Screening Of Manganese Tolerant Strains And Their Effectiveness In Enhancing The Remediation Of Heavy Metal Contaminated Soil By Koelreuteria Paniculata

Phytoremediation is widely used in tailings restoration because it is an environment-friendly and cost effective technology.However,plants usually have disadvantages of slow growth,low biomass and low heavy metal enrichment capacity under heavy metal stress.Phyto-microbial remediation techniques can improve the effect of remediation by promoting the absorption and enrichment of heavy metals of plants.In this study,16 Mn-tolerant strains were screened from the rhizosphere soils of plants growing in Xiangtan manganese mine waste site.The promoting properties and manganese(Mn)adsorption characteristics of the strains were determined to identify the candidate strains for phyto-microbial remediation.Then,a 90-day pot experiment was carried out by planting Koelreuteria paniculata in four kinds of growth substrates,including uncontaminated soil,100%manganese tailing sludge,and mixed contaminated soils(adding 80%or 90%manganese tailing sludge to uncontaminated soils).The effects of EA-1 and BN-4 cultures on the growth,photosynthetic characteristics,physiological response,heavy metal(Mn,Pb and Cu)accumulation and transport of Koelreuteria paniculata in the different substrates were studied.The influence and the potential mechanisms of the two strains(EA-1 and BN-4)in improving the phytoremediation of manganese tailings by Koelreuteria paniculata were discussed.The main results were as follows:(1)Three bacterial strains that were highly tolerant to manganese were screened from manganese mine waste sites,and were identified as Bacillus cereus strain EA-1,Stenotrophomonas sp.BP-2 and Priestia megaterium BN-4.All these strains were capable of producing IAA,phosphorus solubilisation and iron carriers,but strains EA-1 and BN-4 showed better promoting properties.The maximum adsorption rates were observed at Mn2+concentration of 250 mg/L,which were 91.96%of EA-1,88.40%of BP-2 and 90.82%of BN-4,respectively.The Mn2+adsorption rates of the strains decrease with increasing Mn2+concentrations.SEM analysis revealed that there were fold breakages on the surface of the strains and flocculent precipitations between the bacteria after Mn adsorption,indicating that extracellular precipitation is likely to be an important pathway for bacterial adsorption of Mn.FTIR analysis indicated that different chemical groups,such as hydroxyl,carboxyl,carbonyl,amide,phosphate and hydrocarbon groups on the cell surface of the strains,may be involved in the adsorption of Mn.Strains EA-1 and BN-4 were finally identified for subsequent pot experiment according to the promoting properties,growth curves and Mn removal rates.(2)The pot experiment showed that the growth of Koelreuteria paniculata was individually inhibited with increasing of manganese tailing sludge addition in soils.Compared with the treatment without microbial inoculation(CK),inoculation with EA-1 and BN-4 significantly improved the growth status,net photosynthetic rate as well as the chlorophyll content in the leaves of Koelreuteria paniculata growing in four kinds of growth substrates.The strain EA-1 had better performance in enhancing photosynthesis of Koelreuteria paniculata in comparison with BN-4.With increasing of manganese tailing sludge addition,the activity of antioxidant enzyme in the fine roots of Koelreuteria paniculata decreased but the contents of MDA significantly increased.The inoculation of EA-1 and BN-4 effectively enhanced the regulation of oxidative stress cells,alleviated lipid peroxidation in the cell membrane,reduced oxidative damage and improved tolerance to heavy metal stress by enhancing the activity of root antioxidant enzymes of Koelreuteria paniculata.(3)In four kinds of growth substrates,inoculation with EA-1 and BN-4 enhanced the uptake of Mn,Pb and Cu in all parts of Koelreuteria paniculata,and the enrichment coefficients of Mn,Pb and Zn by Koelreuteria paniculata increased with the addition of manganese tailing sludges.The net photosynthetic rate significantly affected the enrichment coefficients of Mn and Cu as well as the transport coefficients of Mn and Pb.The POD enzyme activity of fine roots had a significant effect on the enrichment coefficients of Mn,Pb and Cu as well as the transport coefficients of Mn and Cu.The MDA content of leaves significantly affected the enrichment of Mn and Cu,and the MDA content of roots had a significant effect on the enrichment coefficients of Mn and Cu as well as the transport coefficients of Mn and Pb.(4)This study demonstrated that strains EA-1 and BN-4 can effectively improve the growth characteristics of Koelreuteria paniculata under the stress of manganese tailing sludges,reduce the toxic level of heavy metals and promote the uptake and enrichment of heavy metals by Koelreuteria paniculata.Based on the growth of Koelreuteria paniculata and its enrichment and transport of heavy metals,Koelreuteria paniculata inoculation with EA-1 combined with pure manganese tailings sludge was an efficient mode for manganese tailings remediation from the view of saving remediation cost.