Endolithic microbial communities inside sediment-hosted carbonate nodules within and below the sulfate-methane transition zone

Porous, authigenic carbonate concretions (nodules) precipitate in marine sediments as a result of anaerobic oxidation of methane (AOM) and are observed in marine methane seep sites around the world. These incipient rocks can also host active endolithic communities capable of performing AOM. The sequences in this study integrate the biological profiles of seep sediments within and below the SMTZ with the biological profiles of their seep carbonate nodules. By characterizing and quantifying similarities between nodules and host sediments, we explore shared formation histories and evidence for continued exchange after formation. Furthermore, depth-dependent transitions in community identity provide insights into the in situ geochemical conditions and anticipated niche adaptations where these endolithic communities occur. Together, these data expand our understanding of the endolithic communities in seep sediment-hosted carbonate nodules and how they may be shaped by past and present conditions.