Fractionation of Fe isotopes during Fe(II) oxidation by a marine photoferrotroph is controlled by the formation of organic Fe-complexes and colloidal Fe fractions

Abstract

Much interest exists in finding mineralogical, organic, morphological, or isotopic biosignatures for Fe(II)-oxidizing bacteria (FeOB) that are retained in Fe-rich sediments, which could indicate the activity of these organisms in Fe-rich seawater, more common in the Precambrian Era. To date, the effort to establish a clear Fe isotopic signature in Fe minerals produced by Fe(II)-oxidizing metabolisms has been thwarted by the large kinetic fractionation incurred as freshly oxidized aqueous Fe(III) rapidly precipitates as Fe(III) (oxyhydr)oxide minerals at near neutral pH. The Fe(III) (oxyhydr)oxide minerals resulting from abiotic Fe(II) oxidation are isotopically heavy compared to the Fe(II) precursor and are not clearly distinguishable from minerals formed by FeOB isotopically. However, in marine hydrothermal systems and Fe(II)-rich springs the minerals formed are often isotopically lighter than expected considering the fraction of Fe(II) that has been oxidized and experimentally-determined fractionation factors. We measured the Fe isotopic composition of aqueous Fe (Fe aq ) and the final Fe mineral (Fe ppt ) produced in batch experiment using the marine Fe(II)-oxidizing phototroph Rhodovulum iodosum. The d 56 Fe aq data are best described by a kinetic fractionation model, while the evolution of d 56 Fe ppt appears to be controlled by a separate fractionation process. We propose that soluble Fe(III), and Fe(II) and Fe(III) extracted from the Fe ppt may act as intermediates between Fe(II) oxidation and Fe(III) precipitation. Based on 57 Fe Mö ssbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray total scattering, we suggests these Fe phases, collectively Fe(II/III) interm , may consist of organic-ligand bound, sorbed, and/or colloidal Fe(II) and Fe(III) mineral phases that are isotopically lighter than the final Fe(III) mineral product. Similar intermediate phases, formed in response to organic carbon produced by FeOB and inorganic ligands (e.g., SiO 4 4À or PO 4 3À ), may form in many natural Fe(II)-oxidizing environments. We propose that the formation of these intermediates is likely to occur in organic-rich systems, and thus may have controlled the ultimate isotopic composition of Fe minerals in systems where Fe(II) was being oxidized by or in the presence of microbes in Earth's past.

Geochemical and iron isotopic insights into hydrothermal iron oxyhydroxide deposit formation at Loihi Seamount

“…Instead,Croal et al (2004) suggested that an Fe(III)-organic ligand species may be in isotopic equilibrium with aqueous Fe(II). A similar mechanism was proposed bySwanner et al (2015) to explain the fractionation of Fe isotopes during Fe(II) oxidation by marine photoferrotrophs. In particular, a range of Fe isotope fractionation factors between Fe(II) aq and Fe(III) s (-0.40 ‰ to +2.22 ‰) was obtained and could not be modeled by a simple kinetic Rayleigh fractionation model.…”

Preservation of Archaeal Surface Layer Structure During Mineralization

“…Nucleation of mineral phases can occur at both intracellular and extracellular locations, sometimes as one of many mechanisms used to reduce the toxic effects of high metal concentrations. Whereas bacteria encrustation by iron minerals has most often been explored in the context of microbial Fe-based metabolisms 26 27 28 29 30 , encrustation of microorganisms, particularly by iron minerals, has been observed in hydrothermal environments as well 31 32 .…”

Binding of heavy metal ions in aggregates of microbial cells, EPS and biogenic iron minerals measured in-situ using metal- and glycoconjugates-specific fluorophores

“…Differences in the mineral product between the present study and previous work are likely due to an 180 days ageing process of the ferrihydrite in batch cultures. During this time, any residual Fe 2+ could have reacted with the ferrihydrite leading to a transformation to goethite over time (Hansel et al, 2005), or alternatively the transformation could have taken place due to the presence of bicarbonate or drying (Swanner et al, 2015;Wu et al, 2014).…”