Biomolecules have significant impacts on the fate and transport of contaminant metals in soils and natural waters. Siderophores, Fe(iii)-binding agents that are exuded by microbes and plants, may form strong complexes with and promote the dissolution of contaminant metal ions, such as Co(iii), U(iv), or Pu(iv). Although aqueous Cr(iii)-siderophore complexes have been recognized in the laboratory setting for almost 40 years, few studies have explored interactions of siderophores with Cr-bearing minerals or considered their impacts on environmental chemistry. To better understand the possible effects of siderophores on chromium mobility, we conducted a series of dissolution experiments to quantify the dissolution rates of Cr(iii)(OH)3 in the presence of hydroxamate, catecholate, and α-hydroxycarboxylate siderophores over a range of environmentally relevant pH values. At pH = 5, dissolution rates in the presence of siderophores are similar to control experiments, suggesting a predominantly proton-promoted dissolution mechanism. At pH = 8, the sorption of the siderophores desferrioxamine B and rhizoferrin can be modeled by using Langmuir isotherms. The dissolution rates for these siderophores are proportional to the surface concentrations of sorbed siderophore, and extended X-ray absorption fine structure spectra of dissolution products indicates the formation of Cr(iii)HDFOB(+) and Cr(iii)rhizoferrin(3-) complexes, suggesting a ligand-promoted dissolution mechanism at alkaline pH. Because siderophores promote Cr(iii)(OH)3 dissolution at rates similar in magnitude to those of iron hydroxides and the resulting Cr(iii)-siderophore complexes may be persistent in solution, siderophores could potentially contribute to the mobilization of Cr in soils and sediments where it is abundant due to geological or anthropogenic sources.

Field studies indicate an intensification of mineral weathering with advancement from arbuscular mycorrhizal (AM) to later-evolving ectomycorrhizal (EM) fungal partners of gymnosperm and angiosperm trees. We test the hypothesis that this intensification is driven by increasing photosynthate carbon allocation to mycorrhizal mycelial networks using 14CO2-tracer experiments with representative tree–fungus mycorrhizal partnerships. Trees were grown in either a simulated past CO2 atmosphere (1500 ppm)—under which EM fungi evolved—or near-current CO2 (450 ppm). We report a direct linkage between photosynthate-energy fluxes from trees to EM and AM mycorrhizal mycelium and rates of calcium silicate weathering. Calcium dissolution rates halved for both AM and EM trees as CO2 fell from 1500 to 450 ppm, but silicate weathering by AM trees at high CO2 approached rates for EM trees at near-current CO2. Our findings provide mechanistic insights into the involvement of EM-associating forest trees in strengthening biological feedbacks on the geochemical carbon cycle that regulate atmospheric CO2 over millions of years.

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Siderophores, a family of biogenic metal chelating agents, play critical roles in the biogeochemical cycling of Fe and other metals by facilitating their solubilization and uptake in circumneutral to alkaline oxic environments. However, because of their small concentrations (ca. nM) and large number of molecular structures, siderophore detection and quantification in environmental samples requires specialized equipment and expertise, and often requires pre-concentration of samples, which may introduce significant bias. The "universal" CAS assay, which was originally designed for use in bacterial cultures, quantifies the iron chelating function of a pool of siderophores but only at concentrations (>2 µM) well above the concentrations estimated to be present in marine, freshwater, and soil samples. In this manuscript, we present a high sensitivity modification of this universal assay (HS-CAS) suitable for detecting and quantifying siderophore activity in the nM concentration range, allowing for direct quantitation of siderophore reactivity in transparent aqueous samples.