The Ecological Barrier Effect Of Rhizosphere Bacteria To Soil Heavy Metals

The fragmentation of economic development and environmental protection has accelerated the enrichment of heavy metals in agricultural soils,which endangers food security and national livelihoods.Most of the existing remediation methods have little effect or potential ecological risk on the remediation of heavy metal pollution in agricultural fields,and making full use of rhizosphere microorganisms to block the migration of heavy metals to crops effectively is in line with the national conditions of production during remediation and sustainable development in China.This study focuses on the life activities of rhizosphere bacteria in response to heavy metal stress,such as free cells,biofilm,and metabolic mineralization.Starting from screening rhizosphere bacteria and generalizing their overall characteristics,we newly defined the ecological barrier effect of rhizosphere microorganisms based on the stress transformation of planktonic cells and biofilm,dynamic interactions between biofilm and soil minerals,mineralization and corrosion of soil minerals by bacterial metabolism,and the core species of functional integration.The main findings are as follows:1)Overall characterization perspective: 38 rhizosphere bacteria with significant differences in traits and species were isolated from heavy metal-contaminated soil,and the intrinsic link between genetic developmental distance and essential bacterial traits/heavy metal fixation was explored.The results showed that although fluctuations within categories could not be ignored,significant differences usually existed between categories.Among the adsorption fixation of heavy metals,Class II(Bacillales,Micrococcales and Caulobacterales)bacteria demonstrated the best cell surface adsorption capacity,Class I(Burkholderiales and Xanthomonadales)had a higher intracellular enrichment capacity,and Class III(Pseudomonadales)bacteria enhanced the immobilization of heavy metals by forming biofilms.In contrast,IV(Aeromonadales and Enterobacterales)bacteria were relatively balanced.Notably,Pseudomonas aeruginosa H13 and Brevundimonas diminuta H16,which were more excellent in promoting plant growth,and Serratia marcescens S14,which showed the best biofilm formation ability and urease activity,deserve further investigation.2)Stress transformation perspective: Two strains of plant growth-promoting bacteria,P.aeruginosa H13 and B.diminuta H16,were used to explore how they responded to external stresses and fixed heavy metals.The results showed that heavy metals inhibited the ability of H13 and H16 to promote plant growth but stimulated the production of bacterial extracellular polysaccharides,unstable inorganic sulfides and enhanced biofilm formation,which significantly facilitated the removal of heavy metals by H13 and H16.The immobilization mechanisms of planktonic cells for heavy metals are mainly ion exchange and functional group complexation.However,biofilms have a more substantial immobilization capacity for heavy metals,mainly through the complexation of active functional groups,especially C-OH and P=O groups associated with the extracellular matrix.3)Dynamic interactions perspective: The mechanisms by which soil minerals(montmorillonite,kaolinite and acanthite)and Cd(II)affected biofilm formation and Cd(II)fixation were investigated using S.marcescens S14 with an excellent biofilm formation ability.Biofilm formation was significantly enhanced after co-culture with soil minerals and/or Cd(II)with S14 and reached 2.3-7.3 times higher than the control,especially in the montmorillonite group.Mineral-induced cell rupture and expression of fim A,bsm A and eps were the leading causes of biofilm formation.Cd(II)and montmorillonite and their combinations regulated biofilm formation by interfering with the ratio of polysaccharides in the biofilm extracellular matrix,resulting in a 1.5-fold increase in biofilm adsorption capacity for Cd(II)under optimal conditions.In addition,the adsorption capacity of soil for Cd(II)was significantly enhanced by adding S14-biofilm.4)Metabolic corrosion perspective: The role of microbial erosion of minerals in influencing the biotoxicity,migration and transformation of heavy metals was elucidated using rhizosphere indigenous flora and urea metabolizing strain S.marcescens S14.Metabolic erosion by urea-degrading microorganisms produced multiple concave surfaces of phosphate minerals,thus increasing their surface area(by a factor of 3)and exposing more internal active sites.The increased Ca(II)exchange sites and active functional groups,as well as the more accessible release of phosphate that co-precipitated with Cd(II),enhanced their sorption capacity for Cd(II)by a factor of about 10.For Cd(II)-containing wastewater purification and Cd(II)-contaminated soil remediation,microbially modified phosphate minerals show good application potential.5)Functional complementarity perspective: Multispecies growth and biofilm formation effects on Cd(II)immobilization were explored at the direct and multiple interaction levels through random combinations of two to three rhizosphere bacteria.More than 65% of the co-cultures exhibited induced biofilm formation under Cd(II)stress,which further enhanced the role of biofilm in Cd(II)immobilization,especially with increasing species richness.Notably,the excellent biofilm-forming ability of P.putida c favoured the promotion of multispecies biofilm formation and Cd(II)adsorption;a weak biofilmforming bacterium B.diminuta f also played a central role in multispecies biofilms through good social induction.These two core species significantly increased soil microbial colonization of rice roots compared to the control,resulting in a 40% reduction in Cd(II)uptake by rice.Although this study focuses only on the easily neglected and subtle biological barrier processes in agricultural soils,elucidating the ecological barrier mechanism will provide new ideas for the rhizosphere microbial remediation of heavy metal-contaminated agricultural fields.It will help develop and apply new microbial agents,which is of great significance to the remediation of polluted farmland,rebuilding ecosystems,and maintaining human health.