Microbe Mediated Molecular Speciation And Transformation Of Sulfur And Heavy Metals In Paddy Soil

In paddy soils, changes of redox conditions in different zones led to microbially mediated sulfur transformation, thus affecting heavy metal behavior. Microbe-mineral interactions play an important role in affecting geochemical transformations of heavy metals in the soil environment. With the developing of molecular environmental science and the evolution of molecular biology technologies in recent years, a late hotspot in the environmental science is how to understand the reaction mechanisms of heavy metals in environment medium at molecular scale. Based on the traditional analysis methods, the synchrotron radiation X-ray absorption spectroscopy (XAS), the synchrotron radiation X-ray fluorescence (XRF), as well as PCR-DGGE, FISH and molecular cloning were used in present study. The aim of our study was to clarify heavy metals and sulfur transformation and the relations between these processes under different conditions, and microbial community diversity and changes of biochemistry characters. The main results were as follows:The speciation of sulfur and copper in rice rhizosphere and bulk soil was investigated using integrated approaches including sequential extraction and XANES. Cu speciation exhibited some differences in rhizosphere and bulk soil. Most Cu in the rhizosphere existed as Cu(II), whereas part of Cu transformed to Cu(I) in the bulk soil. Sulfur XANES showed the presence of multiple both oxidized and reduced forms of sulfur, with more oxidized sulfur in the rhizosphere than in the bulk soil. Changes in redox potential and microbial action shifted the sulfur oxidation and reduction reaction and affected the Cu speciation. Combined action of organisms maintained Cu homeostasis through cation binding to bioactive molecules. Cu bond to histidine and citrate groups as well as glutathione in rice could be defenses against toxic copper.The effect of sulfur on the availability of Cu, Zn, Pb and the microbial community composition in rice rhizosphere soil was studied. Under the impact of sulfur, the availibity of Cu and Zn increased dramatically, while Pb did not. Accordingly, PCR-DGGE experiment suggested that sulfur action led to dramactic changes of soil bacteria composition in Cu polluted soil, while there were no obivious differences in Pb polluted soil. Some specific clones found in S addition soils had high similarity to Thiobacillus, which indicated relatively high rates of potential S oxidation.The compositions of SOB and SRB and their relationship with Pb were studied in rice rhizosphere. FISH results showed more SOB than SRB. Combined action of SOB and SRB contribute to transformation of heavy metals. The formation of metal sulfide, which is mediated by SRB through contributing to sulfate reduction is an important pathway for heavy metal stabilization in anoxic soil. The mechanism of SOB and SRB on transformation of sulfur and heavy metals was studied. PCR-DGGE fingerprinting and real-time PCR results showed increasing numbers of SRB with Pb addition, which corresponded with increases in soil pH and reduction in Eh, suggesting the enhancement of sulfur reduction and SRB activity. Sulfur K-edge XANES revealed reduced states of sulfur. The SRB mediated the sulfate reduction and contributed to the formation of reduced sulfur which interacted with Pb, leading to the formation of stable metal sulfide. In return, acclimated SRB populations developed in Pb-polluted conditions. Hence stabilization of reduced sulfur by Pb enhanced the activity of SRB and sulfate reduction in rice rhizosphere.A mesophilic, chemolithoautotrophic, SOB named HT1 was isolated from a rice rhizosphere soil polluted by Pb. The 16S rRNA gene sequence showed that it was a S oxidizing obligate chemolithotroph belonging to Grammproteobacteria, Halothiobacillus utilizing sulfide, elemental sulfur, thiosulfate and sulfite as substrates. Strain HT1 was able to use CO2 as a carbon source and was responsible for the reduction of nitrate to nitrite and represented a halophilic SOB capable of growth within 0 to 3 M NaCl and a heavy-metals-tolerant SOB. The soxB gene could not be detected in strain HT1. Its metabolism pathway was'S4 intermediate'(S4I) pathway. Sulfur globules accumulated in strain HT1 were mainly S8.It was illustrated that cysteine was involved in chelation within strain HT1 when interacted with Cu2+, Pb2+and Zn2+. Strain HT1 could oxidize sulfide in CuS, PbS and ZnS and cysteine was also synthesized for the tolerance against heavy metal. There were two active zones in the biofilm of HT1:air-biofilm interface and medium-biofilm interface. Oxygen played an important role in the distribution of live cells. The biofilm of HT1 presented different spatial distribution when reacted with different metal sulfides. Strain HT1 did not absorb Zn which resulted in more live cells of HT1 on ZnS than on CuS or PbS. XRF results showed that Pb was mainly on the upper layer while Cu was mainly on the under layer of the biofilm. Withμ-XAFS, it was found that ligands bond to Cu were different. Whiel Cu-cysteine was found to be an important ligand both in the middle and edge of the biofilm.