Magnetic flux closure in mutant magnetotactic bacteria elucidates a key signature of magnetofossils

Magnetotactic bacteria (MTB) produce single- or multi-stranded chains of magnetic nanoparticles that contribute to the magnetization of sedimentary rocks. Their magnetic fingerprint can be detected in ancient geological samples, and serve as a unique biosignature of microbial life. However, fossilized assemblages bear contradictory signatures pointing to magnetic components that have distinct origin(s). Here, we produce mutant bacteria to mimic MTB producing multi-stranded chains that cannot be cultivated in the laboratory, and show that the unresolved magnetic signatures are fully compatible with the contribution of MTB synthesizing multi-stranded nanoparticle chains and with fold-collapsed single-stranded chains. These structures generate magnetic flux-closing configurations while maintaining high remanent magnetizations. This work has important paleoclimatic, paleontological and phylogenetic implications, as it provides a novel tool to differentiate distinct MTB lineages (single- vs multi-stranded nanoparticle chains) which will enable the tracking of the evolution of some of the most ancient biomineralizing organisms in a time-resolved manner.