| 其他摘要 | Thallium (Tl) is a toxic heavy metal present at low concentrations, and has attached increasing concerns worldwide for its high toxicity. This thesis aims at the biogeochemistry of microbe-thallium interaction in the case of fungus. The main contents of the present study are composed of five parts as follows: selection of the fungus strains with high thallium resistance, adsorption and bio-concentration of thallium by the fungus, investigation of subcellular distribution of thallium, microbio-flocculantion of the fungus in the surface water, and ITS (internal transcribed spacer) sequence analysis of the fungus strains.
Compared with the background area, the micro-biomass decreased greatly in the Lanmuchang thallium-mineralized area in southwest Guizhou Province, China. The significant negative correlation is observed between thallium concentration and microbial biomass in the sediment and soil under excess thallium threat, reflecting the intoxication of thallium to the microorganisms. The thallium tolerance of the microorganisms is different, that is actinomycete>bacteria>fungus. From the sediments and soils, around two hundred strains with thallium tolerance are isolated, such as Penicillium, Trichoderma and Paecilomyces.
Nine strains with high tolerance of thallium are screened from the two hundred strains at 1000 mg/L Tl level. Adsorption experiments show that the rate of adsorption vary from 4.63 to 16.89% among the nine strains. The increase of thallium contents in culture result in decreased biomass and decreased adsorption rates of Tl, Ca, and K, which is contrary to the adsorption of Na. It is possible that their adsorption mechanisms are different.
The bio-concentration tests show that the bio-concentration rate of thallium in the nine strains decreased with increasing thallium contents in the culture, the maximum at 7189.06 mg/kg and the maximum factor at 7.19. No significant difference is found between the bio-concentration rates of major elements and thallium. However, the increasing thallium contents in the culture lead to increasing bio-concentration rates of thallium.
In the sub-celluar supernatants, the majority of thallium (up to 79.45%) is found in cytoplasmic supernatant, and the concentrations of thallium in the order of priority is as follows: cytoplasmic>cellwall>cell organelles. Cytoplasmic supernatant is apparently a major storage site for thallium, which is a possible detoxification of thallium by the fungus. By this pathway, the fungus pertain to transport Tl into cell wall and cytoplasmic supernatant, and avoid the toxicity of Tl to vital organelles for growth. In the cell wall supernatant, the contents of Na and Ca are positively correlated with Tl. Hence, it is possible that the transportation of Tl+ from the culture into the cell follows the pathwasy of Na+-K+ ATPase and K+-voltage gated channel, and activation of Ca2+ is more conductive to this process.
The culture broth is used as micro-flocculatant after cultured for three days, which is applied to remove thallium in the surface water of Lanmuchang area. The thallium-removal rate is up to be 70.49% and the effect of factors is pH> temperature>stirring time, whereas the maximum flocculation rate is up to 57.32%, and the effect of factors is pH> stirring time > temperature. The nine strains with high tolerance of thallium are thus highly recommended to apply into the treatment of the contaminated water by thallium.
The genomic DNAs are extracted from the nine strains, using CTAB method. Their ITSs are amplified by PCR and sequencing. The ITSs sequence homology searched by Blast methods in Genebank and the phylogenetic tree constructed with the neighbor-joining method． According the phylogenetic tree, the ITSs of the nine strains are different from the corresponding genus high thallium stress. The results of BLAST search in Genebank show that five strains belong to Penicillium and two belong to Trichoderma among the nine strains. It points a good approach to screen out the fungus resources applied into the micro-remediation for thallium-contamination. |
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