Research On Growth Metabolism And Tolerance Mechanism Of Phosphate-Solubilizing Bacteria Under Pb,Cr Stress

Phosphate solubilizing bacteria(PSB)offers an eco-friendly approach to remediate soil heavy metal contamination with its traits of increasing soil fertilities and detoxicating heavy metal ions.This work was firstly carried out by isolating PSB from soil.Strain F2-1 and F3-2 were finally isolated and identified as Pseudomonas sp.and Bacillus sp.respectively by 16S rRNA sequence analysis.In Pikovskaya medium,the total exudated organic acid of strain F2-1 was found to reach 243.5 mg L-1,which mainly consisted of D-gluconic acid,tartaric acid,malonic acid,acetic acid,the maximum phosphate solubilizing capacity reached 302.0 mg L-1;the total exudated organic acid of strain F3-2 was found to reach 200.8 mg L-1,which mainly consisted of D-gluconic acid,malonic acid,succinic acid,the maximum phosphate solubilizing capacity reached 129.3 mg L-1.Major organic acids were inhibited along with the present and increase of Pb(?)and Cr(?),except succinic acid in strain F3-2 which exuded more under stress.By changing exogenous factors,the results shown that the tolerance of strains was obviously related with incubation temperature and pH,while affected by osmotic pressure slightly.A batch of experiments regarding metal distribution on intracellular/extracellular,loose bound extracellular polymeric substances(LB-EPS)/tight bound extracellular polymeric substances(TB-EPS)was conducted,the results turned out different accumulation behavior of lead and chromium.The accumulated lead ions on extracellular accounted for 61.7%-97.3%of the total accumulation,while it's only 1.3%-13.7%for chromium.In addition,the accumulated metal ions on EPS was mainly distributed on LB-EPS.The results from the investigation of organic matters in LB-EPS shown that in strain F2-1:soluble microbial by-product>humic acid>tryptophan>fluvic acid>tyrosine;strain F3-2:soluble microbial by-product>tryptophan>tyrosine>humic acid>fluvic acid.And the FTIR analysis shown that there were more functional groups involved reaction on F3-2.PSB strains had lower growth rate under Cr(?)stress which suggested that Cr(?)is more cytotoxic.A batch of experiments was conducted to investigate the oxidative balance inside cell.The concentration of Malondialdehyde(MDA)in Pb(?)stress reached 17-18 nmol mg-1 protein while being 29-34 nmol mg-1 protein in Cr(?)stress,which implied Cr caused severe oxygen damage.The isolated PSB strains possessed basal levels of SOD activity in the absence of heavy metals,them were for 75 and 90 U mg-1 protein for strain F2-1 and F3-2 respectively,by the time Pb(?)stress presented and increased,SOD activities of PSB strains increased up to 266 and 210 nmol mg-1 protein,respectively;when Cr stress presented and increased,SOD activity was found negatively correlated with Cr concentration.Catalase(CAT)could further reduce oxides that produced by SOD and oxygen reactive species,it increased under relatively high metal concentration.It was found out from intracellular ROS investigation that both lead and chromium stress induced ROS but with different behaviors.In this study,two heavy metal-tolerant PSB strains were isolated successfully,and characterized heavy metal effects on its organic acids exudation,capability of phosphate-solubilizing,subcellular level heavy metal accumulation,intracellular oxygen balance.The reason of different performances on heavy metal tolerance of two strains was discussed based on above analysis,explored and discussed the growth metabolism and tolerance mechanism of PSB under heavy metal Pb(?,Cr(?)stress.This study is relevant,which benefits the application of PSB for bioremediation and improvement of PSB tolerance capacity.