Geochemical signatures of magnetite are conserved in diverse populations of environmental magnetotactic bacteria

Magnetotactic bacteria (MTB) synthesize intracellular magnetite nanoparticles in a genetically controlled manner. They may represent some of the earliest biomineralizing organisms on Earth, and a unique window into ancient life and the environments that hosted it. Yet, identification of their mineralogical remains in ancient rocks is highly challenging due to the lack of reliable and lasting proxies. Previous results obtained from laboratory cultures of single strains of MTB revealed trace element and Fe isotope patterns that are not shared by any other source of magnetite, which could be used as reliable proxies for the identification of ancient MTB. However, the environmental relevance of these previous findings for environmental and mixed population of MTB is unknown. We report the trace element and Fe isotope compositions of magnetite extracted from environmental MTB collected in the water column of Lake Pavin, France, and show that the most distinctive geochemical traits (enrichment in Mo, Se, Sn, and light Fe isotopes) of magnetite produced by MTB cultivated in the laboratory are conserved in environmental populations. They represent potential biological signatures that can be searched in ancient rocks and coupled with magnetic and mineralogical tracers to support the presence of MTB and reconstruct their emergence and evolution.