A research team from BIAM, in collaboration with IPGP and l’IMPMC, has shown that the isotopic composition of iron found in certain magnetic crystals—produced by magnetotactic bacteria—could serve as a reliable indicator of biological origin. This work offers a promising new tool to distinguish fossil traces left by the first microorganisms, which appeared over 3 billion years ago, and to better understand the history of life on Earth.
Invisible fossils… but magnetic
The very first forms of life on Earth were fragile unicellular microorganisms that left very few direct traces. However, some had a little-known advantage: the ability to leave behind magnetic crystal traces—these are the magnetotactic bacteria (MTB). Believed to have emerged more than 3 billion years ago, these bacteria are still present today in aquatic environments and are capable of producing magnetic crystals—most often composed of magnetite (Fe₃O₄)—within their own cells. These nanometer-sized crystals, aligned in chains, act as a natural compass, enabling the bacteria to move along Earth’s magnetic field lines.
These structures can remain intact long after the cell death, fossilizing in sediments. Known as “magnetofossils,” these remnants are among the few clues we have for detecting ancient microbial life in geological records. But to be useful, scientists must first distinguish biologically produced magnetite from magnetite formed by purely geological processes.
A promising marker: the iron isotopic signature
This is precisely where the SIGMAG project (ANR-18-CE31-0003) comes in. Led by Vincent Busigny (Université Paris Cité, IPGP) in collaboration with Christopher Lefevre (CNRS, BIAM) and Nicolas Menguy (Sorbonne Université, IMPMC), the project aims to explore a new indicator: the isotopic composition of iron1.
Thanks to optimized cultures of a marine bacterium, Magnetovibrio blakemorei MV-12, by the BEAMM team at BIAM, and high-resolution analyses via electron microscopy at IMPMC and mass spectrometry at IPGP, researchers showed that magnetite of bacterial origin is enriched in light iron isotopes. In contrast, abiotically formed magnetite is richer in heavy isotopes.
“This isotopic difference provides a promising marker to identify, in ancient rocks, the evidence of biological activity,” explains Christopher Lefevre, head of the l’équipe BEAMM.
A biosignature: a new tool for paleontology
These findings strengthen the potential of using iron isotopes as a biosignature, opening new perspectives: “Detecting these specific signatures in ancient sediments will allow scientists to confirm the existence of magnetotactic bacteria several billion years ago, and thus better understand the emergence of early metabolisms on our planet,” the scientist adds.
Beyond the simple hunt for fossils, this kind of research is crucial for reconstructing Earth’s history, its past environments, and the major steps of biological evolution.
What’s Next?
The adventure doesn’t stop here. Building on these results from both marine and earlier freshwater strains3, the researchers are now turning to other environmental bacteria—particularly those isolated from the water column of Lake Pavin, a natural site rich in magnetotactic bacteria4. The goal: to further refine the isotopic signature and make it applicable to ancient rocks, including those dating back to the Archean eon, a critical period in the emergence of life on Earth.
This fundamental research, where the cultivation of microorganisms and high-resolution analyses come together to unlock some of the oldest mysteries surrounding the origin of life on our planet.
Observation of magnetosomes in MV-1 cells using electron microscopy
Images showing magnetic structures called magnetosomes in MV-1 bacteria.
(A) A whole cell containing a chain of magnetosomes.
(B) Isolated chains of magnetosomes: the alignment of the magnetite crystals is preserved thanks to a membrane that still covers them.
(C) Close-up of a magnetosome surrounded by its membrane (black arrow); the dotted line shows the outer contours of the membrane.
(D) Zoom in on a magnetite crystal with its membrane removed.
References:
- Busigny, V. et al. Iron isotope fractionation in magnetite produced by the marine magnetotactic bacterium Magnetovibrio blakemorei. Geochimica et Cosmochimica Acta 398, 83–98 (2025).
- Bazylinski, D. A. et al. Magnetovibrio blakemorei, gen. nov. sp. nov., a new magnetotactic bacterium (Alphaproteobacteria: Rhodospirillaceae) isolated from a salt marsh. Int. J. Syst. Evol. Microbiol. 65, 1824–1833 (2013).
- Amor, M. et al. Mass-dependent and -independent signature of Fe isotopes in magnetotactic bacteria. Science 352, 705–708 (2016).
- Busigny, V. et al. Mass collection of magnetotactic bacteria from the permanently stratified ferruginous Lake Pavin, France. Environ Microbiol 24, 721–736 (2021).