Screening Of Lead-removing Lactic Acid Bacteria And Its Application In Lead-contaminated Soil

Due to the boost of industrialization and urbanization,soil pollution with heavy metals has become a global environmental problem that threatens human health and food safety.Microbial assisted phytoremediation is an effective and environmental friendly technology for remediation of heavy metals contaminated soil,and has a broad prospect of application.However,certain microorganisms that are suitable for collaborative remediation are opportunistic pathogens,which may bring about new risks.Lactic acid bacteria?LAB?are food-grade microorganisms and generally recognized as safe.In addition to safety and traditional probiotic functions,the potential of LAB for lead?Pb?removal has attracted special attention from researchers.This project aimed at screening LAB with potential to remove Pb²⁺from fermented foods,and exploring the characteristics and mechanism of lead removal.Then,the potential application of LAB,associated with ryegrass,for remediation of lead-contaminated soil was studied through pot experiments.The main findings are as follows:Firstly,five strains were screened from fermented foods and considered LAB based on their Gram-positive reaction and absence of oxidase and catalase.Among them,DUTYH₁6120012 and DUTYH₁6120026 can effectively remove Pb²⁺from DeMan-Rogosa-Sharpe?MRS?broth,and the removal rates were 88.51%and 81.03%,respectively.The results of 16S rRNA sequence analysis showed that these two strains are Enterococcus faecium and Enterococcus sp.,respectively.Secondly,the initial pH,initial Pb²⁺concentration and temperature were important factors affecting Pb²⁺removal by E.faecium DUTYH₁6120012 and Enterococcus sp.DUTYH₁6120026.The results of scanning electron microscope equipped with energy dispersive spectrometer?SEM-EDS?analysis and X-ray photoelectron spectroscopy?XPS?analysis confirmed that E.faecium DUTYH₁6120012 can cause Pb²⁺of MRS broth to form the yellow-brown deposits mainly composed of PbO₂ and PbO on the cell pellets.The cell wall played a major role,contributing 80.58%,in Pb²⁺removal by E.faecium DUTYH₁6120012.Fourier transform infrared spectrometer?FTIR?analysis found that some functional groups such as-OH,-C=O,-NH and-CN were involved in the removal of Pb²⁺by E.faecium DUTYH₁6120012.Finally,the results of pot experiment showed that after E.faecium DUTYH₁6120012was added to the lead-contaminated soil?0 to 1000 mg/kg?near the root of ryegrass,the dry weight of ryegrass shoot and root significantly increased?P<0.05?,the content of malondialdehyde?MDA?significantly decreased?P<0.05?,and the activities of superoxide dismutase?SOD?,peroxidase?POD?and catalase?CAT?significantly increased?P<0.05?.More importantly,the Pb²⁺concentration in ryegrass shoot and root significantly increased?P<0.05?,in which the content of ryegrass shoot increased by 14.55%to 19.49%,and the content of ryegrass root increased by 3.48%to 10.86%;meanwhile,the bioconcentration factor?BCF?and transport factor?TF?significantly increased?P<0.05?,and increase rates were 17.50%to 22.62%and 17.86%to 30.00%,respectively.This indicated that the strain significantly improved the remediation efficiency of ryegrass on lead-contaminated soil.