Microbe-Plant Interactions Targeting Metal Stress: New Dimensions for Bioremediation Applications -

Microbe-Plant Interactions Targeting Metal Stress: New Dimensions for Bioremediation Applications

The increase in environmental contamination due to global industrialization, particularly from activities like mining and disposal of hazardous metal effluents, poses a threat to water and soil quality. Heavy metals (HMs), such as lead (Pb), uranium (Ur), nickel (Ni), silver (Ag), and chromium (Cr), can disrupt biological processes in plants and animals, negatively impacting plant growth and overall ecosystem health. The hexavalent form of chromium is particularly hazardous and affects agricultural productivity, soil fertility, and water quality. Heavy metal toxicity hampers plant development, nutritional absorption, metabolism, and physiological processes in soils, posing risks to both human health and the ecosystem.

The study emphasizes the need for remediation strategies to address heavy metal contamination, which affects over 150 million km2 of China’s agricultural land, causing significant economic losses. Heavy metal poisoning can lead to severe health issues, affecting organs such as the brain, liver, and kidneys. The research explores various approaches to clean up metal-contaminated soils, including biological, chemical, and physical methods. Bioremediation, utilizing microorganisms, is highlighted as a practical and cost-effective solution.

The study also discusses the importance of environmental factors such as pH, temperature, oxygen, and moisture in influencing the success of metal remediation processes. The indigenous microbial population of the soil is recognized for its crucial role in plant growth regulation, pest control, soil structure maintenance, nutrient recycling, and pollutant transformation.

The research focuses on the potential of using microorganisms for detoxifying heavy metal-contaminated soils, with an emphasis on employing diverse microbial species for more effective bioremediation. Additionally, the study explores the effects of nanoparticles in remediating heavy metals and their potential impact on the environment.

Effects of HMs on the Environment

High concentrations of heavy metals (HMs) pose severe challenges for global environmental health due to their non-biodegradable nature. Essential metals like Co, Ni, Cu, Mo, Fe, and Mn, in trace amounts, are vital for organismal survival. However, elevated levels of these metals become toxic to living organisms. Certain metals and metalloids, such as Ni, Cd, Cr, Hg, As, and Se, can be hazardous to soil and crop health when their concentrations surpass maximum permissible levels set by regulations like the Environmental Response Compensation and Liability Act (ERCLA), e.g., Cr (0.02 mg L−1). These pollutants contribute significantly to life-threatening human degenerative diseases.

Laboratory tests have demonstrated that increased concentrations of heavy metals have detrimental effects on various biological processes, including respiration, the Electron Transport Chain (ETC), photosynthesis, and cell division. The negative impacts of these heavy metals on the environment necessitate coordinated efforts to remove them and safeguard the ecosystem. Effective strategies are essential for addressing heavy metal pollution and minimizing its adverse effects on both environmental and human health.

HMT-PGP (Heavy Metal Tolerant-Plant Growth Promoting) Microbial Mechanisms for Soil Heavy Metal(loid)s Remediation

Heavy metals (HMs) negatively impact biological molecules, impairing their activities and altering the function and structure of proteins, enzymes, and membrane transporters. Microbial bioremediation techniques involve the use of microbes to remove or detoxify HMs/metalloids from contaminated locations. Bioremediation is employed in agriculture to address diverse HM stresses in various plants and heavily contaminated areas to reduce the impact of pollutants on different life forms.

Heavy metal remediation treatments include electro-dialysis, reverse osmosis, and physical treatments such as extraction, stabilization, immobilization, and soil washing. However, these methods can be costly due to high energy and chemical reagent requirements.

Microorganisms play a crucial role in HM cleanup as they can tolerate metal toxicity in various ways. Research suggests that using consortia of bacterial strains is more effective for HM bioremediation than single-strain cultures. Plant-microbe interaction studies offer promising solutions for sustainable agriculture and the development of bioremediation processes.

HMT-PGP (Heavy Metal-Tolerant Plant Growth-Promoting) bacteria/microorganisms can affect plant development, alter soil physicochemical characteristics, and increase metal bioavailability, leading to fast detoxification or removal of HMs from the soil. Microbial mechanisms for HM bioremediation include toxic metal sequestration, modification of metabolic processes, enzymatic processes, and reduction of intracellular metal concentrations through precise efflux mechanisms.

Microbial-driven redox processes are crucial for the transformation of HMs into non-toxic forms. Microbes use various mechanisms such as chelation, coordination, complexation, micro-precipitation, ion exchange, and entrapment during biosorption. Additionally, microbial-driven redox processes play a vital role in HM detoxification in plants.

Metallothioneins produced by microbes and plants have a high affinity for certain metals, aiding in their detoxification. The final stage of HM detoxification involves sequestration or compartmentalization into various subcellular organelles. HM vacuolar compartmentalization is observed in mycorrhizal fungi.

Exopolysaccharide (EPS) synthesis by some PGP bacteria induces biofilm development, which enhances microbial cell tolerance and converts harmful metal ions into non-toxic forms. EPS, composed of complex organic macromolecules, sequesters various types of HMs. Biofilm formation and exopolysaccharide synthesis are interlinked and essential for biomineralization and metal biosorption processes.

Microbe-plant interactions, including those involving Plant Growth Promoting Bacteria (PGPB), organic acids, biosurfactants, biomethylation, redox processes, phosphate solubilization, nitrogen fixation, and iron sequestration, play a crucial role in lessening metal stressors and supporting biomass production and phytoremediation. Understanding these interactions is essential for effective heavy metal detoxification and sustainable environmental practices.

Strategies for Reconstructed Metabolic Pathways in Bioremediation Techniques

Decades of research leveraging high-tech tools like whole-genome sequencing (WGS), directed evolution, and high-throughput screening have laid the foundation for reconstructing novel metabolic pathways in bioremediation. Two primary methods are employed: in-silico methods, which use computational algorithms to build microbial pathways, and experimental methods that validate in-silico-designed pathways using various molecular biology tools.

Computational techniques utilize sizable databases connected to WGS and data from previously researched natural metabolic pathways. Enzyme-catalyzed biochemical process databases like MetaCyc, KEGG, BRENDA, and Rhea detail enzymes involved in constructing metabolic pathways. These databases serve as references for integrated pathways, and the sequence alignment program BLAST is used to find statistically significant matches between query sequences and databases.

The Genome-Scale Metabolic Model (GSMM) is a genetics-based approach that predicts microbial phenotypes and constructs metabolic networks using software, data resources, and genetic information. Tools like KEGG, BioCyc, and MetCyc provide organism-specific pathways, while MEGAN, KAAS, and MG-RAST assist in high-efficiency route reconstruction. Model SEED and COBRA tools are used to rebuild metabolic networks and anticipate genetic alterations for optimizing metabolite synthesis rates and yields. The MAPLE program analyzes metagenomics data in species dispersion investigations. Overall, these strategies contribute to the efficient design and reconstruction of metabolic pathways for bioremediation.

As Carriers for the Active Component during Bioremediation

Nanomaterials, due to their unique physicochemical features, can be effectively employed in environmental bioremediation by combining them with enzymes. Nanoparticles, such as nanographene, nanotubes, nanofibers, and nanogels, serve as carriers for immobilizing physiologically active compounds, forming nanobiocatalysts. This innovative technology addresses challenges posed by heavy metal contamination in arable soil, where essential metals like Cd, Pb, Zn, and Cu become phytotoxic pollutants beyond critical levels.

Bioremediation, while effective for pollution removal, has limitations, especially in areas with high concentrations of harmful contaminants. Nanobioremediation, involving the production of nanoparticles as a by-product of resistance mechanisms against specific metals, offers a promising alternative. Understanding nanoparticle behavior requires knowledge of morphology, particle size distribution, surface area, charge, and crystallographic characteristics.

Nanobioremediation utilizes microorganisms as nanofactories to produce nanoparticles for environmental cleanup. Three main bioremediation techniques—microbes, flora, and enzymatic remediation—are employed. Panicum maximum in nano-phytoremediation and magnetic nanoparticles for adsorption and catalytic pollution remediation have shown efficacy. Nanomaterial-based modifications, considering factors like cost, efficiency, stability, and environmental impact, offer practical and affordable on-site immobilization of pollutants in contaminated soils. Various nanoparticle methods are employed for bioremediation, targeting the removal of HMs from water and organic/inorganic contaminants from soil.

Nanomaterials as Active Additives for Bioremediation

Nanobioremediation, combining nanotechnology with bioremediation, overcomes limitations of traditional techniques. The application of biogenic nanoparticles or materials from biological sources in nanobioremediation offers advantages due to the smaller size of nanoparticles, resulting in a larger surface area for more efficient reactions. Green nanoparticles, environmentally friendly and economically beneficial, are gaining attention. Nanoparticles can be integrated with phytoremediation and enzyme-based bioremediation for quick degradation of pollutants by bacteria and plants.

Nanoparticles produced by bacteria, including zerovalent nanoparticles, have been effective in removing various metallic contaminants from soil and wastewater effluents. Bacterial nanoparticles can bind to and concentrate dissolved metals, converting them into non-toxic nanoparticles. Extracellular synthesis of biogenic nanoparticles by bacteria, such as palladium, titanium, magnetite, gold, and silver, is efficient and produces easy-to-remove nanoparticles.

Biogenic nanoparticles, including those produced by bacteria, fungi, and algae, offer versatile and diverse strategies for nanobioremediation. Bacteria-produced nanoparticles, acting as biocatalysts, bioscaffolds, and active participants, are used for large-scale production. The application of nanotechnology in heavy metal bioremediation has shown significant improvements. Biosynthesis using bacteria is a reasonable and acceptable large-scale production technology.

Nanotechnology enhances bioremediation processes, offering control, sensing, and remediation of pollutants. Nanoparticles synthesized by fungi, bacteria, and algae have demonstrated effectiveness in removing heavy metals. Nanofibers encapsulating bacteria and nanomaterials produced by living organisms, such as magnetic iron nanoparticles from D. radiodurans and iron nanoparticles from Chlorococcum sp., have shown remarkable capacities for metal and dye remediation. Engineered microbes with enhanced characteristics can contribute to more effective and long-lasting bioremediation in the future.

Future challenges

Future challenges in heavy metal bioremediation include leveraging heavy metal-tolerant microorganisms in combination with host plants for environmentally safe and cost-effective soil treatment. Although this strategy holds promise, there is currently a lack of information hindering its commercialization. The lodgement of metals in plant parts often slows the cleanup process in heavily polluted areas. Studies indicate that heavy metal-tolerant, plant growth-promoting microorganisms with nutrient supplements can enhance effectiveness in polluted soil.

Microcosm-scale phytoextraction studies show that adding thiosulfate products to metal-tolerant bacteria increases mobilization and absorption of metals like Arsenic (Ar) and Mercury (Hg) by plants. Genetically engineered microbes tailored to different biogeographical conditions have demonstrated efficient removal of heavy metals from contaminated soils. Nutrient additions can stimulate the local microbial community, promoting soil healing and detoxification in areas contaminated with heavy metals.

Recent tests have explored the ability of consortia of heavy metal-tolerant bacteria to remove heavy metals from contaminated environments. Entomopathogenic fungi present another avenue for eradicating heavy metals from polluted soils and can be applied for biocontrol and cleanup in contaminated and infected soils. Engineering approaches for creating or modifying microbes and plants offer potential for improving heavy metal removal. Genetically designed microbial sensors provide a rapid detection method for assessing polluted soil accurately. Additionally, exploring plant microbiomes in contaminated soils may enhance phytoremediation methods for heavy metal bioremediation.

 Conclusion

In conclusion, the widespread expansion of agriculture and industry has led to environmental contamination with hazardous wastes such as plastics, heavy metals (HMs), chlorinated biphenyls, and agrochemicals. The use of plant-microbe synergy for soil remediation presents a promising yet experimental approach. This study emphasizes the superiority of microbial methods in heavy metal detoxification over less effective and costlier biophysical techniques that require significant energy inputs.

Plant Growth-Promoting (PGP) microorganisms employ various mechanisms, including precipitation, biosorption, enzymatic metal transformation, complexation, and phytoremediation, for heavy metal removal. Heavy Metal-Tolerant (HMT) PGP microorganisms offer advantages such as enhanced soil quality, removal of toxic compounds, increased plant development, and efficient heavy metal extraction from soil. However, developing appropriate bio-formulations for the remediation and utilization of polluted soils with HMT-PGP microorganisms is crucial.

Advancements in nanotechnology, coupled with a deeper understanding of microbe-nanotechnology interactions, are essential for improving contaminant digestion. Conducting field studies will further contribute to the progress of this sector. The application of microorganisms, as a low-input and sustainable approach, has the potential to extract heavy metals from contaminated soil, ultimately enhancing soil quality and productivity. In summary, microorganisms provide a valuable platform for advancing bioremediation models to address diverse environmental contaminants and manage environmental pollution.

Reference:

SAHARAN, B.S., CHAUDHARY, T., MANDAL, B.S., KUMAR, D., KUMAR, R., SADH, P.K. AND DUHAN, J.S., 2023. Microbe-Plant Interactions Targeting Metal Stress: New Dimensions for Bioremediation Applications. Journal of Xenobiotics, 13(2), pp.252-269.