Magnetotactic bacteria produce chains of magnetite crystals within a cell. Bacterial magnetites have characteristic morphologies and sizes that are strictly biologically controlled. We examined morphologies of fossil bacterial magnetites (magnetofossils) preserved in Pacific deep-sea sediments and their relations to organic carbon fluxes. Isotropic crystals dominate magnetofossils in sediments in relatively oxidized conditions, and anisotropic crystals predominate in more reduced conditions. Our finding has important implications for biomineralization processes and demonstrates the potential of magnetofossil morphology as a paleoenvironmental indicator.

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