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. (15th September 2015)

• Record Type: Journal Article
• Authors: Swanner, Elizabeth D.; Wu, Wenfang; Schoenberg, Ronny; Byrne, James; Michel, F. Marc; Pan, Yongxin; Kappler, Andreas
• Is Part Of: Geochimica et cosmochimica acta. Volume 165(2015:Sep. 15)
• Page Start: 44
• Page End: 61
• Publication Date: 2015-09-15
• Subjects: FeOB Fe(II)-oxidizing bacteria -- EXAFS spectroscopy extended X-ray absorption fine structure spectroscopy -- EPS exopolysaccharides -- SSB standard-sample bracketing -- DS double spike -- PDF pair distribution function; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.05.024
• Languages: English
• ISSNs: 0016-7037

Abstract

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 (Feaq ) and the final Fe mineral (Feppt ) produced in batch experiment using the marine Fe(II)-oxidizing phototroph Rhodovulum iodosum . The δ 56 Feaq data are best described by a kinetic fractionation model, while the evolution of δ 56 Feppt appears to be controlled by a separate fractionation process. We propose that soluble Fe(III), and Fe(II) and Fe(III) extracted from the Feppt may act as intermediates between Fe(II) oxidation and Fe(III) precipitation. Based on 57 Fe Mössbauer 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., SiO4 4− or PO4 3− ), 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.