Microbiological interventions at soil plant interface to induce lead mobility and its remediation

Abstract

Lead (Pb) is a toxic metal whose widespread use has caused extensive environmental contamination and health problems through food chain contamination in many parts of the world. Recently, phytoremediation has appeared as an effective and alternative solution to conventional physiochemical techniques for removal of Pb from contaminated soil. However, lack of understanding and information regarding Pb availability, speciation, uptake and translocation mechanisms, suitable plant species for hyperaccumulating Pb, microbial association that interfere phytoremediation process at plant soil interface are hindering its full-scale application. The aim of the current research was to develop an integrated plant-microbial association system for enhanced remediation of Pb contaminated soils using indigenous biological systems including plants and microorganisms. In the first step, extensive screening of ornamental plants locally grown in Pakistan was done for selection of Pb hyperaccumulator plant. Fortunately, two plants Pelargonium hortorum and Mesembryanthemum criniflorum were selected based on significantly higher Pb accumulation (>1000 mg Pb kg-1 in shoot dry biomass) and better translocation i.e., higher accumulation in shoot compared to root without significant (p<0.05) decrease in plant dry biomass (up to 1500 mg kg-1 soil Pb conc.). The selected plants were further investigated for root induced changes in rhizosphere during three-week culture in special cropping device fabricated locally. Results indicated significant ability of P. hortorum to acidify rhizosphere soil (ΔpH= -0.22 pH units) and increasing dissolved organic compounds (DOC) contents (1.4-1.7 –folds) that induced Pb mobility in soil (1-2 –folds) compared to M. criniflorum and control soil. Plant-microbial association studies for phytoremediation potential were performed in the subsequent step. Indigenous Pb resistant bacteria were isolated from soil collected from battery recycling units in industrial zones of Islamabad and Rawalpindi, Pakistan. Klebsiella xi quasipneumoniae (NCCP-1862), Klebsiella variicola (NCCP-1857), Pseudomonas beteli (NCCP- 1845), Microbacterium paraoxydans (NCCP-1848) and Bacillus tequilensis (NCCP-1860) showed Pb tolerance and solubilization and plant growth promoting (PGP) activity. Plant-bacterial interaction studies exhibited the potential of M. paraoxydans as efficient bio-inoculant for increased Pb phytoextraction. Fungal-soil interaction studies showed the ability of Aspergillus flavus, and Mucor spp. to increase the bioavailable fraction by lowering soil pH. Fungal-plant interaction studies exhibited the potential of Mucor spp. as efficient bio-inoculant for enhanced Pb uptake in P. hortorum (2.21 mg per plant) followed by A. flavus (1.85 mg per plant). In the later step, multiple heavy metal accumulator plants (Pteris vittata) and associated rhizospheric bacteria were studied for Pb phytoextraction in sterile conditions. The results showed decrease in Pb uptake while improving plant growth in both inoculated and un-inoculated P. vittata and P. hortorum. Amplification of metal efflux transporter gene fragments (pbrA, cadA2 and czcR) from Pseudomonas sp. genomic DNA explained molecular mechanisms involved in Pb resistance and detoxification in bacteria. Finally, a novel integrated plant-microbial system was developed through series of experiments by co-inoculating bacteria and fungi. Strong inhibitory effect of Pb on soil enzymatic activities, microbial biomass and respiration and significant restoration by bio-inoculants were achieved. Coinoculation of bacteria and fungi significantly improve soil enzymatic activities. The outcome of this detailed investigation provided optimized, efficient and integrated biological system for enhanced remediation of Pb contaminated sites that could be considered as a potential alternative to synthetic chelators and reduce the associated environmental concerns. The findings of the present study may be helpful in developing a pilot scale treatment facility for contaminated soil in industrial and urban soil.