MOLECULAR FINGERPRINTS OF METHANE SEEPING OUT OF THE SEAFLOOR

In recent years, the extensive study of seep environments has not only furthered our understanding of methanotrophy as a central process fueling diverse and complex ecosystems on the seafloor; it has also elevated the importance of lipid biomarker research in biogeochemical sciences. Seep environments harbor a wealth of molecular types with structural features and carbon isotopic compositions hitherto unknown from “normal” sedimentary environments. The pattern of isotopic ordering among compounds that have incorporated isotopically depleted methane-carbon reveals details on the trophic structure of prokaryotic seep communities. Moreover, structural features of seep biomarkers have provided detailed insights on the identity of the elusive microorganisms that are responsible for the anaerobic mode of methane oxidation the major sink of methane in marine environments. Diagnostic ¹³C-depleted biomarkers isolated from seep environments constituted the first direct line of evidence for an involvement of archaea and sulfate-reducing bacteria in anaerobic methanotrophy; parallel molecular-biological techniques have then laid the foundation to associate the uncultivated microbial players with “name tags”, leaving behind only two archaeal groups, ANME-1 and ANME-2. In between, we have realized that at least one of these groups appears to be present whenever methane is oxidized in anoxic environments. While the majority of lipid biomarkers found at seeps so far does not allow the distinction between these two groups, we have recently identified diagnostic sets of intact polar lipids for each of the two ANME-groups. This paper will highlight the brief history of a success story of biomarker applications to biogeochemical problems; also covered will be molecular signatures in paleoenvironments related to intense seepage during periods of methane hydrate destabilization during the Late Quaternary.