Iron adsorption onto soil and aquatic bacteria: XAS structural study

Although the interaction between Fe and microorganisms has been extensively studied, the main physico-chemical factors controlling the mechanisms of Fe adsorption and precipitation on bacterial cell walls remain poorly understood. In this study, we quantified thermodynamic parameters of the Fe adsorption reaction and characterized the speciation of Fe adsorbed on the surface of cyanobacteria and soil heterotrophic bacteria. For this purpose, the molecular mechanisms of iron interaction with typical aquatic and soil bacteria were investigated by combining batch macroscopic adsorption experiments with atomic-level Fe K-edge X-ray absorption fine structure spectroscopy (XAFS). Three cyanobacteria species (Synechococcus sp., Planktothrix sp. and Gloeocapsa sp.) and aerobic heterotrophic soil rhizobacterium (Pseudomonas aureofaciens) were used for Fe3+ and Fe2+ adsorption experiments. These experiments were carried out for a wide range of initial iron concentration (4.5–57.3μM) and pH (2.0–6.5). Surface adsorption data were rationalized using a Linear Programming Model (LPM), which allowed quantification of the surface adsorption constants and the number of binding sites. XAS (XANES and EXAFS) analysis of adsorbed iron demonstrated the predominance of O-coordinated Fe3+ species. Moreover, XANES data treatment using a linear combination fit of reference compounds suggested that the atomic environment of iron adsorbed onto soil bacterial surfaces was dominated by phosphoryl moieties with a lesser amount of carboxylates and some contribution of Fe(III)-oxy(hydr)oxide component. Complete oxidation of Fe(II) to Fe(III) was observed in the solid phase as determined by XANES analysis. Binding of Fe(III) to carboxylate groups was only significant for capsular cyanobacteria (Gloeocapsa sp.). The relative proportions of various Fe species at the cell surface determined by thermodynamic analysis of the macroscopic data and by XAS are in a good agreement. Our results suggest that, in the presence of surface organic ligands, the oxidation of divalent iron does occur, but the polymerization of formed Fe(III)oxy(hydr)oxides is partially inhibited and adsorbed iron in the form of both Fe–O–Fe polymers and individual Fe atoms attached to phosphoryl moieties. The presence of EPS reduces metal-cell binding capacity and enhances Fe polymerization at the bacterial surface.