Resistance Mechanisms And Remediation Potential For Chromium Contamination In Pseudomonas Sp.,AN-B15

Chromium(Cr),due to its widespread industrial usage,causing serious Cr contamination in soil.Cr in farmland can reduce soil fertility,damage soil heath,and cause a reduction in crop yields.Plant growth-promoting bacteria(PGPB)play an important role in soil health and promoting plant growth,which could be applied to remediation of Cr contaminated farmland.However,the mechanisms involved in Cr(Ⅵ)reduction and resistance in PGPB remain unclear.In this study,a PGPB(Pseudomonas sp.,AN-B15)was used as a representative strain to explore the efficiency and mechanisms of Cr(Ⅵ)reduction.Transcriptome analyses revealed the mechanism of Cr(Ⅵ)resistance in AN-B15 at trancriptional level.The bioremediation potential of Cr(Ⅵ)by strain AN-B15 was confirmed by hydroponic culture experiment of wheat seedlings.The biochar was further used as a matrix to make an immobilized bacteria to remediate Cr(Ⅵ)contaminated farmland.The fraction of Cr in soil,soil enzyme activity,and microbial community were analyzed.This study provides a scientific basis for microremediation of Cr contaminated farmland.The main results of this study are following:(1)The resistance mechanisms and remediation potential of Cr(Ⅵ)in AN-B15 were investigated.Cr(Ⅵ)was transported into the cytoplasm where it was reduce to Cr(Ⅵ)by the soluble enzymes(Chr R and azo R).Transcriptome analyses showed that the molecular mechanisms involved in Cr(Ⅵ)reduction and resistance were collaborative processes involving multiple metabolic systems at the transcriptional level.In response to Cr(Ⅵ)exposure,strain AN-B15 up-regulated genes involved in central metabolism to provide reducing power,including valine,leucine,and isoleucine degradation and tryptophan metabolism in amino acid metabolism,glyoxylate and dicarboxylate metabolism and propanoate metabolism in carbohydrate metabolism,and synthesis and degradation of ketone bodies in lipid metabolism.Genes involved in the alleviation of oxidative stress and DNA repair were up-regulated to neutralize Cr(Ⅵ)-induced toxicity.Additionally,genes involved in organosulfur metabolism and certain ion transporters were up-regulated to counteract the starvation of sulfur,molybdate,iron,and manganese induced by Cr(Ⅵ)stress.(2)Immobilized bacteria play a important role in reducing the content and fractionation of Cr(Ⅵ)in soil.The results showed that EXC-Cr(Ⅵ)was reduced by27.9% and EXC-TCr was reduced by 22.7%.Furthermore,immobilized bacteria significantly affected microbial diversity and microbial community.Immobilized bacteria decreased the abundance of Proteobacteria and increased the abundance of Actinobacteria and Firmicutes.Co-network analysis showed that after the amendment of the immobilized bacteria,the connection between native bacteria was more compart,and Pseudomonas,as a key species,formed a stable community with native bacteria and strengthened the reduction of Cr(Ⅵ).