Iron isotope fractionation in magnetite produced by the marine magnetotactic bacterium Magnetovibrio blakemorei

Magnetotactic bacteria (MTB) possess the ability to precipitate intracellular nanosized magnetite. Their emergence may date back to the early Archean (e.g. 3 Ga) but evidence of such a long history has yet to be provided. MTB identification in the rock record relies on magnetofossils, the residual magnetite crystals that may survive in time to rock deformation and low-grade metamorphism. Several criteria such as size, shape and magnetic properties have been proposed to distinguish magnetofossils from other magnetite sources. Recent experimental work on the freshwater strain Magnetospirillum magneticum AMB-1 revealed that Fe isotope composition represents a promising additional criterion, but isotopic characterization of distinct MTB strains and of their magnetite needs to be provided to evaluate the robustness of iron isotope signatures for paleontological applications. In the present work, we investigate the marine strain Magnetovibrio blakemorei MV-1 under various conditions selected to examine the influence of Fe concentration (50, 100 and 150 μM) and redox state (Fe(II)-ascorbate or Fe(III)-citrate) on bacterial biomineralization and determine associated Fe isotope fractionations. Our results confirm the preferential incorporation of light iron isotopes into magnetite relative to the bacterial growth medium previously observed in AMB-1. Modeling the evolution of the growth medium and magnetite isotope compositions by Rayleigh distillation process yields iron isotope fractionations (i.e. Δ⁵⁶Fe_{growth medium-magnetite} = δ⁵⁶Fe_{growth medium} − δ⁵⁶Fe_magnetite) ranging between 0.2 and 0.9 ‰ with a mean value of 0.55 (±0.35) ‰. This isotope fractionation shows no clear relation with iron concentration or redox state. Importantly, the enrichment in light iron isotope of biological magnetite contrasts with the isotopic characteristics of magnetite formed by abiotic precipitation, the latter being enriched in heavy isotopes. This suggests that Fe isotopes could be used as a biosignature for magnetofossils identification in terrestrial or extraterrestrial samples. Finally, in contrast with previous results obtained on AMB-1, no specific enrichment of ⁵⁷Fe (relative to ⁵⁴Fe, ⁵⁶Fe and ⁵⁸Fe) could be detected in MV-1 magnetite, which we interpret by differences in iron budgets between the two strains.