Gaseous Elemental Mercury [Hg(0)] Oxidation in Poplar Leaves through a Two-Step Single-Electron Transfer Process
Yanwei Liu, Hui Tao, Ying Wang, Yingying Fang, Yuping Xiang, Guangliang Liu, Yingying Guo, Jiyan Liu, Yongguang Yin, Yong Cai, Guibin Jiang
Abstract
Foliar oxidation of gaseous elemental mercury [Hg(0)] determines the foliar Hg uptake rate and accumulation capacity and plays a crucial role in atmospheric Hg deposition and biogeochemical Hg cycle. However, mechanisms of foliar Hg(0) oxidation are poorly understood in plants, particularly in forest trees. Herein, foliar oxidation of Hg(0) to different Hg oxidation states was studied using poplar as a model tree species. Mercuric form [Hg(II)] was the dominant Hg species in foliage after in vivo Hg(0) exposure, and trace amounts of mercurous form [Hg(I)] was also detected. Results from in vitro Hg(0) exposure using a leaf homogenate demonstrated that both Hg(I) and Hg(II) were formed with Hg(II) being the dominant species. However, Hg(I) instead of Hg(II) was detected if a phosphate buffer was used as medium for the in vitro Hg(0) exposure experiment. In the leaf homogenate, after enzymatic inhibition and thiol blocking, formation of Hg(II) from Hg(0) or Hg(I) was significantly reduced, whereas Hg(0) oxidation to Hg(I) increased. This work demonstrates that gaseous elemental mercury oxidation in foliage can occur via a two-step single-electron transfer process, involving Hg(I) and Hg(II) formation mediated by nonenzymatic and enzymatic reactions.