Iron isotope fractionation during Fe(II) and Fe(III) adsorption on cyanobacteria

• Fe(III) adsorption on cyanobacteria leads to cell surface 57Fe/54Fe enrichment by ~ 1‰.
• Fe(II) adsorption and oxidation on cyanobacteria lead to cell surface 57Fe/54Fe enrichment by 2.4–2.9‰.
• Isotope fractionation is consistent with the change of Fe structure inferred from XAS.
• Natural spring with cyanobacteria yielded similar to laboratory Fe isotope fractionation.

The present study aimed at testing the hypothesis that Fe adsorption and precipitation on bacterial phytoplankton likely cause significant isotopic fractionation with preferential adsorption of heavy isotopes on the cell surface. We measured the iron isotopic fractionation during the interaction of aqueous Fe with planktonic cyanobacteria (Gloeocapsa sp., Synehococcus sp., and Planthothrix sp.) in six independent experiments using two distinct Fe oxidation states (Fe(III) and Fe(II)) at pH 3 and 6. Isotopic analyses demonstrated that the Fe adsorption on bacterial planktonic biomass yields a clear enrichment of heavy isotopes on the cell surfaces, producing isotopically light δ57Fe values in solutions. The adsorption experiments with Fe(II) in the initial solution yielded a Δ57Fecell-solution = 2.66 ± 0.14‰, whereas the adsorption experiments with Fe(III) in the initial solution yielded Δ57Fecell-solution = 0.97 ± 0.19‰. Because these data approached closed system equilibrium isotopic fractionation lines rather than Rayleigh curves, the most likely mechanism is a steady state isotopic fractionation, linked with short-term reversible Fe adsorption on cells. In agreement with X-ray Absorption Spectroscopy structural data obtained on the same adsorption experiment samples (González et al., 2014), the preferential enrichment of heavy Fe isotopes on the cell surfaces is attributed to the stronger covalent metal-ligand bonding (FeOC/P) of octahedrally coordinated Fe with phosphoryl or carboxyl groups on the cell walls when compared with the Fe aquacomplexes (OFeO) in solution. The larger isotopic fractionation factor in the experiments starting with Fe(II) results from the iron oxidation to Fe(III) which is itself accompanied by a strong isotopic effect. A natural case study of warm hydrothermal spring depositing Fe oxy(hydr)oxide with and without cyanobacterial biomass yielded an isotopic fractionation between the solid phase and aqueous solution (Δ57Fesolid-solution) of 0.62 ± 0.16‰ (biotic) and 0.80 ± 0.06‰ (abiotic). This natural case study therefore confirms experimental results, albeit of smaller magnitude. These findings suggest that Fe adsorption on cyanobacteria cell surfaces in nature should produce cell enrichments in heavier isotopes relative to the coexisting aqueous solution.