GEOBIOLOGY Mass-dependent and -independent signature of Fe isotopes in magnetotactic bacteria Matthieu Amor, 1,2 * Vincent Busigny, 1 * Pascale Louvat, 1 Alexandre Gélabert, 1 Pierre Cartigny, 1 Mickaël Durand-Dubief, 3 Georges Ona-Nguema, 2 Edouard Alphandéry, 2,3 Imène Chebbi, 3 François Guyot 2 Magnetotactic bacteria perform biomineralization of intracellular magnetite (Fe 3 O 4 ) 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 57 Fe during magnetite biomineralization but not in even Fe isotopes ( 54 Fe, 56 Fe, and 58 Fe), 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. M agnetotactic bacteria synthesize magnet- ite [Fe(II)Fe(III) 2 O 4 ] nanoparticles under a genetically controlled pathway (1). They are morphologically, physiologically, and phylogenetically diverse (2, 3). Nanopar- ticles of biogenic magnetite are produced in these cells in organelles called magnetosomes, consist- ing of a bilayered lipid membrane surrounding the magnetite crystal (4). Magnetosomes are as- sembled in chains inside the cell and provide it with a permanent magnetic dipole (4, 5). Mag- netotactic bacteria are markers of oxic/anoxic transition zones in sediments and aquatic sys- tems (1). In addition, these bacteria have been proposed to represent some of the most ancient microorganisms capable of biomineralization (6, 7), but the identification of their activity in the fossil record remains poorly resolved (8). Specifically, it is challenging to distinguish intracellular mag- netite nanoparticles from abiotic or extracellular biogenic magnetites produced by Fe-metabolizing bacteria [i.e., Fe(III)-reducing and Fe(II)-oxidizing bacteria] (5, 6, 8, 9). Moreover, magnetite nano- particles can experience variable transformations during diagenetic and/or metamorphic processes and may thus have lost some of their original physi- cal characteristics (10). Iron isotopes have been proposed as a tool to infer the presence of Fe(III)- reducing and Fe(II)-oxidizing bacteria in the Pre- cambrian geological record (11–13). However, a previous study reported no fractionation of Fe isotopes (14) in magnetite produced by two strains of magnetotactic bacteria, Magnetospirillum mag- netotacticum MS-1 and Magnetovibrio blakmorei MV-1 (15), and only oxygen isotope measurements showed a temperature-dependent fractionation be- tween magnetite and water. We measured the Fe isotope fractionation pro- duced by another magnetotactic bacterium, M. magneticum strain AMB-1, which was cultured in batches with either Fe(III)-quinate or Fe(II)- ascorbate to investigate the effect of various Fe sources on the Fe isotopic signatures in mag- netite (see the supplementary materials). A small fraction of the growth media was sampled before and after AMB-1 cultures for subsequent Fe isotope analyses. Magnetites were magnetically separated from bacterial lysates (i.e., plasmic membranes, periplasm, and cytoplasm) according to an opti- mized procedure, which ensured that the mineral was analyzed with no organic residue (9). Both organic and biomineralized fractions were ana- lyzed for Fe isotope compositions. Magnetite produced by AMB-1 was strongly depleted in the heavy Fe isotopes relative to the initial growth medium. Two replicates for each culture condition provided consistent results, with d 56 Fe values of –1.00 ± 0.08 per mil (‰) and –1.80 ± 0.13‰ for Fe(II)-ascorbate and Fe(III)- quinate experiments, respectively (Fig. 1 and table S1). Overall, the net Fe isotope fractionation be- tween Fe sources and magnetite ranged between 1.5 and 2.5‰. Iron is usually assumed to reside either in magnetite or in the residual growth me- dium (16). Yet we found that bacterial lysates can represent up to 70% of cellular Fe (supplementary materials). In the two culture conditions of our experiments, bacterial lysate d 56 Fe values were enriched in the heavy isotopes by 0.3 to 0.8‰ relative to initial Fe sources. Although several studies reported heavy Fe(II) adsorbed on solid surfaces or ligated into organic complexes (17, 18), Fe in the lysates is more likely to be present as Fe(III). This is consistent with the low d 56 Fe values of magnetite, which suggest partial reduction of SCIENCE sciencemag.org 6 MAY 2016 • VOL 352 ISSUE 6286 705 1 Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 75238 Paris, France. 2 Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Universités, Université Pierre et Marie Curie, UMR 7590 CNRS, Institut de Recherche pour le Développement UMR 206, Museum National d’Histoire Naturelle, 75252 Paris Cedex 05, France. 3 Nanobactérie SARL, 36 Boulevard Flandrin, 75016 Paris, France. *Corresponding author. Email: amor@ipgp.fr (M.A.); busigny@ ipgp.fr (V.B.) δ 56 Fe (‰) -2 -1 0 +1 Initial iron sources Final growth media Magnetites Bacterial lysates Fig. 1. Iron isotope compositions obtained from cultures of magnetotactic bacteria. d 56 Fe values (relative to international standard IRMM-014) of Fe sources (initial growth media, circles), growth media after AMB-1 cultures (squares), magnetite samples (triangles), and bacterial lysates (diamonds) in exper- iments using either Fe(III)-quinate (solid black symbols) or Fe(II)-ascorbate (open symbols) as Fe sources. The gray bar represents the range of Fe isotope compositions for initial growth media and can be regarded as a reference. Magnetite samples are enriched in light Fe isotopes relative to Fe sources, by ~1.5 to 2.5‰. Growth media are also enriched in light isotopes, with fractionation between 0.5 and 1.5‰. In contrast, bacterial lysates are enriched in heavy isotopes relative to Fe sources,by 0.5 to 0.75‰. RESEARCH | REPORTS on May 5, 2016 http://science.sciencemag.org/ Downloaded from