Influence of microbial hydroxamate siderophores on Pb(II) desorption from α-FeOOH
Introduction
The very low solubility of Fe(III) minerals in oxic environments restricts the activity of Fe3+(aq) to values [e.g. <10−18 at pH 6, Schwertmann (1991)] that are too low to meet microbial demands (Powell et al., 1980); yet it is precisely in such environments that micro-organisms are most abundant and diverse (Crowley, 2001). Given the very low concentrations of available Fe, and its essential role in many metabolic reactions, aerobic and facultative anaerobic micro-organisms have evolved Fe acquisition mechanisms involving the secretion of low molecular weight Fe(III) chelators known as siderophores (Crowley et al., 1991, Winkelmann, 1991, Buckley and Schmidt, 2001).
More than 200 siderophores have been isolated, the majority of which possess either catecholate or hydroxamate functional groups, and all of which form Fe(III) complexes of exceptional stability (Powell et al., 1980, Albrecht and Crumbliss, 1998, Buckley and Schmidt, 2001). With 1:1 stability constants commonly exceeding 1030, siderophores are considered to have higher affinities for Fe(III) than for any other major or trace element dissolved in soil solution (Kraemer et al., 1999, Buckley and Schmidt, 2001). Interestingly, however, several siderophores have affinities for trace metals (e.g. Cu, Zn) that approach those for Fe(III), and certain actinides form siderophore complexes of surprisingly high stability (Hernlem et al., 1996, Whisenhunt et al., 1996, Durbin et al., 1998, Stradling, 1998). Despite the abundance, ubiquity and structural diversity of siderophores in both terrestrial and aquatic ecosystems (Powell et al., 1980, Albrecht and Crumbliss, 1998, Martinez et al., 2000, Barbeau et al., 2001), only very recently have efforts focussed on their possible role in contaminant mobilisation (Kraemer et al., 2002). In view of the affinity of siderophores for inorganic contaminants, such an examination is warranted.
Lead in soils commonly occurs within minerals of low solubility, or as chemisorbed species at colloid surfaces (Bargar et al., 1997, Ostergren et al., 2000) and this toxin is therefore considered largely unavailable to plants and microbial communities, except at low pH. For this reason the principal threat to human health from Pb-polluted terrestrial environments is believed to arise from the ingestion of contaminated soil particles rather than via consumption of Pb-containing plant or animal tissue (Manton et al., 2000). This assumption, that Pb is largely immobile in terrestrial environments, shapes both our understanding of Pb ecotoxicology and also the strategies implemented to manage and remediate Pb-polluted environments. Furthermore, the assumption that Pb is immobile underpins studies which aim to identify past anthropogenic activities by means of interpreting Pb distribution profiles in lake sediments and peat deposits (Shotyk et al., 1998, Weiss et al., 1999). At least one such study presupposes Pb immobility throughout the Holocene (Shotyk et al., 1998). The objective of this paper was to determine the role of microbial siderophores in governing Pb mobility and bioavailability.
Section snippets
Materials
Goethite [α-FeOOH] was the adsorbent of choice for the model systems because of its ubiquity in terrestrial environments and its ability to sorb Pb(II) through strong, inner-sphere complexes (Gunneriusson et al., 1994, Bargar et al., 1997, Bargar et al., 1998). This sorbent was also favoured because it allowed for a direct and unambiguous assessment of each siderophore's cation preference [i.e. Pb(II) vs. Fe(III)]. Goethite was prepared by standard methods (Schwertmann and Cornell, 1991).
Iron solubilisation
In the system comprising only goethite and Pb, Fe is present at concentrations near the analytical detection limit but this concentration appears to increase near pH 4 (Fig. 2), in response to proton-promoted dissolution of the oxide (Stumm and Wollast, 1990, Holmen and Casey, 1996). The presence of DFB in the remaining two systems increased soluble Fe concentrations significantly above background levels, whether DFB was introduced to the system before Pb (squares), or Pb was added before DFB
Discussion and conclusions
The ubiquity of hydroxamate siderophores in terrestrial environments, and their affinity for Pb(II), establish these chelates as key agents governing Pb(II) transport and bioavailability, with important implications. Lead assimilation by plants and micro-organisms is widely reported (Fodor et al., 1998, Samardakiewicz and Wozny, 2000, Keane et al., 2001) yet the uptake mechanisms remain unclear; siderophore involvement now appears plausible. Cell membrane siderophore receptors in many species
Acknowledgements
This work was supported by the NERC. We thank V. Din and T. Jefferies for analytical assistance.
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