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.