Exploiting Microbial Polysaccharides for Biosorption of Trace Elements in Aqueous Environments-Scope for Expansion via Nanomaterial Intervention

Exopolysaccharides from agriculturally important microorganisms: Conferring soil nutrient status and plant health

A wide variety of microorganisms secretes extracellular polymeric substances or commonly known as exopolysaccharides (EPS), which have been studied to influence plant growth via various mechanisms. EPS-producing microorganisms have been found to have positive effects on plant health such as by facilitating nutrient entrapment in the soil, or by improving soil quality, especially by helping in mitigating various abiotic stress conditions. The various types of microbial polysaccharides allow for the compartmentalization of the microbial community enabling them to endure undressing stress conditions. With the growing population, there is a constant need for developing sustainable agriculture where we could use various PGPR to help the plant cope with various stress conditions and simultaneously enhance the crop yield. These polysaccharides have also found application in various sectors, especially in the biomedical fields, manifesting their potential to act as antitumor drugs, play a significant role in immune evasion, and reveal various therapeutic potentials. These constitute high levels of bioactive polysaccharides which possess a wide range of implementation starting from industrial applications to novel food applications. In this current review, we aim at presenting a comprehensive study of how these microbial extracellular polymeric substances influence agricultural productivity along with their other commercial applications.

Towards Computationally Guided Design and Engineering of a Neisseria meningitidis Serogroup W Capsule Polymerase with Altered Substrate Specificity

Heavy metal contamination of drinking water is a public health concern that requires the development of more efficient bioremediation techniques. Absorption technologies, including biosorption, provide opportunities for improvements to increase the diversity of target metal ions and overall binding capacity. Microorganisms are a key component in wastewater treatment plants, and they naturally bind metal ions through surface macromolecules but with limited capacity. The long-term goal of this work is to engineer capsule polymerases to synthesize molecules with novel functionalities. In previously published work, we showed that the Neisseria meningitidis serogroup W (NmW) galactose-sialic acid (Gal-NeuNAc) heteropolysaccharide binds lead ions effectively, thereby demonstrating the potential for its use in environmental decontamination applications. In this study, computational analysis of the NmW capsule polymerase galactosyltransferase (GT) domain was used to gain insight into how the enzyme could be modified to enable the synthesis of N-acetylgalactosamine-sialic acid (GalNAc-NeuNAc) heteropolysaccharide. Various computational approaches, including molecular modeling with I-TASSER and molecular dynamics (MD) simulations with NAMD, were utilized to identify key amino acid residues in the substrate binding pocket of the GT domain that may be key to conferring UDP-GalNAc specificity. Through these combined strategies and using BshA, a UDP-GlcNAc transferase, as a structural template, several NmW active site residues were identified as mutational targets to accommodate the proposed N-acetyl group in UDP-GalNAc. Thus, a rational approach for potentially conferring new properties to bacterial capsular polysaccharides is demonstrated.

Application of Chitosan/Alginate Nanocomposite Incorporated with Phycosynthesized Iron Nanoparticles for Efficient Remediation of Chromium

Biopolymers and nanomaterials are ideal candidates for environmental remediation and heavy metal removal. As hexavalent chromium (Cr⁶⁺) is a hazardous toxic pollutant of water, this study innovatively aimed to synthesize nanopolymer composites and load them with phycosynthesized Fe nanoparticles for the full Cr⁶⁺ removal from aqueous solutions. The extraction of chitosan (Cht) from prawn shells and alginate (Alg) from brown seaweed (Sargassum linifolium) was achieved with standard characteristics. The tow biopolymers were combined and cross-linked (via microemulsion protocol) to generate nanoparticles from their composites (Cht/Alg NPs), which had a mean diameter of 311.2 nm and were negatively charged (−23.2 mV). The phycosynthesis of iron nanoparticles (Fe-NPs) was additionally attained using S. linifolium extract (SE), and the Fe-NPs had semispherical shapes with a 21.4 nm mean diameter. The conjugation of Cht/Alg NPs with SE-phycosynthesized Fe-NPs resulted in homogenous distribution and stabilization of metal NPs within the polymer nanocomposites. Both nanocomposites exhibited high efficiency as adsorbents for Cr⁶⁺ at diverse conditions (e.g., pH, adsorbent dose, contact time and initial ion concentration) using batch adsorption evaluation; the most effectual conditions for adsorption were a pH value of 5.0, adsorbent dose of 4 g/L, contact time of 210 min and initial Cr⁶⁺ concentration of 75 ppm. These factors could result in full removal of Cr⁶⁺ from batch experiments. The composited nanopolymers (Cht/Alg NPs) incorporated with SE-phycosynthesized Fe-NPs are strongly recommended for complete removal of Cr⁶⁺ from aqueous environments.

Photo induced mechanistic activity of GO/Zn(Cu)O nanocomposite against infectious pathogens: Potential application in wound healing

Treating infection causing microorganisms is one of the major challenges in wound healing. These may gain resistance due to the overuse of conventional antibiotics. A promising technique is antimicrobial photodynamic therapy (aPDT) used to selectively cause damage to infectious pathogenic cells via generation of reactive oxygen species (ROS). We report on biocompatable nanomaterials that can serve as potential photosensitizers for aPDT. GO/Zn(Cu)O nanocomposite was synthesized by co-precipitation method. Graphene Oxide (GO) is known for its high surface to volume ratio, excellent surface functionality and enhanced antimicrobial property. ZnO nanoparticle induces the generation of reactive oxygen species (ROS) under light irradiation and it leads to recombination of electron-hole pair. Nanocomposites of GO and Cu doped ZnO increases visible light absorption and enhances the photocatalytic property. It generates more ROS and increases the bacterial inhibition. GO/Zn(Cu)O nanocomposite was tested against Staphylococcus aureus (S. aureus), Enterococcus faecium (E. faecium), Escherichia coli (E. coli), Salmonella typhi (S. typhi), Shigella flexneri (S. flexneri) and Pseudomonas aeruginosa (P. aeruginosa) by well diffusion method, growth curve, colony count, biofilm formation under both dark and visible light condition. Reactive Oxygen Species assay (ROS), Lactate dehydrogenase leakage (LDH) assay, Protein estimation assay and membrane integrity study proves the mechanism of inhibition of bacteria. Inhibition kinetics shows the sensitivity between bacteria and GO/Zn(Cu)O nanocomposite.

Biosorption of Pb (II) and Zn (II) from aqueous solution by Oceanobacillus profundus isolated from an abandoned mine

The present study investigated biosorption of Pb (II) and Zn (II) using a heavy metal tolerant bacterium Oceanobacillus profundus KBZ 3-2 isolated from a contaminated site. The effects of process parameters such as effect on bacterial growth, pH and initial lead ion concentration were studied. The results showed that the maximum removal percentage for Pb (II) was 97% at an initial concentration of 50 mg/L whereas maximum removal percentage for Zn (II) was at 54% at an initial concentration of 2 mg/L obtained at pH 6 and 30 °C. The isolated bacteria were found to sequester both Pb (II) and Zn (II) in the extracellular polymeric substance (EPS). The EPS facilitates ion exchange and metal chelation-complexation by virtue of the existence of ionizable functional groups such as carboxyl, sulfate, and phosphate present in the protein and polysaccharides. Therefore, the use of indigenous bacteria in the remediation of contaminated water is an eco-friendly way of solving anthropogenic contamination.

Biosorption of heavy metals by dry biomass of metal tolerant bacterial biosorbents: an efficient metal clean-up strategy

Heavy metals discharge at an unrestrained rate from various industries into the environment pose serious human health problems. Considering this, the present study aimed at exploring the metal biosorbing potentials of bacterial strains recovered from polluted soils. The bacterial strains (CPSB1, BM2 and CAZ3) belonging to genera Pseudomonas, Bacillus and Azotobacter expressing multi-metal tolerance ability were identified to species level as P. aeruginosa, B. subtilis and A. chroococcum, respectively, by 16S rRNA partial gene sequence analysis. The biosorption of cadmium, chromium, copper, nickel, lead and zinc by three dead bacterial genera were studied as a function of metal concentration, variable pH of the medium and reaction (contact) time. The three bacterial strains exhibited a tremendous metal removal ability which continued even at the highest tested concentration of some metals. Later, a decline in the percentage of biosorbed metals was recorded as the metal concentration was increased with the simultaneous generation of a driving force to overcome mass transfer resistance for movement of metal ions between the solution and the surface of adsorbent. Among test bacteria, B. subtilis biosorbed a maximum of 96% chromium at 25 μg mL^-1 while the maximum percentage (91%) of biosorbed metals recorded at 400 μg Cd mL^-1 was observed for P. aeruginosa. The sorption of metal ions by dead biomass of three bacterial genera at optimum conditions followed the order-(i) B. subtilis BM2: Pb > Cu > Ni > Cd > Cr, (ii) A. chroococcum CAZ3: Cr > Cd > Cu > Ni > Pb and (iii) P. aeruginosa CPSB1: Cd > Cr > Ni > Cu > Pb > Zn. It was found that the optimum pH for metal adsorption ranged between pH 8 and 9 which, however, declined substantially at pH 5.0 for all three bacterial strains. In general, the biosorption of Cd, Cr, Cu, Ni and Pb by B. subtilis and A. chroococcum and such metals along with Zn by P. aeruginosa occurred maximally up to 60 min of bacterial growth. The adsorption data with regard to five metals provide an outstanding fit to the Langmuir and Freundlich isotherms. The biosorptive ability of three bacterial genera correlated strongly (r² > 0.9) with each metal. The bacteria belonging to two Gram-negative genera Pseudomonas (P. aeruginosa) and Azotobacter (A. chroococcum) and one Gram-positive genus Bacillus (B. subtilis) demonstrated exceptional metal removal efficiency and, hence, provides a comprehensive understanding of metal-bacteria sorption process which in effect paves the way for detoxifying/removing metals from contaminated environment.

Unveiling the role of bioturbation on bacterial activity in metal-contaminated sediments

The processes permeating the relationships between bioturbation and microorganisms remain poorly understood due to the difficulty of traditional techniques in quantifying their two- and three-dimensional aspects. We used cutting-edge technologies to address the macro- and microorganisms' interactions under metal-contamination. Bioturbation (mucus-lined gallery perimeter, mucus-lined gallery surface area, and gallery water volume) positively influence the carbohydrate consumption rate by the bacterial consortium, elevating bacterial metabolic activity, despite metal-contamination. Synchrotron-based 2D-μXRF revealed that the mucous lining by marine worm during bioturbation as the primary carbon source enhances metal immobilization by bacterial biofilm, improving the bacterial metabolic activity. Bioturbation thus can positively affect bacterial consortium that can use the mucus as a carbon source, which enhances the resistance to metals through biofilm formation in metal-contaminated sediments.

Synthetic biology approaches towards the recycling of metals from the environment

Metals are a finite resource and their demand for use within existing and new technologies means metal scarcity is increasingly a global challenge. Conversely, there are areas containing such high levels of metal pollution that they are hazardous to life, and there is loss of material at every stage of the lifecycle of metals and their products. While traditional resource extraction methods are becoming less cost effective, due to a lowering quality of ore, industrial practices have begun turning to newer technologies to tap into metal resources currently locked up in contaminated land or lost in the extraction and manufacturing processes. One such technology uses biology for the remediation of metals, simultaneously extracting resources, decontaminating land, and reducing waste. Using biology for the identification and recovery of metals is considered a much 'greener' alternative to that of chemical methods, and this approach is about to undergo a renaissance thanks to synthetic biology. Synthetic biology couples molecular genetics with traditional engineering principles, incorporating a modular and standardised practice into the assembly of genetic parts. This has allowed the use of non-model organisms in place of the normal laboratory strains, as well as the adaption of environmentally sourced genetic material to standardised parts and practices. While synthetic biology is revolutionising the genetic capability of standard model organisms, there has been limited incursion into current practices for the biological recovery of metals from environmental sources. This mini-review will focus on some of the areas that have potential roles to play in these processes.

Heavy metal induced stress on wheat: phytotoxicity and microbiological management

Among many soil problems, heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety. Due to these problems, soil biologists/agronomists in recent times have also raised concerns over heavy metal pollution, which indeed are unpleasantly affecting agro-ecosystems and crop production. The toxic heavy metals once deposited beyond certain permissible limits, obnoxiously affect the density, composition and physiological activities of microbiota, dynamics and fertility of soil leading eventually to reduction in wheat production and via food chain, human and animal health. Therefore, the metal induced phytotoxicity problems warrant urgent and immediate attention so that the physiological activities of microbes, nutrient pool of soils and concurrently the production of wheat are preserved and maintained in a constantly deteriorating environment. To mitigate the magnitude of metal induced changes, certain microorganisms have been identified, especially those belonging to the plant growth promoting rhizobacteria (PGPR) group endowed with the distinctive property of heavy metal tolerance and exhibiting unique plant growth promoting potentials. When applied, such metal-tolerant PGPR have shown variable positive impact on wheat production, even in soils contaminated with metals, by supplying macro and micro nutrients and secreting active biomolecules like EPS, melanins and metallothionein (MTs). Despite some reports here and there, the phytotoxicity of metals to wheat and how wheat production in metal-stressed soil can be enhanced is poorly explained. Thus, an attempt is made in this review to better understand the mechanistic basis of metal toxicity to wheat, and how such phytotoxicity can be mitigated by incorporating microbiological remediation strategies in wheat cultivation practices. The information provided here is likely to benefit wheat growers and consequently optimize wheat production inexpensively under stressed soils.

Among many soil problems, heavy metal accumulation is one of the major agronomic challenges that has seriously threatened food safety.

Putative Role of Bacterial Biosorbent in Metal Sequestration Revealed by SEM-EDX and FTIR

Bacterial exopolysaccharides (EPS) play a critical role in sequestration of metals from contaminated environment. Considering these, this study was aimed at extracting EPS from metal tolerant Pseudomonas aeruginosa CPSB1 and Azotobacter chroococcum CAZ3 and to ascertain its role in metal removal. P. aeruginosa CPSB1 and A. chroococcum CAZ3 secreted 1306.7 and 1660 µg mL-1 EPS, respectively in the presence of 200 and 100 µg mL-1 Pb, respectively with glucose as C source. The binding of metal ions to bacterial EPS was validated by SEM and EDX. The functional group involved in metal chelation was revealed by FT-IR. The metal ions were adsorbed onto EPS and hence, EPS could play a crucial role in metal detoxification. Due to this novel trait, P. aeruginosa CPSB1 and A. chroococcum CAZ3 could be developed as bioinoculant to cleanup metal contaminated sites.

Bioreduction of toxicity influenced by bioactive molecules secreted under metal stress by Azotobacter chroococcum

Heavy metal pollution destruct soil microbial compositions and functions, plant's performance and subsequently human health. Culturable microbes among many metal abatement strategies are considered inexpensive, viable and environmentally safe. In this study, nitrogen fixing bacterial strain CAZ3 recovered from chilli rhizosphere tolerated 100, 1000 and 1200 µg mL^-1 of cadmium, chromium and nickel, respectively and was identified as Azotobacter chroococcum by 16S rDNA sequence analysis. Under metal stress, cellular morphology of A. chroococcum observed under SEM was found distorted and shrinkage of cells was noticed when grown with 50 µg mL^-1 of Cd (cell size 1.7 µm) and 100 of µg mL^-1 Ni (cell size 1.3 µm) compared to untreated control (cell size 1.8 µm). In the presence of 100 µg mL^-1 of Cr, cells became elongated and measured 1.9 µm in size. Location of metals inside the cells was revealed by EDX. A dose dependent growth arrest and consequently the death of A. chroococcum cells was revealed under CLSM. A. chroococcum CAZ3 secreted 320, 353 and 133 µg EPS mL^-1 when grown with 100 µg mL^-1 each of Cd, Cr and Ni, respectively. The EDX revealed the presence of 0.4, 0.07 and 0.24% of Cd, Cr and Ni, respectively within EPS extracted from metal treated cells. Moreover, a dark brown pigment (melanin) secreted by A. chroococcum cells under metal pressure displayed tremendous metal chelating activity. The EDX spectra of melanin extracted from metal treated cells of A. chroococcum CAZ3 displayed 0.53, 0.22 and 0.12% accumulation of Cd, Cr and Ni, respectively. The FT-IR spectra of EPS and melanin demonstrated stretching vibrations and variations in surface functional groups of bacterial cells. The C-H stretching of CH₃ in fatty acids and CH₂ groups, stretching of N-H bond of proteins and O-H bond of hydroxyl groups caused the shifting of peaks in the EPS spectra. Similar stretching vibrations were recorded in metal treated melanin which involved CHO, alkyl, carboxylate and alkene groups resulting in significant peak shifts. Nuclear magnetic resonance (NMR) spectrum of EPS extracted from A. chroococcum CAZ3 revealed apparent peak signals at 4.717, 9.497, 9.369 and 9.242 ppm. However, ¹H NMR peaks were poorly resolved due largely to the impurity/viscosity of the EPS. The entrapment of metals by EPS and melanin was confirmed by EDX. Also, the induction and excretion of variable amounts of metallothioneins (MTs) by A. chroococcum under metal pressure was interesting. Conclusively, the present findings establish- (i) cellular damage due to Cd, Cr and Ni and (ii) role of EPS, melanin and MTs in adsorption/complexation and concurrently the removal of heavy metals. Considering these, A. chroococcum can be promoted as a promising candidate for supplying N efficiently to plants and protecting plants from metal toxicity while growing under metal stressed environment.

Arsenic Bioremediation by Indigenous Heavy Metal Resistant Bacteria of Fly Ash Pond

Fly ash (FA), the major by-product of coal-fired thermal power plants, causes significant environmental degradation owing to its injurious heavy metal contents. Leaching of arsenic (As) from ash ponds is especially significant as As released from FA can increase As concentration of drinking water above maximum contaminant level of 10 ppb. The aim of this paper was demonstration of As bioremediation potential of indigenous As resistant bacteria present in the weathered pond ash sample. Ten isolates belonging to Bacillus, Micrococcus, Kytococcus and Staphylococcus genera were characterized. Biochemical tests showed reduction of relatively non toxic arsenate to more toxic arsenite by two strains while four strains showed oxidation of arsenite to arsenate. Two exoplolysaccharide producing strains were shown to absorb As within their biomass. Total heterotrophs versus As resistant heterotrophs counting performed showed that FA was enriched with As resistant heterotrophs. Column leaching based microcosm study revealed overall As detoxification potential of the isolated microbes.