Life in a Benthic Mirror‐World: Evidence for Deep‐Living Magnetotactic Bacteria in Hydrothermally Bottom‐Up Oxygenated Pacific Ridge Flank Sediments

Abstract Low‐temperature hydrothermal fluids in crustal rocks of the Clarion‐Clipperton‐Zone (East Pacific) supply dissolved oxygen into the sediment from below. Diffusive upward transport led to formation of an inverse oxygen gradient zone in the overlying sediments. The resultant oxic/suboxic transition zone could provide suitable conditions for a deep, mirrored habitat for microaerophilic magnetotactic bacteria that were so far only found in the shallow oxygen gradient zone beneath the sediment‐water interface. Previously, the presence of such deep‐living MTB was only inferred from paleo‐ and rock‐magnetic proxies, but here it is evidenced by electron microscopy showing intact multi‐stranded, large (>120 nm diameter) magnetofossil chains from the deep past oxic/suboxic transition zone. Sediment magnetic parameters indicate localized zones of biogenic magnetite accumulation affirming the existence of living MTB at around 8 m sediment depth. This finding is the first evidence for MTB in bottom‐up oxygenated sediments near the sediment/crust interface.