Positive effects of bio-nano Pd (0) toward direct electron transfer in Pseudomona putida and phenol biodegradation

Unboxing PGPR-mediated management of abiotic stress and environmental cleanup: what lies inside?

Abiotic stresses including heavy metal toxicity, drought, salt and temperature extremes disrupt the plant growth and development and lowers crop output. Presence of environmental pollutants further causes plants suffering and restrict their ability to thrive. Overuse of chemical fertilizers to reduce the negative impact of these stresses is deteriorating the environment and induces various secondary stresses to plants. Therefore, an environmentally friendly strategy like utilizing plant growth-promoting rhizobacteria (PGPR) is a promising way to lessen the negative effects of stressors and to boost plant growth in stressful conditions. These are naturally occurring inhabitants of various environments, an essential component of the natural ecosystem and have remarkable abilities to promote plant growth. Furthermore, multifarious role of PGPR has recently been widely exploited to restore natural soil against a range of contaminants and to mitigate abiotic stress. For instance, PGPR may mitigate metal phytotoxicity by boosting metal translocation inside the plant and changing the metal bioavailability in the soil. PGPR have been also reported to mitigate other abiotic stress and to degrade environmental contaminants remarkably. Nevertheless, despite the substantial quantity of information that has been produced in the meantime, there has not been much advancement in either the knowledge of the processes behind the alleged positive benefits or in effective yield improvements by PGPR inoculation. This review focuses on addressing the progress accomplished in understanding various mechanisms behind the protective benefits of PGPR against a variety of abiotic stressors and in environmental cleanups and identifying the cause of the restricted applicability in real-world.

Biodegradation of phenolic pollutants and bioaugmentation strategies: A review of current knowledge and future perspectives

The widespread use of phenolic compounds renders their occurrence in various environmental matrices, posing ecological risks especially the endocrine disruption effects. Biodegradation-based techniques are efficient and cost-effective in degrading phenolic pollutants with less production of secondary pollution. This review focuses on phenol, 4-nonylphenol, 4-nitrophenol, bisphenol A and tetrabromobisphenol A as the representatives, and summarizes the current knowledge and future perspectives of their biodegradation and the enhancement strategy of bioaugmentation. Biodegradation and isolation of degrading microorganisms were mainly investigated under oxic conditions, where phenolic pollutants are typically hydroxylated to 4-hydroxybenzoate or hydroquinone prior to ring opening. Bioaugmentation efficiencies of phenolic pollutants significantly vary under different application conditions (e.g., increased degradation by 10-95% in soil and sediment). To optimize degradation of phenolic pollutants in different matrices, the factors that influence biodegradation capacity of microorganisms and performance of bioaugmentation are discussed. The use of immobilization strategy, indigenous degrading bacteria, and highly competent exogenous bacteria are proposed to facilitate the bioaugmentation process. Further studies are suggested to illustrate 1) biodegradation of phenolic pollutants under anoxic conditions, 2) application of microbial consortia with synergistic effects for phenolic pollutant degradation, and 3) assessment on the uncertain ecological risks associated with bioaugmentation, resulting from changes in degradation pathway of phenolic pollutants and alterations in structure and function of indigenous microbial community.

Use of nanoparticle-coated bacteria for the bioremediation of organic pollution: A mini review

Nanoparticle (NP)-coated (immobilized) bacteria are an effective method for treating environmental pollution due to their multifarious benefits. This review collates a vast amount of existing literature on organic pollution treatment using NP-coated bacteria. We discuss the features of bacteria, NPs, and decoration techniques of NP-bacteria assemblies, with special attention given to the surface modification of NPs and connection mechanisms between NPs and cells. Furthermore, the performance of NP-coated bacteria was examined. We summarize the factors that affect bioremediation efficiency using coated bacteria, including pH, temperature, and agitation, and the possible mechanisms involving them are proposed. From future perspectives, suitable surface modification of NPs and wide application in real practice will make the NP-coated bacterial technology a viable treatment strategy.

Towards BioMnOx-mediated intra/extracellular electron shuttling for doxycycline hydrochloride metabolism in Bacillus thuringiensis

Doxycycline hydrochloride (DCH) could be continuously removed by Bacillus thuringiensis S622 with the in-situ biogenic manganese oxide (BioMnOx) via oxidizing/regenerating. The DCH removal rate was significantly increased by 3.01-fold/1.47-fold at high/low Mn loaded via the integration of biological (intracellular/extracellular electron transfer (IET/EET)) and abiotic process (BioMnOx, Mn(III) and •OH). BioMnOx accelerated IET via activating coenzyme Q to enhance electrons transfer (ET) from complex I to complex III, and as an alternative electron acceptor for respiration and provide another electron transfer transmission channel. Additionally, EET was also accelerated by stimulating to secrete flavins, cytochrome c (c-Cyt) and flavin bounded with c-Cyt (Flavins & Cyts). To our best knowledge, this is the first report about the role of BioMnOx on IET/EET during antibiotic biodegradation. These results suggested that Bacillus thuringiensis S622 incorporated with BioMnOx could adopt an alternative strategy to enhance DCH degradation, which may be of biogeochemical and technological significance.

High-resolution particle size and shape analysis of the first Samarium nanoparticles biosynthesized from aqueous solutions via cyanobacteria Anabaena cylindrica

Samarium (Sm) is one of the most sought-after rare earth metals. Price trends and dwindling resources are making recovery increasingly attractive. In this context, the use of cyanobacteria is highly promising. For Sm it was unclear whether Anabaena cylindrica produces particles through metabolically active Sm³⁺ uptake. High-resolution (HR) imaging now clearly demonstrates microbe generated biosynthesis of Sm nano-sized particles (Sm NPs) in vivo. Furthermore, a simple method to determine particle size and shape with high accuracy is presented. Digital image analysis with ImageJ of HR-TEMs is used to characterize Sm NPs revealing a nearly uniform local size distribution. Assuming round particles, the overall average area size is 135.5 nm², resp. 11.9 nm diameter. In HR, where different cell sections of the same cell are averaged, the mean particle is smaller, 76.7 nm² resp. 8.9 nm diameter. The reciprocal aspect ratio is 0.63. The Feret major axis ratio is calculated as shape factor, with 35% of the particles between 1.2 and 1.4. A roundness classification shows that 38% of particles are fairly round and 41% are very round. Consequently, A. cylindrica represents a suitable microorganism for possible Sm recovery and biosynthesis of roundish nano-sized particles.

Biotechnological synthesis of Pd-based nanoparticle catalysts

Palladium metal nanoparticles are excellent catalysts used industrially for reactions such as hydrogenation and Heck and Suzuki C-C coupling reactions. However, the global demand for Pd far exceeds global supply, therefore the sustainable use and recycling of Pd is vital. Conventional chemical synthesis routes of Pd metal nanoparticles do not meet sustainability targets due to the use of toxic chemicals, such as organic solvents and capping agents. Microbes are capable of bioreducing soluble high oxidation state metal ions to form metal nanoparticles at ambient temperature and pressure, without the need for toxic chemicals. Microbes can also reduce metal from waste solutions, revalorising these waste streams and allowing the reuse of precious metals. Pd nanoparticles supported on microbial cells (bio-Pd) can catalyse a wide array of reactions, even outperforming commercial heterogeneous Pd catalysts in several studies. However, to be considered a viable commercial option, the intrinsic activity and selectivity of bio-Pd must be enhanced. Many types of microorganisms can produce bio-Pd, although most studies so far have been performed using bacteria, with metal reduction mediated by hydrogenase or formate dehydrogenase enzymes. Dissimilatory metal-reducing bacteria (DMRB) possess additional enzymes adapted for extracellular electron transport that potentially offer greater control over the properties of the nanoparticles produced. A recent and important addition to the field are bio-bimetallic nanoparticles, which significantly enhance the catalytic properties of bio-Pd. In addition, systems biology can integrate bio-Pd into biocatalytic processes, and processing techniques may enhance the catalytic properties further, such as incorporating additional functional nanomaterials. This review aims to highlight aspects of enzymatic metal reduction processes that can be bioengineered to control the size, shape, and cellular location of bio-Pd in order to optimise its catalytic properties.

Prompting the FDH/Hases-based electron transfers during Pt(IV) reduction mediated by bio-Pd(0)

Due to the excellent hydrogen affinity and high conductivity, palladium nanoparticles (Pd NPs) were considered as a potential strategy to regulate bacterial electron transfer and energy metabolism. Herein, Citrobacter freundii JH, capable of in-situ biosynthesizing Pd(0) NPs, was employed to promote Pt(IV) reduction. The results showed that the Pt(IV) reduction to Pt(II) was accomplished mainly via the flavins-mediated extracellular electron transfer (EET) process, while Pt(II) reduction to Pt(0) was limit step, and proceeded via two intracellular respiratory chains, including FDH/Hases-based short chain (S-chain) and typical CoQ-involved long respiratory chain (L-chain). Noteworthily, the incorporation of Pd(0) NPs mainly diverted the electrons to S-chain (as high as 71.7%-73.4%) by improving the hydrogenases (Hases) activity. Furthermore, Pd(0) NPs could stimulate the secreting of flavins and the combination between flavins and cytochrome c (c-Cyt), which converted electron transfer manner of L-chain. Additionally, Pd(0) NPs might also act as alternative proton channels to improve the energy metabolism. These findings provided significant insights into the promotion by Pd(0) NPs in terms of electron generation, electron consumption and proton translocation.

Intra/extracellular electron transfer for aerobic denitrification mediated by in-situ biosynthesis palladium nanoparticles

The slow electron transfer rate is the bottleneck to the biological wastewater treatment process, and the nanoparticles (NPs) has been verified as a feasible strategy to improve the biological degradation efficiency by accelerating the electron transfer. Here, we employed the Gram-positive Bacillus megaterium Y-4, capable of synthetizing Pd(0), to investigate the intra/extracellular electron transfer (IET/EET) mechanisms mediated by NPs in aerobic denitrification for the first time. Kinetic and thermodynamic results showed that the bio-Pd(0) could significantly promote the removal of both nitrate and nitrite by improving affinity and decreasing activation energy. The enzymic activity and the respiration chain inhibition experiment indicated that the bio-Pd(0) could facilitate the nitrate biotic reduction by improving the Fe-S center activity and serving as parallel H carriers to replace coenzyme Q to selectively increase the electron flux toward nitrate in IET, while promoting the nitrite reduction by abiotic catalysis. Most importantly, the detection of DPV peak at -226~-287 mV proved that the one-electron EET via multiheme cytochrome-bound flavins also occurred in Gram-positive bacteria and enhanced in Pd-loaded cells. In addition, the remarkable increase of the formal charge in EPS indicated that the bio-Pd(0) could act as an electron shuttle to increase the redox site in EPS, eventually accelerating the electron hopping in long-distance electron transfer. Overall, this study expanded our understanding of the roles of bio-Pd(0) on the aerobic denitrification process and provided an insight into the IET/EET of Gram-positive strains.

Unlocking the potential of plant growth-promoting rhizobacteria on soil health and the sustainability of agricultural systems

The concept of soil health refers to specific soil properties and the ability to support and sustain crop growth and productivity, while maintaining long-term environmental quality. The key components of healthy soil are high populations of organisms that promote plant growth, such as the plant growth promoting rhizobacteria (PGPR). PGPR plays multiple beneficial and ecological roles in the rhizosphere soil. Among the roles of PGPR in agroecosystems are the nutrient cycling and uptake, inhibition of potential phytopathogens growth, stimulation of plant innate immunity, and direct enhancement of plant growth by producing phytohormones or other metabolites. Other important roles of PGPR are their environmental cleanup capacities (soil bioremediation). In this work, we review recent literature concerning the diverse mechanisms of PGPR in maintaining healthy conditions of agricultural soils, thus reducing (or eliminating) the toxic agrochemicals dependence. In conclusion, this review provides comprehensive knowledge on the current PGPR basic mechanisms and applications as biocontrol agents, plant growth stimulators and soil rhizoremediators, with the final goal of having more agroecological practices for sustainable agriculture.

Electron transfer involved in bio-Pd (0) synthesis by Citrobacter freundii at different growth phases

Gram-negative Citrobacter freundii with high Pd (II) reduction capacity was isolated from electroplating wastewater, and the electron transfer involved in Pd (II) bio-reduction by C. freundii JH was investigated in phosphate buffer saline solution with sodium formate as sole electron donor under anaerobic condition. FTIR spectra indicated that hydroxyl and amine groups on cell wall participated Pd (II) bio-sorption. TEM, XRD, XPS results confirmed that Pd (0) nanoparticles (NPs) could be bio-synthesized intra/extracellularly. Meanwhile, pH turn-over were observed owing to the reduction of cytochrome c (c-Cyt) in bio-reduction process. EPR spectra indicated that free radicals (OH) was generated from high concentration Pd (II), which would cause seriously damage to cell. Despite of the lower tolerance to Pd (II), the cells at logarithmic phase exhibited higher Pd (II) reduction capacity (72.21%) than that at stationary phase (56.21%), which might be related to the relatively stronger proton motive force (PMF) created by the substrate oxidation and the electron transfer, as evidenced by electrochemical experiments (CV, DPV, amperometric I-t curves) and protein denaturalization experiments. Additionally, c-Cyt and riboflavin were confirmed to be important participants in electron transfer. Finally, a putative synthesis mechanism of Pd (0)-NPs was deduced. This study contributed to further understanding the electron transfer in Pd (II) reduction, and provided more information for the bio-synthetic of metal nanoparticles.

Soil enzyme response to bisphenol F contamination in the soil bioaugmented using bacterial and mould fungal consortium

The concept of the study resulted from the lack of accurate data on the toxicity of bisphenol F (BPF) coinciding with the need for immediate changes in the global economic policy eliminating the effects of environmental contamination with bisphenol A (BPA). The aim of the experiment was to determine the scale of the previously unstudied inhibitory effect of BPF on soil biochemical activity. To this end, in a soil subjected to increasing BPF pressure at three contamination levels of 0, 5, 50 and 500 mg BPF kg^-1 DM, responses of soil enzymes, dehydrogenases, catalase, urease, acid phosphatase, alkaline phosphatase, arylsulphatase and β-glucosidase, were examined. Moreover, the study suggested a potentially effective way of biostimulating the soil by means of bioaugmentation with a consortium of four bacterial species: Pseudomonas umsongensis, Bacillus mycoides, Bacillus weihenstephanensis and Bacillus subtilis, and the following fungal species: Mucor circinelloides, Penicillium daleae, Penicillium chrysogenum and Aspergillus niger. It was found that BPF was a controversial BPA analogue due to the fact that it contributed to the inhibition of all the enzyme activities. Dehydrogenases proved to be the most sensitive to bisphenol contamination of the soil. The addition of 5 mg BPF kg^-1 DM of soil triggered an escalation of the inhibition comparable to that for the other enzymes only after exposing them to the effects of 50 and 500 mg BPF kg^-1 DM of soil. Moreover, BPF generated low activity of urease, acid phosphatase, alkaline phosphatase and β-glucosidase. Bacterial inoculum increased the activity of urease, β-glucosidase, catalase and alkaline phosphatase. Seventy-six percent of BPF underwent biodegradation during the 5 days of the study.

Immobilization of horseradish peroxidase on electrospun magnetic nanofibers for phenol removal

In this study, Fe₃O₄/polyacrylonitrile (PAN) magnetic nanofibers (MNFs) were fabricated by electrospinning method to immobilize the horseradish peroxidase (HRP), making which a complex platform for phenol removal application. Results indicated that, the average diameter of MNFs was about 200-400 nm and the maximum saturation magnetic induction was 19.03 emu/g. Compared with the free HRP, the modified HRP showed no change in optimum pH, but showed higher catalytic activity. Moreover, HRP immobilized MNFs (H-MNFs) with 40% Fe₃O₄ nanoparticles loading had the lowest HRP loading, but had the highest relative activity, because of the magnetic synergy with the presence of MNPs. Subsequently, the 40% H-MNFs was used for the remediation of phenol wastewater, achieved the removal efficiency of phenol to 85% in the first round use, and remained 52% of efficiency after 5 recycles using. It was expected that the H-MNFs could be a potential application in wastewater treatment such as phenol removal.

The effect of biotic and abiotic environmental factors on Pd(II) adsorption and reduction by Bacillus megaterium Y-4

In this study, we screened a new aerobic bacterium (Bacillus megaterium Y-4) that can efficiently reduce Pd(II) with different electron donors. The best electron donor was sodium formate and the best reduction of Pd(II) were by log growth phase cells. The high removal capacity of Pd(II) (1658.3 mg/g) was obtained with 30 mg/L dry cell weight and 50 mg/L Pd (II) in the presence of 5 mM sodium formate. The removal amount of Pd(II) increased with initial Pd(II) concentrations ranging from 50 to 200 mg/L with 100 mg/L Pd(II) being completely removed by 148 mg/L dry cell weight in 6 h. The cell wall, periplasmic space and intracellular contents of B. megaterium Y-4 contains different kinds of enzymes for reducing Pd(II). In addition, the activity of extracellular and periplasmic enzymes was more sensitive to temperature than intracellular enzymes. XRD and XPS analysis revealed that the enzyme for reducing Pd(II) in B. megaterium Y-4 can tolerate a broad range of temperatures (20-60 °C) and pH (2.0-7.0) but was sensitive to oxygen. TEM analysis showed that biogenic palladium nanoparticles (Pd-NPs) were mainly distributed evenly in the periplasmic space of the live cells and were released from cells into aqueous solution, which reduced the toxicity of Pd(II), allowing Pd-NP recovery without cell destruction. B. megaterium Y-4 is a potential bacterium for efficient treatment and reclamation of Pd(II) pollution and formation of Pd-NPs.