Abstract
In this study, we couple iron isotope analysis to microscopic and mineralogical investigation of iron speciation during circumneutral Fe(II) oxidation and Fe(III) precipitation with photosynthetically produced oxygen. In the presence of the cyanobacterium Synechococcus PCC 7002, aqueous Fe(II) (Fe(II)(aq)) is oxidized and precipitated as amorphous Fe(III) oxyhydroxide minerals (iron precipitates, Fe(ppt)), with distinct isotopic fractionation (ε(56)Fe) values determined from fitting the δ(56)Fe(II)(aq) (1.79‰ and 2.15‰) and the δ(56)Fe(ppt) (2.44‰ and 2.98‰) data trends from two replicate experiments. Additional Fe(II) and Fe(III) phases were detected using microscopy and chemical extractions and likely represent Fe(II) and Fe(III) sorbed to minerals and cells. The iron desorbed with sodium acetate (Fe(NaAc)) yielded heavier δ(56)Fe compositions than Fe(II)(aq). Modeling of the fractionation during Fe(III) sorption to cells and Fe(II) sorption to Fe(ppt), combined with equilibration of sorbed iron and with Fe(II)(aq) using published fractionation factors, is consistent with our resulting δ(56)Fe(NaAc). The δ(56)Fe(ppt) data trend is inconsistent with complete equilibrium exchange with Fe(II)(aq). Because of this and our detection of microbially excreted organics (e.g., exopolysaccharides) coating Fe(ppt) in our microscopic analysis, we suggest that electron and atom exchange is partially suppressed in this system by biologically produced organics. These results indicate that cyanobacteria influence the fate and composition of iron in sunlit environments via their role in Fe(II) oxidation through O(2) production, the capacity of their cell surfaces to sorb iron, and the interaction of secreted organics with Fe(III) minerals.