Magnetotactic bacterial abundance in pelagic marine environments is limited by availability of dissolved iron and organic carbon flux

Despite the fact that magnetotactic bacteria intracellularly biomineralize magnetite of an ideal grain size for recording paleomagnetic signals, bacterial magnetite is not widely reported in the pre-Quaternary geological record. Most magnetotactic bacteria are gradient organisms that live in chemically stratified environments near the oxic-anoxic interface. Thus, when magnetofossils are eventually buried within anoxic environments, the fine-grained magnetite undergoes dissolution and is therefore not preserved in the geological record. Pelagic carbonate sediments provide optimal environments for preserving bacterial magnetite because they have expanded pore-water redox zonations that often do not become anoxic for hundreds of meters. Nevertheless, the biogeochemical factors that control magnetotactic bacterial populations in such settings are not well known. We document bacterial magnetite preservation throughout Eocene carbonates from the southern Kerguelen Plateau, Southern Ocean. We provide evidence that iron fertilization, via increased eolian dust deposition, enhanced primary productivity, which controlled bacterial magnetite abundance via export of organic carbon to the seafloor. Increased burial of eolian iron-bearing phases also delivered iron to the seafloor, some of which became bioavailable through iron reduction. Increased organic carbon fluxes to the seafloor provided nutrients needed for bacterial metabolism and increased bioavailable iron needed for magnetosome mineralization. These factors therefore appear to have provided important controls on the concentration of magnetotactic bacteria, which increased with enhanced delivery of organic carbon and bioavailable iron to the seafloor. While several other limiting factors also influence primary productivity in oligotrophic waters, our results suggest that magnetotactic bacterial populations are particularly sensitive to iron and organic carbon limitation.