| Rare earth elements(REEs)are important strategic resources,which is the essential raw materials used in industrial fields such as sophisticated electronics,aerospace and new materials.Southern Jiangxi,enjoying the reputation of “the kingdom of the rare earth elements”,is one of the most important national bases and the main producing area of ion-absorbed REE resources in China.Since the discovery and exploitation of REEs in the area in 1969,about 300,000 tons of the REE resources have been extracted here,accounting for more than 70 % of the ion-absorbed REE amount in China.The REE exploitation had brought considerable economic benefits,but also caused serious ammonia-nitrogen polluted soils and water.Recent techniques for mining REEs inevitably lead to severely pollute soils and water induced by the leaching solution(ammonium sulfate).The over high concentration of ammonia nitrogen may result in hardening soils,hindering the respiration of plants and use of water.In particular,when ammonia nitrogen in water is converted to the nitrite nitrogen,poisoning from drinking water and cancers will be caused in local residents.Therefore,it is imminent to artificially remediate the ammonia-nitrogen pollution in the tailings of REE mines.However,the techniques of traditional physical and chemical remediation still have the limitations that the pollutants can not be extensively removed,and the secondary pollution may easily be induced.The microbial remediation,as an effective safe tool to remediate polluted environments,recently attracts our increasing attention.In this paper,we performed the filed observations and systimatic sampling in the tailings of REE mines in southern Jiangxi,analyzing the microbial diversity in the soils/sediments by the DGGE technique,screening an indigenous strain Gan-35 with the outstanding ammonia nitrogen-degrading ability from the tailings of REE mines in Gannan,optimizing the ammonia nitrogen-degrading conditions,anatomizing the degradation mechanisms of ammonia nitrogen by microbes,performing the high-throughput sequencing for strain Gan-35,and analyzing the ammonia-nitrogen degrading pathways and straong salt tolarence at the gene level.These results may provide the scientific evidence and technic support for constructing high-efficency ammonia nitrogen-degrading strain and performing micorbial remediation in the future.The innovative results are described in detail as bellow:(1)The ammonia-nitrogen pollution is very serious in the tailings of the REE mines in Gannan,as shown that the content of ammonia nitrogen in water was up to 1835.7 mg/L,and the highest ammonia-nitrogen content in the tailings reached 899.4 mg/L with an average of 724.6 mg/L.The microbial diversity analysis in soils shows that there had the species diversity to certain extent in the area,but not rich;and fungi were better than that of the bacterial diversity.(2)One strain,named as Gan-35,with the outstanding ammonia nitrogen-degrading ability in the concentration of 1,000 mg/L was screened from the tailings polluted by ammonia nitrogen.Based on the physiological,biochemical,and molecular biological features,the strain was identified as Burkholderia sp.Up to date,no bacterium with the ammonia nitrogen-degrading ability has been reported by screening from this kind of the polluted tailings.The determination of nitrite-and nitrate-nitrogen in the culture broth produced by Gan 35 in the ammonia-degrading process suggests that the strain had the ability to convert ammonia nitrogen into the matters utilized by itself or beneficial substances to environments with the production of minor nitrate and nitrite.(3)The optimization of ammonia nitrogen-degrading conditions for Burkholderia Gan-35 suggests that the optimal carbon source is glucose;pH is 7.0;the inoculation quantity is 10%;the fermentation time is 48 h;temperature is 30 ?C;C/N ratio is 10:1;and the rotation rate is 150 rpm.In the above optimal conditions,the maximum ammonia-degrading rate was up to 68.6%.During the 12-days experiment,we found that Gan-35 not only had a strong ammonia-nitrogen degrading ability,but also evidently promoted the plant growth in the concentration of 1,000 mg/L.Therefore,Gan-35 may be an important potential engineering strain for treating the ammonia nitrogen-polluted tailings of REE mines in Gannan.(4)The complete genome sequence of Gan-35 was spliced and assembled into 47 chromosome scaffold sequences and 2 plasmids,and 7,519 genes were predicted.Of which 5,596 proteins have been COG numbering,suggesting that most of them are involved in basic metabolisms.The GO annotation shows that the total of 5,078 proteins had the GO forecasting information.The genetic prediction for the Gan-35 genome suggests that there is the tRNA of selenocysteine.The complete denitrification enzymes can be found in the Gan-35 genome,including nitrate reductase,periplasmic nitrate reductase,nitrite reductase and nitric oxide reductase,but no enzymes of the classical nitrification pathway.The identification of the above-mentioned functional genes may lay the basis for revealing the molecular mechanisms of the ammonia-nitrogen degradation by Gan-35 and successfully constructing the outstanding ammonia nitrogen-degrading engineering strain. |
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