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

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.