Mechanisms Research On Removal Of Heavy Metal Cr(?) By Dissimilatory Iron-reducing Bacteria From Marine Sediment

Dissimilatory iron-reducing bacteria is a general term for microorganisms that transfer electrons generated by metabolism of intracellular organics to extracellular and reduce Fe(?)to Fe(?).The microbial-mediated dissimilatory iron reduction process can reduce heavy metal pollution.Dissimilatory iron-reducing bacteria are widespread in anaerobic habitats such as marine sediments.In this experiment,Clostridium butyricum LQ25,a dissimilatory iron-reducing bacteria isolated from marine sediments,was used as the research object to analyze the effect of microbial dissimilated Fe(?)reduction process on the reduction of Cr(?)and the mechanism of dissimilated Fe(?)reduction.The main research results included the following aspects:(1)Analysis of the characteristics of dissimilated Fe(?)reduction by strain LQ25,Fe(?)concentration was 0?40 mmol/L,Under Fe(?)concentration 10 mmol/L,The strain growth A600 and the cumulative Fe(?)concentration reached the peak,respectively(1.62±0.01)and(5.41±0.21)mmol/L.Continued increase of Fe(?)concentration would inhibit the strain growth and reduce its iron reducing ability.The optimal Fe(?)concentration for strain LQ25 growth and dissimilated Fe(?)reduction is 10 mmol/L.(2)At the end of the cultivation of strain LQ25,collected the bacterial cells to prepare biomagnetite.Morphological observation through electronic microscope and XRD analysis of the characterization properties of biomagnetite.At the same time,setting en?ronmental conditions to analyze the influencing factors of heavy metal Cr(?)removal by biomagnetite.The the range of pH was 3.0 to 9.0,As the pH increased,Cr(?)removal rate showed a downward trend.When pH was 3.0,the Cr(?)removal rate was the highest,which could reach 100%.In the range of 20 to 160 mg,as the dosage increased,the Cr(?)removal rate continued to increase,The Cr(?)removal rate at a dosage of 160 mg is 7.4 times the Cr(?)removal rate at a dosage of 20 mg.When the initial Cr(?)concentration was 5?30 mg/L,biomagnetite could effectively remove Cr(?).(3)Screening mutant of strain LQ25 by UV mutagenesis,high-concentration Cr(?)tolerance mutagenesis,and genome shuffling.Based on two rounds of UV mutagenesis and high-concentration Cr(?)tolerant mutagenesis,9 mutant strains were obtained,and 9 mutant strains were used as parent strains for protoplast fusion and obtained 10 fusion strains.Using the reduction efficiency of dissimilated Fe(?)as the screening index,obtained two mutant strains C2 and R9,whose cumulative Fe(?)concentration increased by 6.08%and decreased by 80.10%compared with original bacteria.(4)The analysis of the whole genome sequencing results showed that the electron transfer process of fermentative dissimilatory iron-reducing bacteria LQ25 is very different from the typical cytochrome c of breath-type dissimilatory iron-reducing bacteria,and there was a large amount of iron-sulfur protein.The analysis suggested that it could be iron-sulfur protein replaced cytochrome c in the electron transfer process.The genome contained a variety of genes related to electron transport.The annotated protein functions included cytochrome c551,cytochrome c biogenic protein ccdA,type ? pili assembly protein PilA,electron transfer flavin protein,riboflavin transporter,riboflavin synthase.(5)The results of transcriptome sequencing analysis showed that the mutant strain C2 had three significantly differentially expressed genes relative to the original strain LQ25,and all were up-regulated.The mutant strain R9 had 65 significantly differentially expressed genes relative to the original strain LQ25,including 23 up-regulated genes and 42 down-regulated genes.The significant difference in the transcriptome level between the mutant strain R9 and the original strain LQ25 made the two strains have significantly different iron dissimilation reduction efficiency.