The Photochemical Demethylation Of Methylmercury And The Bacterial Methylation Of Mercury

This paper mainly and preliminarily discuss and research the mechanism of photochemical demethylation of methylmercury and the process of the bacterial methylation of mercury in surface water and its sediment environment.For the study of the photochemical demethylation?photodegradation?of methylmercury,we select three representative environmental water?sea water,creek water,dissolved organic matter aqueous solution?as the research object based on the research of the photodegradation of methylmercury in Florida Everglades water previously and research the photodegradation of methylmercury in different condition by the experiment of adding singlet oxygen,hydroxyl radical and hydrated electronm scavenger,competitive ligand?N₃^-?,complexing agent?Cysteine?,purging oxygen and nirogen.Furthermore,we also research the change of photodegradation rate constant of methylmercury in different proportion of methylmercury and dissolved organic matter.The results show that the photodegradation of methylmercury is inhibited when purging oxygen and promoted when purging nitrogen in this three kinds of environmental water,adding hydroxyl radical and hydrated electronm scavenger hardly influence the photodegradation of methylmercury,but adding singlet oxygen scavenger can inhibit the photodegradation of methylmercury obviously.From the experiment result of adding cysteine,the reason of inhibition is not because sodium azide scavenge singlet oxygen,but because sodium azide will compete methylmercury's blinding site with dissolved organic matter and cysteine,and produce MeHg-N₃?photoactivity is smaller,photodegradation can't happen easily?.In addition,the photodegradation rate constant of methylmercury remain approximate constant first,then decrease rapidly after increase rapidly with the proportion of methylmercury and dissolved organic matter rise.All in all,direct photodegradation MeHg-DOM complex by intramolecularly electron transfer may be an universal mechanism for photodegradation of methylmercury in common environmental water.For the study of the bacterial methylation of mercury,we select two kinds of bacteria—sulfate reducing bacteria?Desulfovibro desulfuricans subsp?DD?and Desulfovibrio africanus?DA??and iron reducing bacteria?Geobacter sulfurreducens?GS??which can methylate mercury and are reported the most in domestic and foreign paper now.Firstly,we optimize cultivate condition?temperature,the composition and proportion of anaerobic gas,medium component and so on?over and over again,then cultivating three kinds of bacteria based on optimum cultivate condition,and confirm the appropriate time?adding medicate when bacteria in the logarithmic phase?and concentration of adding Hg???by determinating growth curve and mercury dose-effect curve,after that determinating both bacteria intracellular?precipitum?and extracellular?supernate?content of mercury and methylmercury after a period of mercury exposure,and comparing the capacity of absorbing and methylating mercury among different bacteria,and then discussing bacterial methylation of mercury's process preliminarily.The results show that DA and GS have similar growth rate that are slower than DD,but DA and DD have similar final bacterial concentration that are smaller that GS.Intracellular mercury content is more than extracellular mercury content,but the content of methylmercury is adverse.DA and GS have the capacity of methylating mercury,but the methylation rate of DA is lower,DD may not have the capacity of methylating mercury.From the result of agarose gel electrophoresis,we can see DA has hgcA?methylation gene?,but DD dosen't have.The process of bacterial methylation of mercury may be mercury capture methyl and then produce methylmercury within bacteria by some process of bacterial physiological metabolism?protein synthesis?after bacteria absorb mercury into its body,then methylmercury release out of bacteria quickly.