WARC DISCOVERIES

A breakthrough in our understanding of the iron isotope fingerprint that iron-eating bacteria may leave in the rock record

So far, the record of bacterial life in the most ancient rocks on Earth largely lies in microfossils and organic carbon, as well as the isotopic fingerprints left in carbon- and sulfur-bearing materials in sedimentary rocks. Iron-metabolizing bacteria, including the Fe(III)-reducing bacteria, are thought to lie deep in the tree of life based on phylogenetic analysis, and yet scientists have only recently begun to search for this important metabolism in the ancient rock record. A key to finding the “fingerprints” left by “iron-eating” microbes is understanding the mechanisms behind the apparently anomalous iron isotope signatures that they leave. WARC researchers Clark Johnson, Brian Beard, and Eric Roden have completed a detailed study of the iron isotope fractionations that are produced by Fe(III)-reducing bacteria that provides a mechanistic understanding of the isotopic signatures left by these microbes. Published in Geobiology, WARC researchers showed that the unusual iron isotope compositions of aqueous iron that is produced by iron-reducing bacteria is generated by iron atom exchange between aqueous iron and iron atoms on the surface of oxide minerals, where this exchange is catalyzed by electron pumping by the bacteria to the mineral surface during respiration. Future work by WARC researchers will explore a wider range of bacteria and mineral surfaces, and investigate the effects of complex aqueous solutions that are more reflective of natural environments, including ancient seawater.

Citation:
Crosby, HA, Roden, EE, Johnson, CM, and Beard, BL (2007) The mechanisms of iron isotope fractionation produced during dissimilatory Fe(III) reduction by Shewanella putrefaciens and Geobacter sulfurreducens (2007) Geobiology 5:169-189.  A PDF copy of the paper can be found here.

Posted January 11, 2008