HR: 11:50h
AN: B11E-07    [PDF]
TI: Mechanism of Non-Steady State Dissolution of Goethite in the Presence of Siderophores
AU: * Reichard, P U
EM: reichard@ito.umnw.ethz.ch
AF: Institute of Terrestrial Ecology ETH Zuerich, Grabenstrasse 3, Schlieren, ZH 8952 Switzerland
AU: Kretzschmar, R
EM: kretzschmar@ito.umnw.ethz.ch
AF: Institute of Terrestrial Ecology ETH Zuerich, Grabenstrasse 3, Schlieren, ZH 8952 Switzerland
AU: Kraemer, S M
EM: stephan.kraemer@ito.umnw.ethz.ch
AF: Institute of Terrestrial Ecology ETH Zuerich, Grabenstrasse 3, Schlieren, ZH 8952 Switzerland
AB: Iron is an essential micronutrient for almost all known organisms. Bacteria, fungi, and graminaceous plants are capable of exuding siderophores as part of an iron acquisition strategy. The production of these strong iron chelating ligands is induced by iron limited conditions. Grasses under iron stress, for example, exude phytosiderophores into the rhizosphere in a special diurnal rhythm (Roemheld and Marschner 1986). A few hours after sunrise the exudation starts, culminates around noon and is shut down again until about 4 hours after noon. The phytosiderophores diffuse into the rhizosphere (Marschner et al. 1986) and are passively back transported to the plants by advective flow induced by high transpiration around noon. Despite a fairly short residence time of the phytosiderophores in the rhizosphere, it is a very effective strategy for iron acquisition. To investigate the effect of such pulse inputs of siderophores on iron acquisition, we studied the dissolution mechanism of goethite (alpha-FeOOH), a mineral phase common in soils, under non-steady state conditions. In consideration of the chemical complexity of the rhizosphere, we also investigated the effect of other organic ligands commonly found in the rhizosphere (e. g. oxalate) on the dissolution kinetics. The dissolution experiments were conducted in batch reactors with a constant goethite solids concentration of 2.5 g/l, an ionic strength of 0.01 M, a pH of 6 and 100 microM oxalate. To induce non-steady state conditions, 3 mM phytosiderophores were added to a batch after the goethite-oxalate suspension reacted for a certain time period. Before the siderophore was added to the goethite-oxalate suspension, no dissolution of iron was observed. But, with the addition of the siderophore, a high rate was observed for the iron mobilization under these non-steady state conditions that subsequently was followed by a slow steady state dissolution rate. The results of these non-steady state experiments are very reproducible and the magnitude of dissolved iron corresponds to the reaction time of goethite with oxalate. Analogous non-steady state experiments were conducted, but with two other siderophores or citrate to induce non-steady state conditions: 40 microM of the bacterial siderophore desferrioxamine B (DFO-B), 40 microM of the fungal siderophore Ferrichrome, and 3 mM of citrate. Fast dissolution of iron was observed as a response to non-steady state. We also substituted the non-siderophore ligand oxalate by 500 microM citrate or 750 microM malonate and again observed fast dissolution after the non-steady state siderophore additions. Independent of the type of the ligands, a reproducible fast dissolution of iron followed by steady state dissolution was observed after the addition of the non-steady state ligand concentrations. Thus it can be said that the reproducible fast dissolution of iron under non-steady state conditions represents a general geochemical mechanism and an important process in the context of biological iron acquisition in natural systems. References Marschner, H., Roemheld, V. et al. (1986). "Different Strategies in Higher-Plants in Mobilization and Uptake of Iron". Journal of Plant Nutrition 9(3-7): 695-713. Roemheld, V. and Marschner, H. (1986)." Evidence for a Specific Uptake System for Iron Phytosiderophore in Roots of Grasses". Plant Physiology 80(1): 175-180.
DE: 0400 Biogeosciences
DE: 1045 Low-temperature geochemistry
DE: 1875 Unsaturated zone
DE: 1886 Weathering (1625)
DE: 4805 Biogeochemical cycles (1615)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting