Cold seeps, also known as hydrocarbon seeps or methane seeps, provide a direct window into the exchange of matter and energy between the geosphere to the exosphere (i.e., biosphere, hydrosphere, and atmosphere). They occur across the global ocean where methane (CH₄), hydrogen sulfide (H₂S), and other hydrocarbon-rich fluids and/gases escape through cracks or fissures in the seafloor (Boetius and Wenzhöfer, 2013). In the cold seep ecosystem, syntrophic aggregates of anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) or metal-reducing bacteria (MRB) can carry out anaerobic oxidation of methane (AOM) coupled to sulfate or iron (III) reduction, respectively (Beal et al., 2009; Boetius and Wenzhöfer, 2013). At the same time, the AOM was accompanied by the promotion of authigenic mineral deposition such as carbonate rock and pyrite.
Magnetotactic bacteria (MTB) are an important group of gradient-dwelling microorganisms often found at the oxic-anoxic transition zone (OATZ) in diverse aquatic environments (Lefèvre and Bazylinski, 2013). MTB share the hallmark ability to biomineralize membrane-enveloped nanocrystals of magnetite (Fe₃O₄) or greigite (Fe₃S₄), referred to as magnetosomes. These magnetosomes are typically arranged in intracellular chain or chains that act as a biological compass, allowing MTB to align and swim along Earth’s magnetic field. Through their metabolic flexibility and magnetosome biomineralization, MTB contribute to the biogeochemical cycling of C, N, P, S, Si and Fe in aquatic OATZs.
MTB have been observed worldwide, in both freshwater and nearshore marine habitats (Lefèvre and Bazylinski, 2013). Nevertheless, their distribution, diversity, and biogeochemical roles in cold seep ecosystems remain insufficiently characterized, largely due to limited sampling opportunities and methodological challenges. In this study, we performed metagenomic, magnetic, and microscopic analyses on deep-sea sediments collected from two active cold seeps in the South China Sea (SCS), and complemented these efforts with a large-scale metagenomic survey of MTB in cold seep environments worldwide. Our results revealed the diversity, abundance, and potential functions of MTB in these ecosystems, highlighting their roles in Fe and S cycle, as well as in anaerobic methane oxidation. This work broadens our understanding of MTB in deep-sea settings and offers new insights into the biogeochemical significance of MTB in cold seep environments.

Boetius, A. and Wenzhöfer, F., 2013. Seafloor oxygen consumption fuelled by methane from cold seeps. Nature Geoscience 6(9), 725-734.
Beal, E.J., House, C.H. and Orphan, V.J., 2009. Manganese- and Iron-Dependent Marine Methane Oxidation. Science 325(5937), 184-187.
Boetius, A. and Wenzhöfer, F., 2013. Seafloor oxygen consumption fuelled by methane from cold seeps. Nature Geoscience 6(9), 725-734.
Lefèvre, C.T. and Bazylinski, D.A., 2013. Ecology, diversity, and evolution of magnetotactic bacteria. Microbiology and Molecular Biology Reviews 77(3), 497-526.
 

Cold seep,Electron microscopy,Metagenomics,Sediment magnetism