Iron Isotope Fractionation during Fe(II) Oxidation Mediated by the Oxygen-Producing Marine Cyanobacterium Synechococcus PCC 7002

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, Feppt), with distinct isotopic fractionation (ε56Fe) values determined from fitting the δ56Fe(II)aq (1.79‰ and 2.15‰) and the δ56Feppt (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 (FeNaAc) yielded heavier δ56Fe compositions than Fe(II)aq. Modeling of the fractionation during Fe(III) sorption to cells and Fe(II) sorption to Feppt, combined with equilibration of sorbed iron and with Fe(II)aq using published fractionation factors, is consistent with our resulting δ56FeNaAc. The δ56Feppt 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 Feppt 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 O2 production, the capacity of their cell surfaces to sorb iron, and the interaction of secreted organics with Fe(III) minerals.

Pervasively anoxic surface conditions at the onset of the Great Oxidation Event: New multi-proxy constraints from the Cooper Lake paleosol

“…All stable isotope preparation and measurement was performed at the University of Tübingen using the protocols described in Schoenberg and von Blanckenburg (2005) and Swanner et al (2015) (Kurzweil et al, 2016;Moeller et al, 2014;Swanner et al, 2017;Swanner et al, 2015;Wu et al, 2017), and the Tüb-Fe standard returned a mean δ 56/54 Fe value of -0.372 ± 0.032 ‰ (2sd, n=3), agreeing well with the longer term laboratory mean of -0.376 ± 0.049 ‰ (2sd, n=31).…”

Iron Isotope Fractionation during Bio- and Photodegradation of Organoferric Colloids in Boreal Humic Waters

“…Although the fractionation of Fe between bacterial cells and organo-ferric colloids has not been studied previously, the adsorption of metal cations such as Zn 2+ at the microorganism cell surface is known to favor the heavier isotopes 51 and both Fe 2+ and Fe 3+ ions follow this rule 30,31 . The heavier Fe isotopes are preferentially adsorbed onto solid surfaces during equilibrium isotope fractionation processes 36 .…”

“…Other experiments producing goethite reported a fractionation factor of +1.7±0.14‰, corresponding to the Fe(III)solid being enriched in heavier isotopes 41 . As for the bacterially-induced colloid transformation, two main processes controlling the isotope fractionation during metal-live cell interaction are 1) fast adsorption of heavier Fe isotope on the cell surfaces 30,31 , and 2) long-term assimilation of heavier Fe isotopes as recently shown for phytoplankton 42 and magnetotactic bacteria 33 . Note however that, in contrast to the fairly good knowledge of isotope fractionation of ionic metals and their organic complexes interaction with microbial cells, virtually nothing is known on colloidal metalcell interaction reactions.…”

“…The Fe isotope approach is an efficient tool for deciphering elementary processes involving microbes and aqueous solutions [29][30][31][32][33] , DOM -Fe 34 and mineral -Fe 35,36 interaction including colloids 3,6,37 . The latter study demonstrated an unusual enrichment of heavier Fe isotopes in the LMW (< 1-10 kDa) fraction relative to remaining colloidal fraction (by up to +3 to +4 ‰ in  57 Fe) and suggested that bio-and/or photo-transformation of colloids may be responsible for heavy Fe isotope signatures in LMW fraction of boreal waters.…”

Microbial nitrification and acidification of lacustrine sediments deduced from the nature of a sedimentary kaolin deposit in central Japan

“…Numerous studies on Fe isotopes to date have shown that Fe isotope fractionation occurs in biotic or abiotic 35,36 . The studies on Fe isotope fractionation in soil have reported that Fe isotope compositions produced by the dissolution of silicates tended to be slightly lighter than the original isotope ratios 37,38 , whereas other studies on microbial Fe isotope fractionation with Fe oxidation have shown that Fe isotope compositions of Fe-hydroxide precipitates were generally heavier than that of ferrous iron in aqueous solution 39,40 . In the case of Motoyama deposits, it is difficult to identify at what stage Fe isotope fractionation occurred effectively from the present data, and more detailed examination is required.…”