Mechanistic studies on the bioremediation of Cr(VI) using Sphingopyxis macrogoltabida SUK2c, a Cr(VI) tolerant bacterial isolate

Reduction of Cr(VI) by Bacillus toyonensis LBA36 and its effect on radish seedlings under Cr(VI) stress

Chromium, being among the most toxic heavy metals, continues to demand immediate attention in the remediation of Cr-contaminated environments. In this study, a strain of LBA36 (Bacillus toyonensis) was isolated from heavy metal contaminated soil in Luanchuan County, Luoyang City, China. The reduction and adsorption rates of LBA36 in 30 mg·L-1 Cr-containing medium were 97.95% and 8.8%, respectively. The reduction mechanism was confirmed by Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). Cr(VI) reduction by this strain predominantly occurred outside the cell, with hydroxyl, amide, carboxyl, C-N group, carbonyl, and sulfur carbonyl as the main reaction sites. XPS analysis revealed the presence of Cr2p1/2 and Cr2p3/2. Furthermore, the hydroponic experiment showed that the fresh weight and plant height of radish seedlings increased by 87.87% and 37.07%, respectively, after inoculation with LBA36 strain under 7 mg·L-1 Cr(VI) stress. The levels of chlorophyll, total protein, malondialdehyde, superoxide dismutase and catalase were also affected to different degrees. In conclusion, this study demonstrated the potential of microbial and phytoremediation in the treatment of heavy metal toxicity, and laid the foundation for the development of effective bioremediation methods for Cr(VI) pollution.

Adsorption of Hg2+/Cr6+ by metal-binding proteins heterologously expressed in Escherichia coli

BACKGROUND

Removal of heavy metals from water and soil is a pressing challenge in environmental engineering, and biosorption by microorganisms is considered as one of the most cost-effective methods. In this study, the metal-binding proteins MerR and ChrB derived from Cupriavidus metallidurans were separately expressed in Escherichia coli BL21 to construct adsorption strains. To improve the adsorption performance, surface display and codon optimization were carried out.

RESULTS

In this study, we constructed 24 adsorption engineering strains for Hg2+ and Cr6+, utilizing different strategies. Among these engineering strains, the M'-002 and B-008 had the strongest heavy metal ion absorption ability. The M'-002 used the flexible linker and INPN to display the merRopt at the surface of the E. coli BL21, whose maximal adsorption capacity reached 658.40 μmol/g cell dry weight under concentrations of 300 μM Hg2+. And the B-008 overexpressed the chrB in the intracellular, its maximal capacity was 46.84 μmol/g cell dry weight under concentrations 500 μM Cr6+. While in the case of mixed ions solution (including Pb2+, Cd2+, Cr6+ and Hg2+), the total amount of ions adsorbed by M'-002 and B-008 showed an increase of up to 1.14- and 4.09-folds, compared to the capacities in the single ion solution.

CONCLUSION

The construction and optimization of heavy metal adsorption strains were carried out in this work. A comparison of the adsorption behavior between single bacteria and mixed bacteria systems was investigated in both a single ion and a mixed ion environment. The Hg2+ absorption capacity is reached the highest reported to date with the engineered strain M'-002, which displayed the merRopt at the surface of chassis cell, indicating the strain's potential for its application in practical environments.

A sustainable solution for alleviating hexavalent chromium from water streams using Lactococcus lactis AM99 as a novel Cr(VI)-reducing bacterium

Chromium, extensively used in various industries, poses significant challenges due to its environmental impact. The threat of Cr(VI) causes critical concerns in aquatic ecosystems as a consequence of the fluidity of water. The conventional approach for the treatment of effluents containing Cr(VI) is reducing Cr(VI) to low-noxious Cr(III). This research is related to a Gram positive bacterium newly isolated from tannery effluent under aerobic conditions. To characterize functional groups on the isolate, Fourier transform infrared spectroscopy was utilized. The effect of different factors on Cr(VI) bioreduction was investigated, including temperature, initial Cr(VI) concentration, acetate concentration, and Tween 80 surfactant. Under optimal conditions (37 °C and 0.90 g/L sodium acetate), the bioreduction rate of the isolate, identified as Lactococcus lactis AM99, achieved 88.0 % at 300 mg/L Cr(VI) during 72 h (p < 0.05). It was observed that Cr(VI) bioreduction was enhanced by the acetate in both the quantity and intensity, while Tween 80 had no impact on the reaction. The strain AM99 exhibited remarkable characteristics, notably a marginal decrease in growth at elevated concentrations of hexavalent chromium and an exceptional potential to reduce Cr(VI) even at very low biomass levels, surpassing any prior findings in the associated research. Furthermore, The isolate could tolerate 1400 mg/L Cr(VI) in a solid medium. These distinctive features make the isolate a promising and well-suited candidate for remediating Cr(VI)-polluted environments. Additionally, the impact of biogenic extracellular polymer produced by the strain AM99 on reduction was examined at different temperatures.

Synthesis of graphite/rGO-modified fungal hyphae for chromium (VI) bioremediation process

Bioremediation is a promising technology that can eliminate the drawbacks of conventional treatment methods in removing harmful toxic metals including chromium(VI). Therefore, in this study, fungal hyphae modified with graphite and reduced graphene oxide were synthesized and assessed for their potential to bioremediate heavy metals for the first time in the literature. The effects of the carbon-based materials on microbial structure were characterized using scanning electron microscopy analysis. Thermogravimetric, RAMAN, X-ray diffraction, and enzymatic analyzes were performed to determine the role of functional groups. In addition, batch adsorption experiments utilizing response surface methodology were conducted to optimize operating parameters such as time (1-11 h), chromium (10-50 mg/L), and graphite/reduced graphene oxide (0.1-1 g/L). The maximum adsorption capacity with the graphene fungal hyphae was determined to be 568 mg.g-1, which is 9.7 times that of the crude fungal hyphae. The Cr(VI) removal for fungal hyphae-graphite and fungal hyphae-reduced graphene oxide biocomposites was 98.25% and 98.49%, respectively. The isothermal and kinetic results perfectly matched the 2nd order pseudo-model and Langmuir model in terms of the nature of the adsorption process. The laboratory scale test results indicate that fungal hyphae modified with graphite and reduced graphene oxide have a high adsorption capacity, suitable for the removal of chromium (VI) from wastewater.

Performance and mechanism of chromium reduction in denitrification biofilm system with different carbon sources

In the process of biological reduction of Cr(VI), the type of carbon sources affects the rate and effect of Cr(VI) reduction, but its specific performance and influencing mechanism have not yet been explored. In this study, four denitrification biofilm reactors were operated under four common carbon sources (C6H12O6, CH3COONa, CH3OH, CH3COONa:C6H12O6 1:1) to reveal the impact of carbon sources on Cr(VI) reduction. Through preliminary experimental concentration research, 75 mg/L Cr(VI) was selected as the dosing concentration. In long-term operation, the composite carbon sources of CH3COONa and C6H12O6 demonstrated excellent stability and achieved an impressive Cr(VI) removal efficiency of 99.5 %. The following sequence was C6H12O6, CH3COONa, and CH3OH. Among them, CH3OH was less competitive and the system was severely unbalanced with lowest Cr(VI) reduction efficiency. The toxicity reactions, changes in EPS and its functional groups, and electron transfer revealed the reduction and fixation mechanism of chromium on denitrification biofilm. The changes in microbial communities indicated that microbial communities in composite carbon sources can quickly adapt to the high toxic environment. The proportion of Trichococcus reached 43.6 %, which played an important role in denitrification and Cr(VI) reduction. Meanwhile, the prediction of microbial COG function reflected its excellent metabolic ability and defense mechanism.

Bioremediation of Cr(VI) using indigenous bacterial strains isolated from a common industrial effluent treatment plant in Vishakhapatnam

The present study focuses on removing hexavalent chromium (Cr(VI)) using indigenous metal-resistant bacterial strains isolated from a common industrial effluent treatment plant, a contaminated site in Vishakhapatnam. Three high metal-resistant isolates were screened by growing them in nutrient agar media containing different Cr(VI) concentrations for 24 h at 35 ± 2 °C. The three strains' minimum inhibitory concentrations of Cr(VI) were examined at neutral pH and 35 ± 2 °C temperature. Morphological, biochemical, and molecular characterizations were carried out, and the strains were identified as Bacillus subtilis NITSP1, Rhizobium pusense NITSP2, and Pseudomonas aeruginosa NITSP3. Elemental composition and functional group analysis of the native and metal-loaded cells were done using energy-dispersive spectroscopy and Fourier-transform infrared spectroscopy, respectively. The operating conditions were optimized using a one-factor-at-a-time analysis. When compared with three bacterial isolates, maximum Cr(VI) removal (80.194 ± 4.0%) was observed with Bacillus subtilis NITSP1 with an initial Cr(VI) concentration of 60 mg/L, pH 7.0, an inoculum size of 2% (v/v), and an incubation period of 24 h. The logistic model was used to predict the variation of biomass growth with time. The present study can be extended to remove heavy metals from industrial wastewater in an environmental-friendly manner.

Mechanism of Cr(VI) removal by efficient Cr(VI)-resistant Bacillus mobilis CR3

Cr(VI) is a hazardous environmental pollutant that poses significant risks to ecosystems and human health. We successfully isolated a novel strain of Bacillus mobilis, strain CR3, from Cr(VI)-contaminated soil. Strain CR3 showed 86.70% removal capacity at 200 mg/L Cr(VI), and a good Cr(VI) removal capacity at different pH, temperature, coexisting ions, and electron donor conditions. Different concentrations of Cr(VI) affected the activity of CR3 cells and the removal rate of Cr(VI), and approximately 3.46% of total Cr was immobilized at the end of the reaction. The combination of SEM-EDS and TEM-EDS analysis showed that Cr accumulated both on the cell surface and inside the cells after treatment with Cr(VI). XPS analysis showed that both Cr(III) and Cr(VI) were present on the cell surface, and FTIR results indicated that the presence of Cr on the cell surface was mainly related to functional groups, such as O-H, phosphate, and -COOH. The removal of Cr(VI) was mainly achieved through bioreduction, which primarily occurred outside the cell. Metabolomics analysis revealed the upregulation of five metabolites, including phenol and L-carnosine, was closely associated with Cr(VI) reduction, heavy metal chelation, and detoxification mechanisms. In addition, numerous metabolites were linked to cellular homeostasis exhibited differential expression. Cr(VI) exerted inhibitory effects on the division rate and influenced critical pathways, including energy metabolism, nucleotide metabolism, and amino acid synthesis and catabolism. These findings reveal the molecular mechanism of Cr(VI) removal by strain CR3 and provide valuable insights to guide the remediation of Cr(VI)-contaminated sites.

A review on chromium health hazards and molecular mechanism of chromium bioremediation

Living beings have been devastated by environmental pollution, which has reached its peak. The disastrous pollution of the environment is in large part due to industrial wastes containing toxic pollutants. The widespread use of chromium (Cr (III)/Cr (VI)) in industries, especially tanneries, makes it one of the most dangerous environmental pollutants. Chromium pollution is widespread due to ineffective treatment methods. Bioremediation of chromium (Cr) using bacteria is very thoughtful due to its eco-friendly and cost-effective outcome. In order to counter chromium toxicity, bacteria have numerous mechanisms, such as the ability to absorb, reduce, efflux, or accumulate the metal. In this review article, we focused on chromium toxicity on human and environmental health as well as its bioremediation mechanism.

Changes in Lolium perenne L. rhizosphere microbiome during phytoremediation of Cd- and Hg-contaminated soils

The contamination of soil and water by metals such as mercury (Hg) and cadmium (Cd) has been increasing in recent years, because of anthropogenic activities such as mining and agriculture, respectively. In this work, the changes in the rhizosphere microbiome of Lolium perenne L. during the phytoremediation of soils contaminated with Hg and Cd were evaluated. For this, two soil types were sampled, one inoculated with mycorrhizae and one without. The soils were contaminated with Hg and Cd, and L. perenne seeds were sown and harvested after 30 days. To assess changes in the microbiome, DNA isolation tests were performed, for which samples were subjected to two-step PCR amplification with specific 16S rDNA V3-V4 primers (337F and 805R). With mycorrhizae, changes had been found in the absorption processes of metals and a new distribution. While with respect to microorganisms, families such as the Enterobacteriaceae have been shown to have biosorption and efflux effects on metals such as Hg and Cd. Mycorrhizae then improve the efficiency of removal and allow the plant to better distribute the absorbed concentrations. Overall, L. perenne is a species with a high potential for phytoremediation of Cd- and Hg-contaminated soils in the tropics. Inoculation with mycorrhizae modifies the phytoremediation mechanisms of the plant and the composition of microorganisms in the rhizosphere. Mycorrhizal inoculation and changes in the microbiome were associated with increased plant tolerance to Cd and Hg. Microorganism-assisted phytoremediation is an appropriate alternative for L. perenne.

Chromium Toxicity in Plants: Signaling, Mitigation, and Future Perspectives

Plants are very often confronted by different heavy metal (HM) stressors that adversely impair their growth and productivity. Among HMs, chromium (Cr) is one of the most prevalent toxic trace metals found in agricultural soils because of anthropogenic activities, lack of efficient treatment, and unregulated disposal. It has a huge detrimental impact on the physiological, biochemical, and molecular traits of crops, in addition to being carcinogenic to humans. In soil, Cr exists in different forms, including Cr (III) "trivalent" and Cr (VI) "hexavalent", but the most pervasive and severely hazardous form to the biota is Cr (VI). Despite extensive research on the effects of Cr stress, the exact molecular mechanisms of Cr sensing, uptake, translocation, phytotoxicity, transcript processing, translation, post-translational protein modifications, as well as plant defensive responses are still largely unknown. Even though plants lack a Cr transporter system, it is efficiently accumulated and transported by other essential ion transporters, hence posing a serious challenge to the development of Cr-tolerant cultivars. In this review, we discuss Cr toxicity in plants, signaling perception, and transduction. Further, we highlight various mitigation processes for Cr toxicity in plants, such as microbial, chemical, and nano-based priming. We also discuss the biotechnological advancements in mitigating Cr toxicity in plants using plant and microbiome engineering approaches. Additionally, we also highlight the role of molecular breeding in mitigating Cr toxicity in sustainable agriculture. Finally, some conclusions are drawn along with potential directions for future research in order to better comprehend Cr signaling pathways and its mitigation in sustainable agriculture.

Strategies for microbial bioremediation of environmental pollutants from industrial wastewater: A sustainable approach

Heavy metals are hazardous and bring about critical exposure risks to humans and animals, even at low concentrations. An assortment of approaches has been attempted to remove the water contaminants and keep up with water quality, for that microbial bioremediation is a promising way to mitigate these pollutants from the contaminated water. The flexibility of microorganisms to eliminate a toxic pollutant creates bioremediation an innovation that can be applied in various water and soil conditions. This review insight into the sources, occurrence of toxic heavy metals, and their hazardous human exposure risk. In this review, significant attention to microbial bioremediation for pollutant mitigation from various ecological lattices has been addressed. Mechanism of microbial bioremediation in the aspect of factors affecting, the role of microbes and interaction between the microbes and pollutants are the focal topics of this review. In addition, emerging strategies and technologies developed in the field of genetically engineered micro-organism and micro-organism-aided nanotechnology has shown up as powerful bioremediation tool with critical possibilities to eliminate water pollutants.

Mechanism of Cr(VI) reduction by an indigenous Rhizobium pusense CR02 isolated from chromite mining quarry water (CMQW) at Sukinda Valley, India

Toxicological assessment of CMQW generated due to chromite mining activities at Sukinda Valley has revealed high chromium contamination along with Zn and Fe. The present study focused on the mechanism of chromate reduction by an indigenous multi-metal tolerant bacterium, Rhizobium pusense CR02, isolated from CMQW. The isolated strain has shown resistance up to 520 mg/L of Cr(VI) with an IC50 value of 385.4 mg/L. The highest reduction rate 8.6 × 10^-2/h was recorded with 20 mg/L of initial concentration of Cr(VI). Extracellular (3.06 ± 0.012 U/mL), intracellular (3.60 ± 0.13 U/mL), and membrane (1.89 ± 0.01 U/mL) associated chromate reductases were found to be involved for reduction. The extracellular polymeric substances (EPS) produced by the isolate also enhanced reduction activity of 46.32 ± 1.69 mg/L after 72 h with an initial concentration of 50 mg/L. FTIR analysis revealed the involvement of functional groups -OH, -CO, and -NH for Cr(VI) biosorption whereas P=O, -CO-NH- and -COOH interacted with Cr(III). Zeta potential with less negative surface charge favored reduction of Cr(VI). Treatment of CMQW by bacterial isolate detoxified Cr(VI) minimizing chromosomal aberrations in root cells of Allium cepa L., suggesting the role of Rhizobium pusense CR02 as a promising bio-agent for Cr(VI) detoxification.

Chromium in plant-soil nexus: Speciation, uptake, transport and sustainable remediation techniques

Heavy metal (HM) pollution has become a serious global problem due to the non-biodegradable nature of the HMs and their persistence in the environment. Agricultural soil is a non-renewable resource that requires careful management so that it can fulfill the increasing demand for agricultural food production. However, different anthropogenic activities have resulted in a large-scale accumulation of HMs in soil which is detrimental to soil and plant health. Due to their ubiquity, increased bioavailability, toxicity, and non-biodegradable nature, HM contamination has formed a roadblock in the way of achieving food security, safety, and sustainability in the future. Chromium (Cr), specifically Cr(VI) is a highly bioavailable HM with no proven role in the physiology of plants. Chromium has been found to be highly toxic to plants, with its toxicity also influenced by chemical speciation, which is in turn controlled by different factors, such as soil pH, redox potential, organic matter, and microbial population. In this review, the different factors that influence Cr speciation were analyzed and the relationship between biogeochemical transformations of Cr and its bioavailability which may be beneficial for devising different Cr remediation strategies has been discussed. Also, the uptake and transport mechanism of Cr in plants, with particular reference to sulfate and phosphate transporters has been presented. The biological solutions for the remediation of Cr contaminated sites which offer safe and viable alternatives to old-style physical and chemical remediation strategies have been discussed in detail. This review provides theoretical guidance in developing suitable approaches for the better management of these remediation strategies.

Recent advances in bioremediation of heavy metals and persistent organic pollutants: A review

Heavy metals and persistent organic pollutants are causing detrimental effects on the environment. The seepage of heavy metals through untreated industrial waste destroys the crops and lands. Moreover, incineration and combustion of several products are responsible for primary and secondary emissions of pollutants. This review has gathered the remediation strategies, current bioremediation technologies, and their primary use in both in situ and ex situ methods, followed by a detailed explanation for bioremediation over other techniques. However, an amalgam of bioremediation techniques and nanotechnology could be a breakthrough in cleaning the environment by degrading heavy metals and persistant organic pollutants.

Unraveling diverse survival strategies of microorganisms to vanadium stress in aquatic environments

Worldwide vanadium contamination is posing serious risks to ecosystems. Although abilities of microbial communities to cope with vanadium stress using specific survival strategies have been reported, little is known regarding their relative importance and the underlying detoxification/tolerance mechanisms. Herein, we investigated the potential survival strategies of microbial communities and associated pathways in aquatic environments based on geochemistry and molecular biology. High vanadium content was observed for both water (12.6 ± 1.15 mg/L) and sediment (1.18 × 10³ ± 10.4 mg/kg) in the investigated polluted stream. Co-occurrence network investigation implied that microbial communities showed cooperative interactions to adapt to the vanadium-polluted condition. Vanadium was also characterized as one of the vital factors shaping the community structure via redundancy analysis and structural equation models. Based on the metagenomic technology, three survival strategies including denitrification pathway, electron transfer, and metal resistance in innate microbes under the vanadium stress were revealed, with comprehensively summarized vanadium detoxification/tolerance genes. Remarkable role of electron transfer genes and the prevalent existence of resistance genes during detoxifying vanadium were highlighted. Overall, these findings provide novel insights into survival strategies under the vanadium contamination in aquatic environments, which can be of great significance for the identification, isolation, and application of vanadium reducing bacteria in vanadium bioremediation.

Changes of distribution and chemical speciation of metals in hexavalent chromium loaded algal-bacterial aerobic granular sludge before and after hydrothermal treatment

Algal-bacterial aerobic granular sludge (AGS) was applied for hexavalent chromium (Cr(VI)) biosorption from wastewater and the dynamic distribution and mobility of different metals in granules were systematically examined before and after hydrothermal treatment. The loaded Cr on algal-bacterial AGS was found to mainly localize in microbial cells and mineral particles; little Cr was detected in extracellular polymeric substances (EPS) after 6 h contact, which increased to 5.1% after 24 h biosorption. Along with Cr localization, 9.3-22.8% of Mg and 11.5-26.4% of Ca in algal-bacterial AGS were released from loosely bound EPS, then replenished to maintain their stable proportion in EPS, probably contributing to granular stability. In addition, chemical fractionation showed that the Cr mobility in algal-bacterial AGS, indicated by a low mobility factor of 4.7%, decreased to 1.4% with the co-existence of mineral salts and acetate during biosorption, which was further declined to 0 after hydrothermal treatment.

Health hazards of hexavalent chromium (Cr (VI)) and its microbial reduction

Industrial effluents/wastewater are the main sources of hexavalent chromium (Cr (VI)) pollutants in the environment. Cr (VI) pollution has become one of the world’s most serious environmental concerns due to its long persistence in the environment and highly deadly nature in living organisms. To its widespread use in industries Cr (VI) is highly toxic and one of the most common environmental contaminants. Cr (VI) is frequently non-biodegradable in nature, which means it stays in the environment for a long time, pollutes the soil and water, and poses substantial health risks to humans and wildlife. In living things, the hexavalent form of Cr is carcinogenic, genotoxic, and mutagenic. Physico-chemical techniques currently used for Cr (VI) removal are not environmentally friendly and use a large number of chemicals. Microbes have many natural or acquired mechanisms to combat chromium toxicity, such as biosorption, reduction, subsequent efflux, or bioaccumulation. This review focuses on microbial responses to chromium toxicity and the potential for their use in environmental remediation. Moreover, the research problem and prospects for the future are discussed in order to fill these gaps and overcome the problem associated with bacterial bioremediation’s real-time applicability.

Bioreduction performances and mechanisms of Cr(VI) by Sporosarcina saromensis W5, a novel Cr(VI)-reducing facultative anaerobic bacteria

This study reported a novel facultative anaerobic Cr(VI)-reducing bacteria (Sporosarcina saromensis W5) and investigated its Cr(VI) removal performances and removal mechanisms. The strain W5 was able to grow and reduce Cr(VI) under aerobic and anaerobic environment, and exhibited considerable Cr(VI) reduction capabilities under a wide range of pH (8.0-13.0), temperature (20-40 °C) and initial Cr(VI) concentration (50-800 mg/L). The addition of Cd²⁺ severely inhibited its growth and Cr(VI) removal, while Cu²⁺ and Fe³⁺ significantly enhanced the removal efficiencies. The strain W5 could utilize various electron donors and mediators to accelerate Cr(VI) reduction. Aerobic Cr(VI) reduction mainly occurred in cytoplasm and the final products were soluble organo-Cr(III) complexes. Anaerobic Cr(VI) reduction was located in both cytoplasm and membrane, and the reduction products were soluble organo-Cr(III) complexes and Cr(III) precipitates. The functional groups of hydroxyl, carboxyl and phosphoryl on cell surface participated in the combination with Cr(III). Due to its facultative anaerobic property, S. saromensis W5 offers itself as a promising engineering strain for the bioremediation of Cr(VI)-contaminated areas, especially in hypoxia environments.

Chromium pollution and its bioremediation mechanisms in bacteria: A review

Environment pollution is at its peak and is creating havoc for living beings. Industrial wastes containing toxic pollutants have contributed to a great extent in this disastrous environment pollution. Chromium (Cr³⁺/Cr⁶⁺) is highly toxic and one of the most common environmental pollutants because of its extensive use in industries especially tanneries. Lack of efficient treatment methods has resulted in extensive chromium pollution. Bioremediation of chromium using bacteria is very thoughtful due to its eco-friendly and cost-effective outcome. Bacteria possess numerous mechanisms such as biosorption, reduction, efflux or bioaccumulation, naturally or acquired to counter the toxicity of chromium. This review focuses on the bacterial responses against chromium toxicity and scope for their application in bioremediation. The differences and similarities between Gram negative and positive bacteria against chromium are also highlighted. Further, the knowledge gap and future prospects are also discussed in order to fill these gaps and overcome the problem associated with real-time applicability of bacterial bioremediation.

Bioremediation of hexavalent chromium from wastewater using bacteria-a green technology

Hexavalent chromium has high toxic effect on the ecological system. The aim of the present study is to isolate and characterize the bacteria that can reduce the toxicity of hexavalent chromium from liquid effluent. The bacterial isolate was identified as Bacillus sp. ltds1 after 16 S rRNA gene sequencing, and annotation has been submitted in National Center for Biotechnology Information (NCBI) GenBank. The bacterial strain was found able to grow in Luria Broth medium at 100 mg/L Cr⁶⁺ concentration. A maximum Cr⁶⁺ bioremediation (95.24 ± 2.08 %) could be achieved using the said isolate at 40 mg/L, pH 7, and inoculum concentration 4 % at 24 h. The residual chromium was found in the form of less toxic trivalent chromium (Cr³⁺), which confirms that the bacterial isolate can transform toxic Cr⁶⁺ to non-toxic Cr³⁺. Fourier Transform Infra-Red (FTIR) study was performed to analyze the functional groups and overall nature of chemical bonds involved in the remediation process, whereas, Energy-Dispersive Spectroscopy (EDS) studies of native and treated cells showed the changes in elemental composition in response to metal stress. Artificial Neural Network (ANN) based prediction model is developed based on experimental points. The developed model was found to predict the bioremediation of Cr⁶⁺ at various operating conditions. Particle Swarm Optimization (PSO) is used to optimize the variables like the initial concentration of metal, pH, temperature, and inoculum concentration for the said bacterial strain. The results showed that the isolate could be applied as a potential bioremediation agent for Cr⁶⁺ removal.

Effects of thorium on bacterial, microalgal and micromeiofaunal community structures in a periphytic biofilm

Few ecotoxicity studies are available on thorium (Th) which hinders the ability to evaluate its ecotoxicological risk. Its release in the environment is often associated with the extraction of rare earth elements and uranium, as well as the field applications of phosphate fertilizers. This study investigates the effects of Th on microbial communities of periphytic biofilms. Ceramic plates were left to colonize for one month in the laboratory with a biofilm sampled from Cap Rouge river (QC, Canada). Plates were randomly placed in channels containing culture media representing three different conditions: a control condition (C0; background Th concentrations of 0.004 ± 0.002 nM), a low Th concentration condition (C1; 0.18 ± 0.09 nM Th) and a moderately high Th condition (C10; 8.7 ± 3.4 nM) for up to 4 weeks. The presence of Th modified the diatom community by changing its taxonomic structure, reducing diversity and increasing cell density. The taxonomic structure of the bacterial community, followed by 16S metabarcoding analysis, was affected with a significant decrease in Pseudanabaena and Shingopyxis genera in the two Th exposed conditions. No direct toxic effect of Th was observed on counted micromeiofauna but the changes in diatom and bacterial communities could explain the higher number of individual diatoms and micromeiofauna observed in Th-exposed conditions. This work shows that low concentrations of Th can modify biofilm structure, which, in turn, could disturb its ecologically key functions.

Hexavalent chromium reducing bacteria: mechanism of reduction and characteristics

As a common heavy metal, chromium and its compounds are widely used in industrial applications, e.g., leather tanning, electroplating, and in stainless steel, paints and fertilizers. Due to the strong toxicity of Cr(VI), chromium is regarded as a major source of pollution with a serious impact on the environment and biological systems. The disposal of Cr(VI) by biological treatment methods is more favorable than traditional treatment methods because the biological processes are environmentally friendly and cost-efficient. This review describes how bacteria tolerate and reduce Cr(VI) and the effects of some physical and chemical factors on the reduction of Cr(IV). The practical applications for Cr(VI) reduction of bacterial cells are also included in this review.

Study on detoxification and removal mechanisms of hexavalent chromium by microorganisms

Extensive industrial activities have led to an increase of the content of chromium in the environment, which causes serious pollution to the surrounding water, soil and atmosphere. The enrichment of chromium in the environment through the food chain ultimately affects human health. Therefore, the remediation of chromium pollution is crucial to development of human society. A lot of scholars have paid attention to bioremediation technology owing to its environmentally friendly and low-cost. Previous reviews mostly involved pure culture of microorganisms and rarely discussed the optimization of bioreduction conditions. To make up for these shortcomings, we not only introduced in detail the conditions that affect microbial reduction but also innovatively introduced consortium which may be the cornerstone for future treatment of complex field environments. The aim of this study is to summary chromium toxicity, factors affecting microbial remediation, and methods for enhancing bioremediation. However, the actual application of bioremediation technology is still facing a major challenge. This study also put forward the current research problems and proposed future research directions, providing theoretical guidance and scientific basis for the application of bioremediation technology.

Adapting the algal microbiome for growth on domestic landfill leachate

We aimed to improve algal growth rate on leachate by optimising the algal microbiome. An algal-bacterial consortium was enriched from landfill leachate and subjected to 24 months of adaptive laboratory evolution, increasing the growth rate of the dominant algal strain, Chlorella vulgaris, almost three-fold to 0.2 d^-1. A dramatic reduction in nitrate production suggested a shift in biological utilisation of ammoniacal-N, supported by molecular 16S rRNA taxonomic analyses, where Nitrosomonas numbers were not detected in the adapted consortium. A PICRUSt approach predicted metagenomic functional content and revealed a high number of sequences belonging to bioremediation pathways, including degradation of aromatic compounds, benzoate and naphthalene, as well as pathways known to be involved in algal-bacterial symbiosis. This study enhances our understanding of beneficial mechanisms in algal-bacterial associations in complex effluents, and ultimately enables the bottom-up design of optimised algal microbiomes for exploitation within industry.