Morphology, structure, and metal binding mechanisms of biogenic manganese oxides in a superfund site treatment system

Manganese oxides, which may be biogenically produced in both pristine and contaminated environments, have a large affinity for many trace metals. In this study, water and Mn oxide-bearing biofilm samples were collected from the components of a pump and treat remediation system at a superfund site. To better understand the factors leading to their formation and their effects on potentially toxic metal fate, we conducted a chemical, microscopic, and spectroscopic characterization of these biofilm samples. Scanning electron microscopy revealed the presence of Mn oxides in close association with biological structures with morphologies consistent with fungi. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) revealed the oxides to be a mixture of layer and tunnel structure Mn(iv) oxides. In addition, XAS suggested that Ba, Co, and Zn all primarily bind to oxides in the biofilm in a manner that is analogous to synthetic or laboratory grown bacteriogenic Mn oxides. The results indicate that Mn oxides produced by organisms in the system may effectively scavenge metals, thus highlighting the potential utility of these organisms in designed remediation systems.

Ectomycorrhizal fungi and past high CO2 atmospheres enhance mineral weathering through increased below-ground carbon-energy fluxes

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.

Siderophore-promoted dissolution of chromium from hydroxide minerals

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.