Response Of Soil Bacterial Community To Chromate Pollution Along The Lanzhou Reach Of The Yellow River And Mechanisms Of Bacterial Chromate Redcution

Heavy metal pollutions receive worldwide concern and threaten the ecosystem and human health.Microorganisms are an important part of ecological system and play crucial role in geochemical cycles of heavy metals.With the industrial development of Lanzhou in the last century,the environment was destroyed by heavy metal contaminants at different levels.However,the influence of long-term heavy metal pollutions on bacterial communities is still unclear.The mechanisms of bacterial communities response to severe heavy metals pollution remain to be explored.This study analyzed soil bacterial communities in the Lanzhou reach of the Yellow River by using highthroughput technologies(MiSeq sequencing of 16 S rRNA gene,GeoChip technology,metagenomics and metatranscriptomics),protein expression,and determination of enzyme activity to investigate the relationships between bacterial community and related environment factors.Moreover,to explore the changes of bacterial communities structure,composition and function during chromate reduction,to monitor the response of bacterial activities to short time chromate stimulus,and to understand the mechanisms of chromate reductase in chromate reduction.The main results and conclusions are as follows:(1)The study of bacterila communities in six sites(LJX,XC,XGU,XG,XGD and CG)with various pollution levels along the Yellow River found that XG area was the most seriously heavy metals contaminated areas among them.The results showed significant difference in bacterial communities structure among these six sites.Microorganisms(such as Geobacter,Rhodoferax,and Thiobacillus),which presented significant positive correlations with heavy metals,could be served as indicator to monitor the heavy metal pollution in Lanzhou reach of the Yellow River.(2)The composition and function structures of soil bacterial communities in XG shifted during chromate reduction.The abundances of bacterial populations and genes related to chromate resistance or reduction increased after chromate incubation and rare species may play an important role in chromate reduction.Meanwhile,the abundance of genus Rhodoferax significantly increased after chromate incubation indicated this microorganism might have a central role in chromate reduction.(3)Further study was performed with rapid chromate stimulus to the bacterial community in XG.Comparative analysis of metagenomics and metatranscriptomic data showed expression level of bacterial populations belong to Proteobacteria is higher in chromate treatment group than control group.The expression level of genes related to reactive oxygen species detoxifying,chromate resistance,chromate reduction,and regulator of transcription also elevated in chromate treatment group.The possible mechanisms responded to chromate stress in Desulfobacter was described.(4)The function of NahAa gene in Pseudomonas brassicacearum LZ-4 during naphthalene degradation and chromate reduction was investigated by RNA interference of NahAa expression and determination of enzyme activity.Results showed that the expression of NahAa gene enhanced the chromate reduction ability of P.brassicacearum LZ-4,and the chromate reduction ability of NahAa protein was 2-fold than that of Yief protein.Therefore,this might be the reason for the chromate reduction ability of P.brassicacearum LZ-4 with naphthalene as the sole carbon source is higher than that with glucose as the sole carbon source could be explained by the different expression level of NahAa gene under these two conditions.In conclusion,the composition,structure and function of bacterial community in the Lanzhou reach of the Yellow River were reported for the first time in this paper.Omics technologies discovered novel bacterial resources and applied for bioremediation of chromate pollution.Meanwhile,this paper found genes involved in naphthalene degradation could reduce chromate,which showed potential for treatment of combined pollutants consisted of PAHs and heavy metals.