Deep-living magnetotactic bacteria in hydrothermally bottom-up oxygenated sediments: a case for a mirror world

Low-temperature hydrothermal fluids circulating through crustal rocks of the Clarion–Clipperton Zone (East Pacific) introduce dissolved oxygen into the overlying sediments from below, generating an inverse oxygen gradient within the sediments. The resulting oxic–suboxic transition zone may create favorable conditions for a deep, mirrored habitat for microaerophilic magnetotactic bacteria (MTB), which have previously been observed only in the shallow oxygen gradient zone beneath the sediment–water interface. Until now, the existence of such deep-dwelling MTB had been inferred solely from paleo- and rock-magnetic proxies. In this study, however, their presence is directly demonstrated by electron microscopy revealing intact, multi-stranded, large magnetofossil chains (>120 nm in diameter) originating from the former deep oxic–suboxic transition zone. Magnetic properties of the sediments further identify localized accumulations of biogenic magnetite, supporting the presence of living MTB at approximately 8 m sediment depth. These results provide the first direct evidence of MTB inhabiting bottom-up oxygenated sediments near the sediment–crust interface.