|Alternative Title||Biological Effects of Mercury on Unicellular Eukaryotic Tetrahymena|
|Thesis Advisor||江桂斌 ; 史建波|
|Place of Conferral||北京|
|Keyword||汞,四膜虫,生物效应,拮抗作用,甲基化 Mercury, Tetrahymena, Biological Effects, Antagonism, Methylation|
考虑到低剂量暴露在实际环境中的重要意义，依赖于流式细胞技术，研究了低浓度二价汞离子和甲基汞暴露对四膜虫生长增殖的影响。结果表明，低浓度的两种汞化合物对三种四膜虫的增殖产生了显著的促进作用（即“毒物兴奋效应”），而对另外两种四膜虫没有影响。同时，选取另外六种常见重金属离子对其中一种四膜虫（T. thermophila SB210）进行低浓度暴露，发现了类似的生长促进作用，并且这种生长促进作用与暴露的起始时间以及暴露的时间长短均有关。
Mercury (Hg) is regarded as a global toxic pollutant due to its long-range transport, environmental persistence, bioaccumulation, and high toxicity. The toxicity of mercury is closely related to its species, and methylmercury (MeHg) is recognized as the most toxic mercury compound. In aquatic environments, mercuric ions (Hg2+) and MeHg are the major species of mercury and they can be transformed into one another by chemical or biological processes. Tetrahymena is a type of unicellular eukaryotic protozoa, which is widespread in fresh water bodies. Located at the bottom of the food chain, Tetrahymena is sensitive to toxic substances in aquatic systems. In the case of mercury, the uptake and accumulation by Tetrahymena can be an important point for mercury to enter the food chain and participate in the biogeochemical cycle. Therefore, studies about the accumulation, transportation and transformation as well as the biological effects of different mercury species in Tetrahymena are environmentally significant.
In this dissertation, the uptake kinetics of Hg2+ and MeHg in Tetrahymena were first studied. The different ingestion rates of Hg2+ and MeHg in Tetrahymena were compared, and the toxic characteristics and potential mechanisms of the two mercury species were further investigated. The growth-promoting effect (known as “hormesis”) of low-dose mercury on Tetrahymena was discovered. Furthermore, the effects of the starting time of exposure on the growth promotion were investigated. Afterwards, one species of Tetrahymena (T. malaccensis) was selected to be exposed to three species of selenium and two species of mercury, and the interactions of selenium and mercury with different species and concentrations in T. malaccensis were studied. In addition, analytical methods for MeHg in Tetrahymena cells were optimized. Based on the analytical methods, the possible transformation of inorganic mercury in Tetrahymena was studied after exposure to a variety of exposure conditions.
First, the accumulation, toxicity and potential mechanisms of Hg2+ and MeHg in five Tetrahymena species were studied. The results of uptake kinetics and cell viability experiments showed that both Hg2+ and MeHg could be absorbed by Tetrahymena and, thus, induce the inhibition on the proliferation of Tetrahymena. Tetrahymena has a faster ingestion rate for MeHg than Hg2+. Further mechanistic studies have shown that potential toxic mechanisms of Hg2+ and MeHg are significantly different, although both of them can cause changes in the mitochondrial membrane potential of Tetrahymena cells. In detail, Hg2+ can significantly increase the reactive oxygen species in Tetrahymena and then leads to the inhibition of cell growth, while MeHg mainly affects the survival of Tetrahymena by destroying the integrity of Tetrahymena cell membranes.
Considering the environmental importance of low-dose exposure, the effects of low-dose Hg2+ and MeHg exposure on the growth and proliferation of Tetrahymena were initially explored relying on flow cytometry. The results showed that the low-dose exposure of both two mercury compounds induced significantly growth-promoting effect (i.e. hormesis) on the proliferation of three Tetrahymena species but showed no effect on the other two Tetrahymena species. Six other common heavy metals were further selected and found to display the similar growth-promoting effects on T. thermophila SB210 after exposure to low concentrations. The hormesis was found to be related to both the onset and the duration of exposure.
One species of Tetrahymena (T. malaccensis) was used to examine the joint effects of selenium and mercury after exposure to different species and concentration combinations. Individual exposure of selenium compounds showed that the biological effects of these selenium species on T. malaccensis were different, with sodium selenite being the most toxic species, followed by selenomethionine, and sodium selenate having almost no toxicity to T. malaccensis. The joint-exposure results showed that the antagonistic effects of mercury and selenium also existed in T. malaccensis and could be defined as mutual detoxification. This mutual detoxification was closely related to the respective species and concentration of selenium and mercury. Compared to other species, MeHg and selenomethionine are more likely to participate in the antagonism of mercury and selenium in T. malaccensis.
Tetrahymena was exposed to inorganic Hg2+ under various exposure conditions. Pretreatment method and multiple analytical methods for the analysis of MeHg in Tetrahymena cells were established and optimized to make an accurate judgment on the presence of MeHg in Tetrahymena after exposure to Hg2+, including gas chromatography coupled with atomic fluorescence, ultra-high performance liquid chromatography coupled with inductively coupled plasma mass spectrometry, isotope labeling method and isotopes dilution method. After repeated experiments, no MeHg production was detected in Tetrahymena, indicating that the methylating effect of Tetrahymena on Hg2+ was not obvious.
|刘成斌. 汞对单细胞真核生物四膜虫的生物效应研究[D]. 北京. 中国科学院生态环境研究中心,2018.|
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