Magnetic Biosignatures of Magnetosomal Greigite From Micromagnetic Calculation

Abstract

Abstract Greigite magnetosomes produced by magnetotactic bacteria (MTB) are widely distributed in natural environments, but large uncertainties remain regarding their magnetic biosignatures. Here, we have constructed micromagnetic models with realistic biogenic greigite particles to quantify the magnetic properties and magnetotaxis efficiency of greigite‐producing MTB cells. Our calculations suggest coercivity ( B c ) of ∼15–21 mT for intact greigite‐producing rod‐shaped MTB and many‐celled magnetotactic prokaryotes, with B c decreasing to ∼11 mT for greigite magnetofossils with clumped particles. These magnetic signatures make biogenic greigite distinguishable from typical biogenic magnetite and inorganic greigite, providing reliable magnetic criteria to detect biogenic greigite in a wide range of environmental and geological settings. Our numerical calculations suggest that rod‐shaped greigite‐producing MTB have a similar magnetotaxis efficiency to magnetite MTB, likely by biomineralizing more greigite crystals to compensate for the lower saturation magnetization of greigite and less ordered chains in greigite MTB cells, demonstrating biological‐controlled optimization of their magnetic nanostructure. Plain Language Summary Magnetic bacteria can produce greigite (Fe 3 S 4 ) or magnetite (Fe 3 O 4 ) nanoparticles arranged in chains and use them as nano‐compass for navigating along the geomagnetic field lines. Dead magnetic bacteria can be fossilized in the geological records that retain important signals of past geomagnetic field, environmental conditions, and biochemical processes in the Earth surface. Until now magnetic biosignatures of bacterial greigite are unclear because it is difficult to obtain pure greigite magnetotactic bacteria samples to measure their magnetic properties. Here, we constructed computer models that mimic those found in living greigite‐producing magnetic bacteria. Model calculations determined robust magnetic fingerprints of biogenic greigite that can be used to search for fossilized biogenic greigite nanoparticles in natural environments. Moreover, according to our calculations, greigite‐producing magnetic bacteria have similar navigational ability to bacterial magnetite counterparts by producing more biogenic greigite crystals in the bacterial cells, suggesting that magnetic nanostructures are optimized by those magnetic microorganisms. Key Points Micromagnetic calculations reveal robust magnetic biosignatures of biogenic greigite that have been difficult to determine experimentally Modeled hysteresis properties of biogenic greigite provide magnetic criteria for their identification in a wide range of environments Greigite‐producing magnetotactic microorganisms likely have optimized their magnetic nanostructure for navigational and other purposes