-51.30% under greenhouse conditions and by 4.83-9.16% in paddy fields) and reduced As damage in rice. Specifically, S10 decreased As accumulation in brown rice in both greenhouse (3.50-26.01%) and paddy-field conditions (9.26-10.50%) by significantly reducing the available As concentration in rhizospheric soil, especially in the Dabaoshan-B field (34.00%). Conversely, in greenhouse condition, strains S6 and S7 increased As concentrations by 6.10-20.10% and 2.14-14.60%, respectively. Our results suggested that PGPR inoculation could be used to decrease As accumulation and promote rice growth in As-contaminated paddy fields. However, as the effects of PGPRs differed by strain and depended on environmental factors, careful selection of the strains and environmental conditions, and pre-testing, will be essential before applying PGPRs to As-contaminated paddy fields.

Implications of plant growth promoting Klebsiella sp. CPSB4 and Enterobacter sp. CPSB49 in luxuriant growth of tomato plants under chromium stress

The present study explores the potential of two chromium tolerant and plant growth promoting bacterial strains, Klebsiella sp. and Enterobacter sp. in luxuriant growth of tomato plants under chromium stress conditions. For the assessment of potentiality of the two selected strains, a pot scale experiment was setup with tomato plant under different levels of chromium contamination. In pot experiment, different plant growth parameters, oxidative stress tolerance and chromium bioremediation potential were studied upon inoculation of the selected bacterial strains. The results of pot experiment showed that both the strains were effective in promotion of plant growth and enhanced the plant biomass but Enterobacter sp. was more prominent in enhancement of root length, shoot length, fresh and dry weight, and nutrient uptake in tomato plant. The enhancement of enzymes to combat oxidative stress in tomato plant under chromium stress was also observed for both the strains. Both strains enhanced the levels of superoxide dismutase, catalase, peroxidase, total phenolic, and ascorbic acid in tomato plant under different levels of chromium stress conditions. The chromium phytoremediation potential of tomato plant upon inoculation of both the strains was also studied. The results of phytoremediation showed greater chromium accumulation in roots with poor translocation in shoot upon inoculation of Klebsiella sp. while no significant enhancement in chromium uptake by tomato plant was observed on inoculation of Enterobacter sp. compared to control. Thus, these two strains can effectively be used in luxuriant growth of tomato plant under metal stress conditions.

A comparative evaluation towards the potential of Klebsiella sp. and Enterobacter sp. in plant growth promotion, oxidative stress tolerance and chromium uptake in Helianthus annuus (L.)

Prevalence of metal pollutants exerts negative effects on human health and environment, thus propounding an urgent need for a safer substitute. This study was conducted to compare the chromium bioremediation and plant growth promotion ability of two bacterial strains, Klebsiella sp. strain CPSB4 (MH266218) and Enterobacter sp. strain CPSB49 (MH532567), isolated from the rhizospheric soils. A pot scale experiment was setup with Helianthus annuus (L.) as a test plant to compare the efficiency of both isolates in enhancement of plant growth, nutrients uptake, anti-oxidative enzymes production, lipid peroxidation, and chromium bioremediation. Inoculation of strains, CPSB4 and CPSB49 enhanced plant biomass, plant growth, nutrient uptake, anti-oxidative enzymes, and chromium bioremediation, while reduction in lipid peroxidation was observed compared to uninoculated control under chromium stress. The maximum increase in plant growth and nutrient uptake was found in treatments inoculated with CPSB49, while maximum chromium uptake by sunflower was observed in treatments inoculated with CPSB4. Moreover, an increase in anti-oxidative enzyme production and decrease in lipid peroxidation was observed on inoculation of the selected strains. Thus, the strains Klebsiella sp. and Enterobacter sp. can be effectively used in chromium bioremediation and plant growth promotion under chromium stress conditions.

Anthropogenic remediation of heavy metals selects against natural microbial remediation

In an era of unprecedented environmental change, there have been increasing ecological and global public health concerns associated with exposure to anthropogenic pollutants. While there is a pressing need to remediate polluted ecosystems, human intervention might unwittingly oppose selection for natural detoxification, which is primarily carried out by microbes. We test this possibility in the context of a ubiquitous chemical remediation strategy aimed at targeting metal pollution: the addition of lime-containing materials. Here, we show that raising pH by liming decreased the availability of toxic metals in acidic mine-degraded soils, but as a consequence selected against microbial taxa that naturally remediate soil through the production of metal-binding siderophores. Our results therefore highlight the crucial need to consider the eco-evolutionary consequences of human environmental strategies on microbial ecosystem services and other traits.

Metal tolerance of arsenic-resistant bacteria and their ability to promote plant growth of Pteris vittata in Pb-contaminated soil

Soils contaminated with Pb and As are difficult to remediate. In this study, the utility of coupling As-hyperaccumulator Pteris vittata with metal-resistant rhizobacteria was explored. Siderophore-producing and P-solubilizing As-resistant bacteria from the P. vittata rhizosphere were screened for resistance to multiple metals. Results indicated Pseudomonas spp. strain PG-12 was most efficient in resisting multiple metals, i.e., up to 0.6 mM Cd and 10 mM Pb. Amplification of gene fragments encoding various metal efflux transporters (PbrA and CadA2) from genomic DNA of PG-12 suggested that metal efflux might play a role in its metal resistance and detoxification. In addition, PG-12 produced significant levels of plant growth hormones including 17.4 μg mL^-1 indole acetic acid and 3.54 μg mL^-1 gibberellin. P. vittata sporophytes inoculated with PG-12 were grown in Pb-contaminated medium and exhibited improved growth, increased P uptake, and reduced Pb uptake into plant tissue compared to the control. Results demonstrated that viable PG-12 cells were responsible for Pb immobilization and plant growth enhancement in P. vittata. The ability of PG-12 cells to solubilize P and display resistance to multiple metals combined with the production of plant hormones indole acetic acid and gibberellin make PG-12 a suitable candidate for plant growth promotion in metal-contaminated soil.

Dissipation characteristics of pyrene and ecological contribution of submerged macrophytes and their biofilms-leaves in constructed wetland

The dissipation characteristics of pyrene and ecological contribution of submerged macrophytes (Vallisneria natans, Hydrilla verticillata and artificial plant) and their biofilms-leaves in constructed wetland system were investigated by high-throughput sequencing. Results showed a non-negligible ecological contribution of submerged macrophytes and their biofilms-leaves in dissipating pyrene in planted sediments (33.25-43.00%) in contrast with unplanted system (23.25-26.50%). Alpha and Beta diversity analysis indicated that bacterial community structures were influenced in order of attached surface > incubation time > spiking concentration of pyrene in sediment. Difference analysis suggested that attached surface tended to affect the rare genera up to significant level compared with incubation time. In addition, RDA analysis showed that PAH-degrading bacteria in biofilms-leaves of submerged macrophytes were positively correlated with the dissipation ratios of pyrene. Interestingly, nitrogen transforming microorganisms were also influenced in the presence of pyrene, while submerged macrophytes contributed to alleviate the impact.

The role of arsenic resistant Bacillus aryabhattai MCC3374 in promotion of rice seedlings growth and alleviation of arsenic phytotoxicity

The biological agents have been utilized as an affordable alternative to conventional costly metal remediation technologies for last few years. The present investigation introduces arsenic (As) resistant plant growth promoting rhizobacteria (PGPR) isolated from the As-contaminated agricultural field of West Bengal, India that alleviates arsenic-induced toxicity and exhibited many plant growth promoting traits (PGP). The isolated strain designated as AS6 has identified as Bacillus aryabhattai based on phenotypic characteristics, physio-biochemical tests, MALDI-TOFMS bio-typing, FAME analysis and 16S rDNA sequence homology. The strain found to exhibit five times more resistance to arsenate than arsenite with minimum inhibitory concentrations (MIC) being 100 mM and 20 mM respectively. The result showed that accumulation of As was evidenced by SEM- EDAX, TEM-EDAX studies. The intracellular accumulation of arsenic was also confirmed as in bacterial biomass by AAS, FTIR, XRD and XRF analyses. The increased rate of As (V) reduction by this strain found to be exploited for the remediation of arsenic in the contaminated agricultural field. The strain also found to exhibit important PGP traits viz., ACC deaminase activity (2022 nmol α-ketobutyrate/mg protein/h), IAA production (166 μg/ml), N₂ fixation (0.32 μgN fixed/h/mg proteins) and siderophore production (72%) etc. Positive influenced of AS6 strain on rice seedlings growth promotion under As stress was observed considering the several morphological, biochemical parameters including antioxidants activities as compared with an uninoculated set. Thus this strain might be exploited for stress amelioration and plant growth enhancement of rice cultivar under arsenic spiked agricultural soil.

Phytate promoted arsenic uptake and growth in arsenic-hyperaccumulator Pteris vittata by upregulating phosphorus transporters

While phosphate (P) inhibits arsenic (As) uptake by plants, phytate increases As uptake by As-hyperaccumulator Pteris vittata. Here we tried to understand the underling mechanisms by investigating the roles of phytate in soil As desorption, P transport in P. vittata, short-term As uptake, and plant growth and As accumulation from soils. Sterile soil was used to exclude microbial degradation on phytate. Results showed that inorganic P released 3.3-fold more As than that of phytate from soil. However, P. vittata accumulated 2-2.5 fold more As from soils with phytate than that in control and P treatment. In addition, different from P suppression on As uptake, solution uptake experiment showed that As uptake in phytate treatment was comparable to that of control under 0.1-7.5 μM As after 1-24 h. Moreover, responding to phytate, P. vittata P transporter PvPht1;3 increased by 3-fold while PvPht1;1 decreased by 65%. The data suggested that phytate upregulated PvPht1;3, thereby contributing to As uptake in P. vittata. Our results showed that, though with lower As release from soil compared to P, phytate induced more As uptake and better growth in P. vittata by upregulating P transporters.

Arsenic-induced nutrient uptake in As-hyperaccumulator Pteris vittata and their potential role to enhance plant growth

It is known that arsenic (As) promotes growth of As-hyperaccumulator Pteris vittata (PV), however, the associated mechanisms are unclear. Here we examined As-induced nutrient uptake in P. vittata and their potential role to enhance plant growth in sterile agar by excluding microbial effects. As-hyperaccumulator P. multifida (PM) and non-hyperaccumulator P. ensiformis (PE) belonging to the Pteris genus were used as comparisons. The results showed that, after 40 d of growth, As induced biomass increase in hyperaccumulators PV and PM by 5.2-9.4 fold whereas it caused 63% decline in PE. The data suggested that As played a beneficial role in promoting hyperaccumulator growth. In addition, hyperaccumulators PV and PM accumulated 7.5-13, 1.4-3.6, and 1.8-4.4 fold more As, Fe, and P than the non-hyperaccumulator PE. In addition, nutrient contents such as K and Zn were also increased while Ca, Mg, and Mn decreased or unaffected under As treatment. This study demonstrated that As promoted growth in hyperaccumulators and enhanced Fe, P, K, and Zn uptake. Different plant growth responses to As among hyperaccumulators PV and PM and non-hyperaccumulator PE may help to better understand why hyperaccumulators grow better under As-stress.

A Genomic Outlook on Bioremediation: The Case of Arsenic Removal

Microorganisms play a major role in biogeochemical cycles. As such they are attractive candidates for developing new or improving existing biotechnological applications, in order to deal with the accumulation and pollution of organic and inorganic compounds. Their ability to participate in bioremediation processes mainly depends on their capacity to metabolize toxic elements and catalyze reactions resulting in, for example, precipitation, biotransformation, dissolution, or sequestration. The contribution of genomics may be of prime importance to a thorough understanding of these metabolisms and the interactions of microorganisms with pollutants at the level of both single species and microbial communities. Such approaches should pave the way for the utilization of microorganisms to design new, efficient and environmentally sound remediation strategies, as exemplified by the case of arsenic contamination, which has been declared as a major risk for human health in various parts of the world.

Effective rhizoinoculation and biofilm formation by arsenic immobilizing halophilic plant growth promoting bacteria (PGPB) isolated from mangrove rhizosphere: A step towards arsenic rhizoremediation

Arsenic (As) uptake by plants is largely influenced by the presence of microbial consortia and their interactions with As. In the coastal region of Bengal deltaic plain of Eastern India, the As-contaminated groundwater is frequently used for irrigation purposes resulting in an elevated level of soil As in agricultural lands. The health hazards associated with As necessitates development of cost-effective remediation strategies to reclaim contaminated agricultural lands. Among the available technologies developed in recent times, bioremediation using bacteria has been found to be the most propitious. In this study, two As-resistant halophilic bacterial strains Kocuria flava AB402 and Bacillus vietnamensis AB403 were isolated, identified and characterized from mangrove rhizosphere of Sundarban. The isolates, AB402 and AB403, could tolerate 35mM and 20mM of arsenite, respectively. The effect of As on the exopolysaccharide (EPS) synthesis, biofilm formation, and root association was evaluated for both the bacterial strains. Arsenic adsorption on the cell surfaces and intracellular accumulation in both the bacterial strains were promising under culture conditions. Moreover, both the strains when used as inoculum, not only promoted the growth of rice seedlings but also decreased As uptake and accumulation in plants.

Bacteria from the rhizosphere and tissues of As-hyperaccumulator Pteris vittata and their role in arsenic transformation

Arsenic (As)-resistant bacteria are abundant in the rhizosphere and tissues of As-hyperaccumulator Pteris vittata. However, little is known about their roles in As transformation and As uptake in P. vittata. In this study, the impacts of P. vittata tissue extracts with or without surface sterilization on As transformation in solutions containing 100 μg L^-1 AsIII or AsV were investigated. After 48 h incubation, the sterilized and unsterilized root extracts resulted in 45% and 73% oxidation of AsIII, indicating a role of both rhizobacteria and endobacteria. In contrast, AsV reduction was only found in rhizome and frond extracts at 3.7-24% of AsV. A total of 37 strains were isolated from the tissue extracts, which are classified into 18 species based on morphology and 16S rRNA. Phylogenic analysis showed that ∼44% isolates were Firmicutes and others were Proteobacteria except for one strain belonging to Bacteroidetes. While most endobacteria were Firmicutes, most rhizobacteria were Proteobacteria. All isolated bacteria belonged to AsV reducers except for an As-sensitive strain and one AsIII- oxidizer PVR-YHB6-1. Since As transformation was not observed in solutions after filtrating or boiling, we concluded that both rhizobacteria and endobacteria were involved in As transformation in the rhizosphere and tissues of P. vittata.

Interactions between Pteris vittata L. genotypes and a polycyclic aromatic hydrocarbon (PAH)-degrading bacterium (Alcaligenes sp.) in arsenic uptake and PAH-dissipation

The effects of two Pteris vittata L. accessions and a polycyclic aromatic hydrocarbon (PAH)-degrading bacterium (Alcaligenes sp.) on arsenic (As) uptake and phenanthrene dissipation were studied. The Alcaligenes sp. survived in the rhizosphere and improved soil As bioavailability with co-exposure. However, bacterial inoculation altered Pteris vittata L. stress tolerance, and substantially affected the As distribution in the rhizosphere of the two P. vittata accessions. Bacterial inoculation was beneficial to protect the Guangxi accession against the toxic effects, and significantly increased plant As and phenanthrene removal ratios by 27.8% and 2.89%, respectively. In contrast, As removal was reduced by 29.8% in the Hunan accession, when compared with corresponding non-inoculated treatments. We conclude that plant genotype selection is critically important for successful microorganism-assisted phytoremediation of soil co-contaminated with As and PAHs, and appropriate genotype selection may enhance remediation efficiency.

Application of drum compost and vermicompost to improve soil health, growth, and yield parameters for tomato and cabbage plants

Utilization of different types of solid wastes through composting is important for environmental sustainability and restoring soil quality. Although drum composting is an efficient technology, the possibility of heavy metal contamination restricts its large-scale use. In this research, a field experiment was conducted to evaluate the impact of water hyacinth drum compost (DC) and traditional vermicompost (VC) on soil quality and crop growth in an agro-ecosystem cultivated intensively with tomato and cabbage as test crops. A substantial improvement in soil health was observed with respect to nutrient availability, physical stability, and microbial diversity due to the application of drum compost and traditional vermicompost. Moreover, soil organic carbon was enriched through increased humic and fulvic acid carbon. Interestingly, heavy metal contamination was less significant in vermicompost-treated soils than in those receiving the other treatments. The use of VC and DC in combination with recommended chemical fertilization effectively stimulated crop growth, yield, product quality, and storage longevity for both tomato and cabbage.

Mechanisms of efficient As solubilization in soils and As accumulation by As-hyperaccumulator Pteris vittata

Arsenic (As) in soils is of major environmental concern due to its ubiquity and carcinogenicity. Pteris vittata (Chinese brake fern) is the first known As-hyperaccumulator, which is highly efficient in extracting As from soils and translocating it to the fronds, making it possible to be used for phytoremediation of As-contaminated soils. In addition, P. vittata has served as a model plant to study As metabolisms in plants. Based on the recent advances, we reviewed the mechanisms of efficient As solubilization and transformation in rhizosphere soils of P. vittata and effective As uptake, translocation and detoxification in P. vittata. We also provided future research perspectives to further improve As phytoremediation by P. vittata.

Copper-tolerant rhizosphere bacteria-characterization and assessment of plant growth promoting factors

Remediation of heavy metal contaminated soil is a major problem or concern worldwide. Heavy metal accumulation in the soil is increasing day by day by industries, mines, agriculture, fuel combustion and municipal waste discharge. Such contaminated soils harbour a large number of resistant microbial populations. Screening and isolation of such microbes would be utilized for natural remediation of metal contaminated soils. Therefore, in the present study, highly copper-tolerant bacteria from rhizosphere soil of Cynodon dactylon grown in brass effluent contaminated soil were isolated and assessed for plant growth promoting factors. A total of 61 isolates were isolated from the rhizosphere of three contaminated sites. Six highly copper-tolerant isolates named as MYS1, MYS2, MYS3, MYS4, MYS5 and MYS6 were isolated through enrichment in copper containing nutrient broth. 16S rRNA analysis revealed that the isolates were from genera Stenotrophomonas and Brevundimonas and belong to classes Alpha Proteobacteriacea and Gamma Proteobacteriacea, respectively. Strain MYS1, MYS2 and MYS4 showed 95-99% similarity with Stenotrophomonas acidaminiphila, strain MYS3 and MYS5 showed 99 and 97% similarity with Stenotrophomonas maltophilia and Stenotrophomonas sp. Strain MYS6 showed 94% similarity with Brevundimonas diminuta. All the rhizobacteria showed plant growth promoting traits such as production of siderophores, indole acetic acid (IAA), phosphate solubilization and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity. From this study, we can conclude that all the isolates possess copper resistance and potential for phytoremediation of copper polluted soils.

Microbial siderophores and root exudates enhanced goethite dissolution and Fe/As uptake by As-hyperaccumulator Pteris vittata

Arsenic (As) in soils is often adsorbed on Fe-(hydro)oxides surface, rendering them more resistant to dissolution, which is undesirable for phytoremediation of As-contaminated soils. Arsenic hyperaccumulator Pteris vittata prefers to grow in calcareous soils where available Fe and As are low. To elucidate its mechanisms of acquiring Fe and As from insoluble sources in soils, we investigated dissolution of goethite with pre-adsorbed arsenate (AsV; As-goethite) in presence of four organic ligands, including two root exudates (oxalate and phytate, dominant in P. vittata) and two microbial siderophores (PG12-siderophore and desferrioxamine B). Their presence increased As solubilization from As-goethite from 0.03 to 0.27-5.33 mg L^-1 compared to the control. The siderophore/phytate bi-ligand treatment released 7.42 mg L^-1 soluble Fe, which was 1.2-fold that of the sum of siderophore and phytate, showing a synergy in promoting As-goethite dissolution. In the ligand-mineral-plant system, siderophore/phytate was most effective in releasing As and Fe from As-goethite. Moreover, the continuous plant uptake induced more As-goethite dissolution. The continued release of As and Fe significantly enhanced their plant uptake (from 0.01 to 0.43 mg plant^-1 As and 2.7-14.8 mg plant^-1 Fe) and plant growth (from 1.2 to 3.1 g plant^-1 fw) in P. vittata. Since microbial siderophores and root exudates often coexist in soil rhizosphere, their synergy in enhancing dissolution of insoluble As-Fe minerals may play an important role in efficient phytoremediation of As-contaminated soils.

Arsenic-enrichment enhanced root exudates and altered rhizosphere microbial communities and activities in hyperaccumulator Pteris vittata

Phytoremediation of arsenic (As)-contaminated soil by hyperaccumulator Pteris vittata is promising. A better understanding of the rhizosphere microbial dynamics that regulate As availability and plant growth is important to optimize the phytoremediation process. In this study, Illumina sequencing of 16S rRNA genes was applied to assess the rhizosphere microbial community structure of P. vittata. Microbial functionality was monitored by soil enzyme activities and MPN-PCR targeting genes of interest. Arsenic (100mgkg^-1 AsV) addition to soil significantly increased DOC, root exudates, As and P uptake and the frond biomass of P. vittata. Moreover, As-enrichment significantly increased soil enzyme activities involved in N, P and S cycling and the gene abundance of As transforming bacteria, Fe- and S-reducing bacteria and N and C fixing bacteria in the rhizosphere of P. vittata. Together, the results revealed that the combined selective pressure of As and rhizosphere resulted in stimulation of microbial community, which most likely has a role in reductive dissolution of Fe and S, As and P mobilization, C degradation and fixation, and N fixation. These changes appeared to have a role in mitigation of As toxicity and to promote growth and the As uptake ability of P. vittata under As-enriched conditions.

Arsenic and phosphate rock impacted the abundance and diversity of bacterial arsenic oxidase and reductase genes in rhizosphere of As-hyperaccumulator Pteris vittata

Microbially-mediated arsenic (As) transformation in soils affects As speciation and plant uptake. However, little is known about the impacts of As on bacterial communities and their functional genes in the rhizosphere of As-hyperaccumulator Pteris vittata. In this study, arsenite (AsIII) oxidase genes (aroA-like) and arsenate (AsV) reductase genes (arsC) were amplified from three soils, which were amended with 50mgkg^-1 As and/or 1.5% phosphate rock (PR) and grew P. vittata for 90 d. The aroA-like genes in the rhizosphere were 50 times more abundant than arsC genes, consistent with the dominance of AsV in soils. According to functional gene alignment, most bacteria belonged to α-, β- and γ-Proteobacteria. Moreover, aroA-like genes showed a higher biodiversity than arsC genes based on clone library analysis and could be grouped into nine clusters based on terminal restriction fragment length polymorphism (T-RFLP) analysis. Besides, AsV amendment elevated aroA-like gene diversity, but decreased arsC gene diversity. Redundancy analysis indicated that soil pH, available Ca and P, and AsV concentration were key factors driving diverse compositions in aroA-like gene community. This work identified new opportunities to screen for As-oxidizing and/or -reducing bacteria to aid phytoremediation of As-contaminated soils.

Identification of arsenic resistant endophytic bacteria from Pteris vittata roots and characterization for arsenic remediation application

Mitigation of arsenic (As) pollution is a topical environmental issue of high R&D priority. The present investigation was carried out to isolate As resistant endophytes from the roots of Indian ecotype Pteris vittata and characterize their As transformation and tolerance ability, plant growth promoting characteristics and their role to facilitate As uptake by the plant. A total of 8 root endophytes were isolated from plants grown in As amended soil (25 mg As kg(-1)). These isolates were studied for minimum inhibitory concentration (MIC), arsenite As(III) - arsenate As(V) transformation ability, plant growth promoting (PGP) characteristics through siderophore, indole acetic acid (IAA) production, phosphatase, ACC deaminase activity, and presence of arsenite oxidase (aox) and arsenite transporter (arsB) genes. On the basis of 16S rDNA sequence analysis, these isolates belong to Proteobacteria, Firmicutes and Bacteroidetes families under the genera Bacillus, Enterobacter, Stenotrophomonas and Rhizobium. All isolates were found As tolerant, of which one isolates showed highest tolerance up to 1000 mg L(-1) concentration in SLP medium. Five isolates were IAA positive with highest IAA production up to 60 mg/L and two isolates exhibited siderophore activity. Phosphatase activity was shown by only one isolate while ACC deaminase activity was absent in all the isolates. The As transformation study by silver nitrate test showed that only two strains had dual characteristics of As(III) oxidation and As (V) reduction, four strains exhibited either of the characteristics while other two didn't confirmed any of the two characteristics. Presence of aox gene was detected in two strains and arsB gene in six isolates. The strain with highest As tolerance also showed highest IAA production and occurrence of arsB gene. Present investigation may open up further scope of utilizing these endophytes for up gradation of phytoextraction process.

Arsenic Induced Phytate Exudation, and Promoted FeAsO4 Dissolution and Plant Growth in As-Hyperaccumulator Pteris vittata

Arsenic hyperaccumulator Pteris vittata (PV) is efficient in taking up As and nutrients from As-contaminated soils. We evaluated the mechanisms used by PV to mobilize As and Fe by examining the impacts of As and root exudates on FeAsO4 solubilization, and As and Fe uptake in four plants: As-hyperaccumulators PV and Pteris multifida (PM), nonhyperaccumulator Pteris ensiformis (PE), and angiosperm plant tomato (Solanum lycopersicum). Phytate and oxalate were dominant in fern plants (>93%), which were 50-83, 15-42, and 0-32 mg kg(-1) phytate and 10-15, 7-26, and 4-12 mg kg(-1) oxalate for PV, PM, and PE respectively, with higher As inducing greater phytate exudation and no phytate being detected in tomato exudates. PV treated with phytate+FeAsO4 had higher As and Fe contents and larger biomass than phytate or FeAsO4 treatment, which were 340 vs 20 and 130 mg kg(-1) As in the fronds and 7900 vs 1600 and 4100 mg kg(-1) Fe in the roots. We hypothesized that As-induced phytate exudation helped PV to take up Fe and As from insoluble FeAsO4 and promoted PV growth. Our study suggests that phytate exudation may be special to fern plants, which may play an important role in enhancing As and nutrient uptake by plants, thereby increasing their efficiency in phytoremediation of As-contaminated soils.

Arsenic uptake, arsenite efflux and plant growth in hyperaccumulator Pteris vittata: Role of arsenic-resistant bacteria

Bacteria-mediated arsenic (As) transformation and their impacts on As and P uptake and plant growth in As-hyperaccumulator Pteris vittata (PV) were investigated under sterile condition. All As-resistant bacteria (9 endophytic and 6 rhizospheric) were As-reducers except one As-oxidizer. After growing two months in media with 37.5 mg kg(-1) AsV, As concentrations in the fronds and roots were 3655-5389 (89-91% AsIII) and 971-1467 mg kg(-1) (41-73% AsIII), corresponding to 22-52% decrease in the As in the media. Bacterial inoculation enhanced As and P uptake by up to 47 and 69%, and PV growth by 20-74%, which may be related to elevated As and P in plants (r = 0.88-0.97, p < 0.05). Though AsV was supplied, 95% of the As in the bacteria-free media was AsIII, suggesting efficient efflux of AsIII by PV roots (120 µg g(-1) root fw). This was supported by the fact that no AsV was detected in media inoculated with As-reducers while 95% of AsV was detected with As-oxidizer. Our data showed that, under As-stress, PV reduced As toxicity by efficient AsIII efflux into media and AsIII translocation to the fronds, and bacteria benefited PV growth probably via enhanced As and P uptake.

Arsenic transformation and plant growth promotion characteristics of As-resistant endophytic bacteria from As-hyperaccumulator Pteris vittata

The ability of As-resistant endophytic bacteria in As transformation and plant growth promotion was determined. The endophytes were isolated from As-hyperaccumulator Pteris vittata (PV) after growing for 60 d in a soil containing 200 mg kg(-1) arsenate (AsV). They were isolated in presence of 10 mM AsV from PV roots, stems, and leaflets, representing 4 phyla and 17 genera. All endophytes showed at least one plant growth promoting characteristics including IAA synthesis, siderophore production and P solubilization. The root endophytes had higher P solubilization ability than the leaflet (60.0 vs. 18.3 mg L(-1)). In presence of 10 mM AsV, 6 endophytes had greater growth than the control, suggesting As-stimulated growth. Furthermore, root endophytes were more resistant to AsV while the leaflet endophytes were more tolerant to arsenite (AsIII), which corresponded to the dominant As species in PV tissues. Bacterial As resistance was positively correlated to their ability in AsV reduction but not AsIII oxidation. The roles of those endophytes in promoting plant growth and As resistance in P. vittata warrant further investigation.