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

Bacteria associated with Comamonadaceae are key arsenite oxidizer associated with Pteris vittata root

Pteris vittata (P. vittata), an arsenic (As) hyperaccumulator commonly used in the phytoremediation of As-contaminated soils, contains root-associated bacteria (RAB) including those that colonize the root rhizosphere and endosphere, which can adapt to As contamination and improve plant health. As(III)-oxidizing RAB can convert the more toxic arsenite (As(III)) to less toxic arsenate (As(V)) under As-rich conditions, which may promote plant survial. Previous studies have shown that microbial As(III) oxidation occurs in the rhizospheres and endospheres of P. vittata. However, knowledge of RAB of P. vittata responsible for As(III) oxidation remained limited. In this study, members of the Comamonadaceae family were identified as putative As(III) oxidizers, and the core microbiome associated with P. vittata roots using DNA-stable isotope probing (SIP), amplicon sequencing and metagenomic analysis. Metagenomic binning revealed that metagenome assembled genomes (MAGs) associated with Comamonadaceae contained several functional genes related to carbon fixation, arsenic resistance, plant growth promotion and bacterial colonization. As(III) oxidation and plant growth promotion may be key features of RAB in promoting P. vittata growth. These results extend the current knowledge of the diversity of As(III)-oxidizing RAB and provide new insights into improving the efficiency of arsenic phytoremediation.

Arbuscular mycorrhizal fungi enhanced the drinking water treatment residue-based vertical flow constructed wetlands on the purification of arsenic-containing wastewater

Arsenic (As) is a toxic metalloid that poses a potential risk to the environment and human health. In this study, drinking water treatment residue (DWTR) and ceramsite-based vertical flow constructed wetlands (VFCWs) were built to purify As-containing wastewater. As a method of bioaugmentation, arbuscular mycorrhizal fungi (AMF) was inoculated to Pteris vittata roots to enhance the As removal of the VFCWs. The results showed that the As removal rates reached 87.82-94.29% (DWTR) and 33.28-58.66% (ceramsite). DWTR and P. vittata contributed 64.33-72.07% and 7.57-29% to the removal of As, while AMF inoculation intensified the As accumulation effect of P. vittata. Proteobacteria, the main As3+ oxidizing bacteria in the aquatic systems, dominated the microbial community, occupying 72.41 ± 7.76%. AMF inoculation increased As-related functional genes abundance in DWTR-based wetlands and provided a reliable means of arsenic resistance in wetlands. These findings indicated that the DWTR-based VFCWs with AMF inoculated P. vittata had a great purification effect on As-containing wastewater, providing a theoretical basis for the application of DWTR and AMF for As removal in constructed wetlands.

Arsenic pollution cycle, toxicity and sustainable remediation technologies: A comprehensive review and bibliometric analysis

Arsenic pollution and its allied impacts on health are widely reported and have gained global attention in the last few decades. Although the natural distribution of arsenic is limited, anthropogenic activities have increased its mobility to distant locations, thereby increasing the number of people affected by arsenic pollution. Arsenic has a complex biogeochemical cycle which has a significant role in pollution. Therefore, this review paper has comprehensively analysed the biogeochemical cycle of arsenic which can dictate the occurrence of arsenic pollution. Considering the toxicity and nature of arsenic, the present work has also analysed the current status of arsenic pollution around the world. It is noted that the south of Asia, West-central Africa, west of Europe and Latin America are major hot spots of arsenic pollution. Bibliometric analysis was performed by using scopus database with specific search for keywords such as arsenic pollution, health hazards to obtain the relevant data. Scopus database was searched for the period of 20 years from year 2003-2023 and total of 1839 articles were finally selected for further analysis using VOS viewer. Bibliometric analysis of arsenic pollution and its health hazards has revealed that arsenic pollution is primarily caused by anthropogenic sources and the key sources of arsenic exposure are drinking water, sea food and agricultural produces. Arsenic pollution was found to be associated with severe health hazards such as cancer and other health issues. Thus considering the severity of the issue, few sustainable remediation technologies such as adsorption using microbes, biological waste material, nanomaterial, constructed wetland, phytoremediation and microorganism bioremediation are proposed for treating arsenic pollution. These approaches are environmentally friendly and highly sustainable, thus making them suitable for the current scenario of environmental crisis.

Bayesian network highlights the contributing factors for efficient arsenic phytoextraction by Pteris vittata in a contaminated field

Phytoextraction is a low-cost and eco-friendly method for removing pollutants, such as arsenic (As), from contaminated soil. One of the most studied As hyperaccumulators for soil remediation include Pteris vittata. Although phytoextraction using plant-assisted microbes has been considered a promising soil remediation method, microbial harnessing has not been achieved due to the complex and difficult to understand interactions between microbes and plants. This problem can possibly be addressed with a multi-omics approach using a Bayesian network. However, limited studies have used Bayesian networks to analyze plant-microbe interactions. Therefore, to understand this complex interaction and to facilitate efficient As phytoextraction using microbial inoculants, we conducted field cultivation experiments at two sites with different total As contents (62 and 8.9 mg/kg). Metabolome and microbiome data were obtained from rhizosphere soil samples using nuclear magnetic resonance and high-throughput sequencing, respectively, and a Bayesian network was applied to the obtained multi-omics data. In a highly As-contaminated site, inoculation with Pseudomonas sp. strain m307, which is an arsenite-oxidizing microbe having multiple copies of the arsenite oxidase gene, increased As concentration in the shoots of P. vittata to 157.5 mg/kg under this treatment; this was 1.5-fold higher than that of the other treatments. Bayesian network demonstrated that strain m307 contributed to As accumulation in P. vittata. Furthermore, the network showed that microbes belonging to the MND1 order positively contributed to As accumulation in P. vittata. Based on the ecological characteristics of MND1, it was suggested that the rhizosphere of P. vittata inoculated with strain m307 was under low-nitrogen conditions. Strain m307 may have induced low-nitrogen conditions via arsenite oxidation accompanied by nitrate reduction, potentially resulting in microbial iron reduction or the prevention of microbial iron oxidation. These conditions may have enhanced the bioavailability of arsenate, leading to increased As accumulation in P. vittata.

Sustainable Applications of Endophytic Bacteria and Their Physiological/Biochemical Roles on Medicinal and Herbal Plants: Review

Bacterial endophytes reside within the tissues of living plant species without causing any harm or disease to their hosts. These endophytes can be isolated, identified, characterized, and used as biofertilizers. Moreover, bacterial endophytes increase the plants' resistance against diseases, pests, and parasites, and are a promising source of pharmaceutically important bioactives. For instance, the production of antibiotics, auxins, biosurfactants, cytokinin's, ethylene, enzymes, gibberellins, nitric oxide organic acids, osmolytes, and siderophores is accredited to the existence of various bacterial strains. Thus, this manuscript intends to review the sustainable applications of endophytic bacteria to promote the growth, development, and chemical integrity of medicinal and herbal plants, as well as their role in plant physiology. The study of the importance of bacterial endophytes in the suppression of diseases in medicinal and herbal plants is crucial and a promising area of future investigation.

As(III) oxidation and kinetic analysis by Herminiimonas arsenicoxydans-loaded electrospinning activated carbon fiber biofilms

In this study, an integrated and assembled recyclable biofilm material was prepared by loading Herminiimonas arsenicoxydans (H. arsenicoxydans) onto electrospun biomass-activated carbon nanofibers (denoted as H. arsenicoxydans-BACFs films). The H. arsenicoxydans-BACFs biofilms showed an approximately 50% increase in As(III) removal rate for 50 mg/L during a 48-h incubation. Furthermore, the biofilms demonstrated satisfactory biocompatibility, ideal catalytic As(III) oxidation and excellent recyclability in cyclic reactions (at least 5 runs). The improved catalytic efficiency is mainly due to a large amount of biomass accumulation and biofilms formation on the surface of the BACF films. More important, the BACF films as an electron transport medium from an oxidized state to a reduced state promote the electron transfer of As(III) oxidation of H. arsenicoxydans. The dual factors can synergistically promote As(III) oxidation efficiency. The oxidation process of As(III) in the H. arsenicoxydans-BACFs composite biofilm reactor was more in line with the first-order kinetic equation, and the oxidation rate of As(III) by H. arsenicoxydans-BACF_0.4 was the fastest. The H. arsenicoxydans-BACF films outperformed conventional catalytic materials and could represent biomaterials for the remediation of As(III)-contaminated wastewater.

Role of plant growth-promoting rhizobacteria in boosting the phytoremediation of stressed soils: Opportunities, challenges, and prospects

Soil is considered as a vital natural resource equivalent to air and water which supports growth of the plants and provides habitats to microorganisms. Changes in soil properties, productivity, and, inevitably contamination/stress are the result of urbanisation, industrialization, and long-term use of synthetic fertiliser. Therefore, in the recent scenario, reclamation of contaminated/stressed soils has become a potential challenge. Several customized, such as, physical, chemical, and biological technologies have been deployed so far to restore contaminated land. Among them, microbial-assisted phytoremediation is considered as an economical and greener approach. In recent decades, soil microbes have successfully been used to improve plants' ability to tolerate biotic and abiotic stress and strengthen their phytoremediation capacity. Therefore, in this context, the current review work critically explored the microbial assisted phytoremediation mechanisms to restore different types of stressed soil. The role of plant growth-promoting rhizobacteria (PGPR) and their potential mechanisms that foster plants' growth and also enhance phytoremediation capacity are focussed. Finally, this review has emphasized on the application of advanced tools and techniques to effectively characterize potent soil microbial communities and their significance in boosting the phytoremediation process of stressed soils along with prospects for future research.

Rhizospheric plant-microbe synergistic interactions achieve efficient arsenic phytoextraction by Pteris vittata

Phytoextraction is a cost-effective and eco-friendly technology to remove arsenic (As) from contaminated soil using plants and associated microorganisms. Pteris vittata is the most studied As hyperaccumulator, which effectively takes up inorganic arsenate via roots. Arsenic solubilization and speciation occur prior to plant absorption in the rhizosphere, which play a key role in As phytoextraction by P. vittata. This study investigated the metabolomic correlation of P. vittata and associated rhizospheric microorganisms during As phytoextraction. Three-month pot cultivation of P. vittata in As polluted soil was conducted. In rhizosphere, an increase of water-soluble As concentration and a decrease of pH was observed in the second month, suggesting acidic metabolites as a possible cause of As solubilization. A correlation network was built to elucidate the interactions among metabolites, bacteria and fungi in the rhizosphere of P. vittata. Our results demonstrate that the plant is the major driving force of rhizospheric microbiota generation, and both microbial community and metabolites in rhizosphere of P. vittata correlate to increased bioavailable As. Multi-omics analysis revealed that pterosins enrich microbes that potentially promote As phytoextraction. This study extends the current view of rhizospheric plant-microbes synergistic effects of hyperaccumulators on phytoextraction, which provides clues for developing efficient As phytoremediation approaches.

Bacterial Microbiome in the Phyllo-Endosphere of Highly Specialized Rock Spleenwort

Bacteria communities associated with plants have been given increasing consideration because they are arguably beneficial to their host plants. To understand the ecological and evolutionary impact of these mutualistic associations, it is important to explore the vast unknown territory of bacterial genomic diversity and their functional contributions associated with the major branches of the tree-of-life. Arguably, this aim can be achieved by profiling bacterial communities by applying high throughput sequencing approaches, besides establishing model plant organisms to test key predictions. This study utilized the Illumina Miseq reads of bacterial 16S rRNA sequences to determine the bacterial diversity associated with the endosphere of the leaves of the highly specialized rock spleenwort Asplenium delavayi (Aspleniaceae). By documenting the bacterial communities associated with ferns collected in natural occurrence and cultivation, this study discovered the most species-rich bacterial communities associated with terrestrial ferns reported until now. Despite the substantial variations of species diversity and composition among accessions, a set of 28 bacterial OTUs was found to be shared among all accessions. Functional analyses recovered evidence to support the predictions that changes in bacterial community compositions correspond to functional differentiation. Given the ease of cultivating this species, Asplenium delavayi is introduced here as a model organism to explore the ecological and evolutionary benefits created by mutualistic associations between bacteria and ferns.

Siderophores: an alternative bioremediation strategy?

Siderophores are small molecular weight iron scavengers that are mainly produced by bacteria, fungi, and plants. Recently, they have attracted increasing attention because of their potential role in environmental bioremediation. Although siderophores are generally considered to exhibit high specificity for iron, they have also been reported to bind to various metal and metalloid ions. This unique ability allows siderophores to solubilise and mobilise heavy metals and metalloids from soil, thereby facilitating their bioremediation. In addition, because of their redox nature, they can mediate the production of reactive oxygen species (ROS), and thus promote the biodegradation of organic contaminants. The aim of this review is to summarise the existing knowledge on the developed strategies of siderophore-assisted bioremediation of metals, metalloids, and organic contaminants. Additionally, this review also includes the biosynthesis and classification of microbial and plant siderophores.

Empirical Evidence of Arsenite Oxidase Gene as an Indicator Accounting for Arsenic Phytoextraction by Pteris vittata

Arsenic (As) is a toxic semi-metallic element that is ubiquitous in the environment and poses serious human health risks. Phytoextraction by Pteris vittata is considered a low-cost and environmentally friendly approach to treat As-contaminated soil. P. vittata mainly absorbs arsenate thus the bioavailability of As to P. vittata depends on the chemical form of As. Microbial redox of As contributes to the biogeochemical cycling of As, and rhizobacterium-assisted phytoextraction by P. vittata was proposed. In this study, this microbe-assisted phytoextraction was applied to two fields, and the effectiveness of phytoextraction was evaluated. The results revealed that P. vittata was able to grow in temperate and subarctic climate zones. The biomass was influenced by the weather, and the As concentration in plants was dependent on the As content in the soil. The ratio of arsenite oxidase genes (aioA-like genes) to 16S rRNA genes was employed to evaluate the effect of As phytoextraction, and the results exhibited that the ratio was related to the As concentration in P. vittata. Our results showed that arsenite oxidation in the rhizosphere might not be achieved by single-strain inoculation, while this study provided empirical evidence that the rhizospheric aioA-like genes could be an indicator for evaluating the effectiveness of As phytoextraction.

Phytoextraction of metal(loid)s from contaminated soils by six plant species: A field study

Phytoextraction is an in situ remediation technique that uses (hyper)accumulator plant species to extract metal(loid)s from contaminated soils. Field studies can help in selecting appropriate plants for phytoextraction and in better understanding their phytoextraction performance. Hence, a field study was conducted using six (hyper)accumulator species (Solanum nigrum L., Bidens pilosa L., Xanthium strumarium L., Helianthus annuus L., Lonicera japonica T. and Pennisetum sinese R.) over two years in Jiaoxi town, Liuyang city, Hunan Province, China, to determine the effect of the (hyper)accumulator rhizospheres on field soils contaminated with multiple metal(loid)s and to analyze the variations in rhizosphere soil microbial community diversity and composition. After two years of field experiments, compared to the other four (hyper)accumulators, Bidens pilosa L. and Xanthium strumarium L. exhibited not only better metal(loid) phytoextraction abilities but also higher shoot biomasses. The contents of diethylenetriaminepentaacetic acid (DTPA)-extractable Pb, Cd and Zn decreased in the rhizosphere soils of all six (hyper)accumulators after repeated phytoextraction. Moreover, our findings illustrated that hyperaccumulator planting helps improve and rebuild the soil bacterial community composition and structure in contaminated soils by shifting the soil physiochemical properties. After repeated planting, the soil bacterial communities were reconstructed and dominated by Proteobacteria, Actinobacteriota, Chloroflexi and Acidobacteriota at the phylum level. The soil fungal communities were dominated by Ascomycota, Basidiomycota and Mortierellomycota at the phylum level. The reconstruction of soil microbial communities may help (hyper)accumulators adapt to metal(loid)-contaminated environments and improve their phytoextraction abilities.

Arsenic Remediation through Sustainable Phytoremediation Approaches

Arsenic contamination of the environment is a serious problem threatening the health of millions of people exposed to arsenic (As) via drinking water and crops grown in contaminated areas. The remediation of As-contaminated soil and water bodies needs to be sustainable, low-cost and feasible to apply in the most affected low-to-middle income countries, like India and Bangladesh. Phytoremediation is an aesthetically appreciable and successful approach that can be used for As decontamination with use of the best approach(es) and the most promising plant(s). However, phytoremediation lacks the required speed and sometimes the stress caused by As could diminish plants’ potential for remediation. To tackle these demerits, we need augment plants’ potential with appropriate technological methods including microbial and nanoparticles applications and genetic modification of plants to alleviate the As stress and enhance As accumulation in phytoremediator plants. The present review discusses the As phytoremediation prospects of soil and water bodies and the usefulness of various plant systems in terms of high biomass, high As accumulation, bioenergy potential, and economic utility. The potential and prospects of assisted phytoremediation approaches are also presented.

Phytoextraction efficiency of Pteris vittata grown on a naturally As-rich soil and characterization of As-resistant rhizosphere bacteria

This study evaluated the phytoextraction capacity of the fern Pteris vittata grown on a natural arsenic-rich soil of volcanic-origin from the Viterbo area in central Italy. This calcareous soil is characterized by an average arsenic concentration of 750 mg kg⁻¹, of which 28% is bioavailable. By means of micro-energy dispersive X-ray fluorescence spectrometry (μ-XRF) we detected As in P. vittata fronds after just 10 days of growth, while a high As concentrations in fronds (5,000 mg kg⁻¹), determined by Inductively coupled plasma-optical emission spectrometry (ICP-OES), was reached after 5.5 months. Sixteen arsenate-tolerant bacterial strains were isolated from the P. vittata rhizosphere, a majority of which belong to the Bacillus genus, and of this majority only two have been previously associated with As. Six bacterial isolates were highly As-resistant (> 100 mM) two of which, homologous to Paenarthrobacter ureafaciens and Beijerinckia fluminensis, produced a high amount of IAA and siderophores and have never been isolated from P. vittata roots. Furthermore, five isolates contained the arsenate reductase gene (arsC). We conclude that P. vittata can efficiently phytoextract As when grown on this natural As-rich soil and a consortium of bacteria, largely different from that usually found in As-polluted soils, has been found in P. vittata rhizosphere.

Keep and promote biodiversity at polluted sites under phytomanagement

The phytomanagement concept combines a sustainable reduction of pollutant linkages at risk-assessed contaminated sites with the generation of both valuable biomass for the (bio)economy and ecosystem services. One of the potential benefits of phytomanagement is the possibility to increase biodiversity in polluted sites. However, the unique biodiversity present in some polluted sites can be severely impacted by the implementation of phytomanagement practices, even resulting in the local extinction of endemic ecotypes or species of great conservation value. Here, we highlight the importance of promoting measures to minimise the potential adverse impact of phytomanagement on biodiversity at polluted sites, as well as recommend practices to increase biodiversity at phytomanaged sites without compromising its effectiveness in terms of reduction of pollutant linkages and the generation of valuable biomass and ecosystem services.

Long-term effectiveness of microbe-assisted arsenic phytoremediation by Pteris vittata in field trials

Phytoremediation is a promising inexpensive method of detoxifying arsenic (As) contaminated soils using plants and associated soil microorganisms. The potential of Pteris vittata to hyperaccumulate As contamination has been investigated widely. Since As(V) is efficiently taken up by P. vittata than As(III), As speciation by associated rhizobacteria could offer enormous possibility to enhance As phytoremediation. Specifically, increased rhizobacteria mediated As(III) to As(V) conversion appeared to be a crucial step in As mobilization and translocation. In this study, Pseudomonasvancouverensis strain m318 with the potential to improve As phytoremediation was inoculated to P. vittata in a field trial for three years to evaluate its long-term efficacy and stability for enhancing As phytoextraction. The biomass, As concentration, and As accumulation of ferns showed to be increased by inoculation treatment. Although this trend occasionally declined which may be accounted to lower As concentration in soil and amount of precipitation during experiments, the potential of inoculation was observed in increased enrichment coefficients. Further, the arsenite oxidase (aioA-like) genes in the rhizosphere were detected to evaluate the influence of inoculation on As phytoremediation. The findings of this study suggested the potential application of rhizosphere regulation to improve phytoremediation technologies for As contaminated soils. However, the conditions which set the efficacy of this method could be further optimized.