Mass-dependent and -independent signature of Fe isotopes in magnetotactic bacteria

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Science  06 May 2016:
Vol. 352, Issue 6286, pp. 705-708
DOI: 10.1126/science.aad7632

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An isotope record of magnetic bacteria

Microorganisms have shaped Earth's oceans and atmosphere over billions of years. Ancient microbes left very little direct morphological evidence of their existence in the rock record, thereby requiring geochemical clues for evidence of their activity. Amor et al. show that magnetotactic bacteria impart a distinct isotopic signature to their internal iron nanoparticles. Cultures of a modern magnetic bacterium fractionated 57Fe isotopes independent of their mass, in contrast to fractionation patterns often observed for other isotopes. Because this signature is not produced abiotically or by other iron-metabolizing bacteria, it could serve as a reliable biomarker of this ancient magnetic microbial lifestyle.

Science, this issue p. 705


Magnetotactic bacteria perform biomineralization of intracellular magnetite (Fe3O4) nanoparticles. Although they may be among the earliest microorganisms capable of biomineralization on Earth, identifying their activity in ancient sedimentary rocks remains challenging because of the lack of a reliable biosignature. We determined Fe isotope fractionations by the magnetotactic bacterium Magnetospirillum magneticum AMB-1. The AMB-1 strain produced magnetite strongly depleted in heavy Fe isotopes, by 1.5 to 2.5 per mil relative to the initial growth medium. Moreover, we observed mass-independent isotope fractionations in 57Fe during magnetite biomineralization but not in even Fe isotopes (54Fe, 56Fe, and 58Fe), highlighting a magnetic isotope effect. This Fe isotope anomaly provides a potential biosignature for the identification of magnetite produced by magnetotactic bacteria in the geological record.

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