Study On Screening,identification And Tolerance Mechanisms Of A Marine Bacterium Tolerant To High Concentration Cesium

137Cs is one of the fission products of nuclear facilities,with a half-life of 30.17 years and a fission yield of 5.9%.It is considered to be one of the biologically most dangerous radionuclides.The biological behavior of cesium is similar to potassium.It enters the food chain through competing K+channels,threatening human health and causing pollution to the ecological environment.At present,the conventional methods for remediation of radionuclide cesium pollution in the environment are not effective and high in cost.Microbial remediation technology has entered the field of vision of domestic and foreign research scholars with the advantages of low cost,obvious effect and not easy to produce secondary pollution.This study enriched and screened a strain of marine bacteria tolerant to high concentration of cesium from marine sediments,and initially explored its tolerance mechanisms.The main conclusions are as follows:(1)In this study,Bacillus sp.Cs-700,a marine bacterium tolerant to high concentrations of cesium,was screened and enriched from sediments in a sea area of Fangchenggang,Guangxi.The bacterium is a gram-positive,moderately halophilic bacterium with an optimal growth salinity of around 11%.It cannot grow when the salinity is 0%,and there are weak signs of growth at 25%,The minimum inhibitory concentration(MIC)of Cs+ against this strain is 1800 mM.The comparison of the model strain Bacillus hwajinpoensis SW-72T,which is closely related to this strain(99%),found that the ability to tolerate high concentrations of Cs+ is unique to Bacillus sp.Cs-700,not the commonality of the Bacillus genus.(2)Under the Cs+ environment,in order to resist the stress of cesium and enhance its tolerance,Bacillus sp.Cs-700 cells are bent,elongated,and clumped to form capsule analogs.EDX spectrum analysis showed that after the bacteria adsorbed Cs+,the K element peak and percentage content were partially replaced by Cs,indicating that Cs+and K+were competitively adsorbed to the bacteria.The binding of bacteria to Cs+ is used to improve their tolerance to Cs+.(3)The Bacillus sp.Cs-700 genome consists of a 4,297,839 bp circular chromosome with GC content of 40.32%.A total of 4,486 genes were identified.Most genes are responsible for the transport and metabolism of amino acids,carbohydrates,inorganic ions,energy production and conversion,replication,transcription,translation,and cell wall/membrane/envelop biosynthesis.From the genome,we predicted 26 genes related to Cs+ transport.It has provided a genetic and molecular basis for the in-depth study of physiological regulation mechanisms and application potential under Cs+ stress environment.(4)The transcriptome data before and after Cs+ treatment were analyzed by RNASeq sequencing.A series of differential genes screened showed that under high concentration of Cs+ stress,osmotic pressure and cation-specific toxicity have overlapping effects.Genes related to osmotic pressure regulator synthesis,defense protein synthesis,amino acid metabolism,cell wall and membrane biosynthesis,small molecule,peptide,metal ion transport,energy synthesis,and oxidative stress process regulation are significantly up-regulated.In addition,genes related to serine protein kinase,ribosomal protein,DNA binding protein,and phosphate regulation have also been up-regulated.All of which contribute to the tolerance of Bacillus sp.Cs-700 to Cs+.(5)The atomic absorption spectrophotometer was used to determine the adsorption and removal ability of Bacillus sp.Cs-700 to Cs+.The results showed that the adsorption of Cs+ by the bacteria was related to the structural characteristics of the cell wall and the cation channel.At 48 h,the maximum removal rate could reach 52.06%.In summary,we screened and obtained the marine bacterium that can tolerate the highest concentration of cesium at present:Bacillus sp.Cs-700,which not only has a strong ability to tolerate cesium,but also has a strong ability to remove cesium,indicating that Bacillus sp.Cs-700 can be used for potential bioremediation of radioactive cesium polluted environment.At the same time,this study provides ideas for exploring the ecological restoration of marine radioactivity.