HR: 17:15h
AN: B14B-05    [Abstracts]
TI: Mercury Emission From Plants Depends on Reduction by Ascorbate
AU: * Halbach, S
EM: halbach@gsf.de
AF: Institute of Toxicology, GSF-Research Center of Environment and Health, POB 1129, Oberschleissheim, D-85758, Germany
AU: Ernst, D
EM: ernst@gsf.de
AF: Institute of Biochemical Plant Patholgy, GSF-Research Center of Environment and Health, POB 1129, Oberschleissheim, D-85758, Germany
AU: Fleischmann, F
EM: fleischmann@wzw.tum.de
AF: Phytopathology of Woody Plants, Technical University Munich, Freising-Weihenstephan, Freising, D-85354, Germany
AU: Battke, F
EM: battke@gsf.de
AF: Institute of Biochemical Plant Patholgy, GSF-Research Center of Environment and Health, POB 1129, Oberschleissheim, D-85758, Germany
AB: The importance of vegetation for the ecological Hg cycle has been recognized recently. One step in this cycle is the poorly understood phytogenic reduction of dissolved Hg(II) to volatile Hg(0) which had initially been reported for common reed growing on Hg-contaminated sediments. The hitherto unknown mechanism of this reduction was the objective of our investigations. Young barley and European-beech plants were cultivated for 24 h and 2 days, respectively, on a sterile hydroponic medium containing 20-40 µM HgCl2. Within 10 min after seclusion in a closed exposure system, the Hg(0) emission from the encapsulated aerial part of the plants reached 10 times the control value in a plant-free system and was proportional to the Hg(II) concentration in the medium. At 20 µM Hg(II) in the medium, a flux of 12.8 µg Hg(0)/m2/h was estimated for beech leaves. The phytogenic Hg(II) reduction was further examined by addition of powderized homogenates from deep-frozen leaves (barley, beech, Arabidopsis thaliana) or from needles (Norway spruce) to solutions of 1-5 µM Hg(II). These samples consistently produced a strong transient Hg(0) release at neutral pH that was even reinforced in alkaline medium and vanished at acidic pH. The very same pH dependence was observed after addition of pure L(+)-ascorbate (AA) instead of plant material to the HgCl2 solutions, whereas the reductants NADPH and GSH produced only little or no Hg(0), respectively. At neutral and alkaline pH, the Hg(II)-reducing capacity of spruce needle homogenates was 2 - 4 times that of beech leaves, which paralleled a 6-fold difference in AA concentrations. Homogenates from whole wildtype-plants of Arabidopsis reduced 8-times more Hg(II) than those from the AA-deficient mutant vtc1-1 (AA concentration 30% of wild type). A comparison of literature data on AA concentrations revealed for wetland plants a range from 0.3 µmol/g DW (Phragmites communis) over 15.0 (Typha latifolia) to < 34.1 (Spartina altiflora), and for trees between 2 and 18 for beech leaves and spruce needles, respectively. CONCLUSIONS The comprehensive model of metabolic mercury volatilization by plants is based on the transpiration flow carrying Hg(II) ions to the leaves for phytogenic reduction in the apoplastic space from where Hg(0) diffuses outward under stomatal control. The reductive regeneration of ascorbate proceeds stepwise in the so called antioxidative defense pathway of plants. Considering the ubiquitous presence of AA in plants, the findings contribute to the basic understanding of the soil-plant-air mercury exchange. Furthermeore, our data support the preliminary conclusion that the mercury reducing capacity of wetland plants is not inferior to that of land-bound vegetation.
DE: 0418 Bioremediation
DE: 0476 Plant ecology (1851)
DE: 0489 Trace element cycling (4875)
DE: 0490 Trace gases
SC: Biogeosciences [B]
MN: 2007 Fall Meeting