Screening Of Resistance To Cadmium,Chromium Strains And Study Effects Of The Rice Rhizosphere Micro-ecology

In this study,we screened indigenous strains resistant to cadmium and chromium from soils contaminated with cadmium and chromium in paddy fields.The effects of these strains on the growth of rice seedlings were studied,and the mechanism of resistance to heavy metals were further studied,which laying a foundation for its application in the field.At the same time,microbial community composition and functional diversity in the rhizosphere soil of rice by the exogenous Cd,Cr in different concentration would be studied and analyzed,which will be obtained the information on soil micro-ecological differences,which will provid a theoretical basis for microbial remediation of heavy metal pollution.The key research findings were as follows:(1)8 resistant strains were screened from cadmium and chromium contaminated paddy soil by acclimation of concentration gradient approach.Five strains could endure 16mmol/L maximum concentration of cadmium(Cd2+),and four strains could endure 1000mg/L maximum concentration of chromium(Cr6+),and strain X29 could endure 16mmol/L Cd2+ and 1000mg/L Cr6+.The strains were identified from morphological examination and 18 S rDNA sequence analysis.Eight filamentous fungi were belonged to Metarhizium anisopliae,Phomopsis sp.,Aspergillus sp.,Trichoderma sp.,Fusarium oxysporum,Saccharomyces cerevisiae,Penicillium sp..Of which strain 52 and strain 55 were all Penicillium sp.,and the remaining 6 strains belonged to different species.(2)Eight heavy metal resistant strains with rice(Oryza sativa)plants of seedling age 5 days were co-cultured in 1/2 Ms.Biological character of rice was compared with the controls after 7 days.The growth for rice had no inhibitory effect with Metarhizium anisopliae and Saccharomyces cerevisiae.The mechanism of resistant to cadmium and chromium was studied for Saccharomyces cerevisiae.Scanning electron microscopy was used to observe the damage of mycelium after treatment with chromium and cadmium,and then apoptosis of fungus was observed.Chromium and cadmium ions were adsorbed by Saccharomyces cerevisiae.The absorption peaks of C=O,C-H and N-H in the FTIR(Fourier Transform Infrared Spectrometer)spectra were obviously changed,which indicated that the adsorption of cadmium and chromium ions was closely related to the amide and carboxyl groups.(3)DGGE showed many common bands under conditions of Cd and Cr pollution,suggesting the presence of similar of bacterial communities.Principal component analysis(PCA)on bacterial diversity data is shown 57.24% of the variance in the data is explained by principal component 1,and 27.98%by principal component 2,which suggests that the rhizosphere bacterial community changed with prolonged exposure to heavy metal pollution.Major bands of DGGE patterns were sequenced.Phylogenetic analysis indicated that the majority of the sequences obtained were affiliated with members of Acidobacteria(18.75%),Myxococcales(12.50%),Gemmatimonas(12.50%),and Thiothrix(9.38%).When the rice plants reached the heading stage,the relative abundance of members of Gemmatimonas increased with Cd concentration,while abundance of members of Gallionellaceae increased with Cr concentration.At the maturation stage,the relative abundance of members of Chlorobiales increased with Cd concentration.These results illuminate the effect on diversity of heavy metal pollution in soil.Further study of combinations of other environmental parameters will clarify heavy metal effects on the activity of microbial communities in rice paddy fields.(4)Of the 46 bacterial genera(relative abundance ratio of genera greater than 0.3%),28 and 31 genera were significantly affected by the Cd treatments and Cr treatments,respectively.Soil containing 20 mg/kg Cd exhibited the highest relative bacterial abundance,bacterial community diversity,and carbon utilization diversity.The relative abundance of Longilinea was significantly higher in control soil than in heavy metal-treated soils at all sampling stages.The relative abundance of Pseudomonas was significantly higher in Cd 2 and Cd 3 soils than in the other heavy metal-treated and control soils.The relative abundance of bacteria and carbon source utilization ability were higher at the heading stage than at the maturing stage.Therefore,certain concentrations of heavy metals can improve microbial community structure and carbon utilization diversity.The heavy metal-resistant strains were screened from heavy-metal-contaminated rice fields,which providing strains for microbial remediation of heavy metals.The microbial community structure and population composition in rice rhizosphere were affected by added to the exogenous cadmium and chromium,which providing a theoretical basis for microbial remediation of rice fields with heavy metal pollution by using microecological balance theory.