Microbial and geochemical architecture of an active Scotian Slope cold seep.

Background: Deep marine cold seeps occurring along the seabed of continental margins are identified by their oasis-like ecosystems, which are largely fueled by the chemical energy of the venting fluids. Seep site 2A-1, situated at ~2,500 m water depth on the Scotian Slope of the North Atlantic was discovered in 2021. The seep hosts a large mussel encrusted, carbonate mound with biogenic methane bubbling up from a single vent. The emitted biogenic methane is primarily sourced from ~1 km below the seafloor within the basin bedrock that resides directly above the crest of an underlying salt diapir. Methods: A 600-m long transect composed of six push cores was collected across the seep structure. Downcore porewater ions and lipidomic profiles of 24 predominantly archaeal in origin lipid classes were tentatively identified and quantified across the transect. Results: The resolved lipidomes comprised of intact polar lipids, core lipids, core lipid degradation products, and photosynthetic pigments. These data were compiled as two-dimensional heatmaps to spatially examine vertical and lateral changes in the subsurface geochemical and microbiological architecture of the seep. Microbially mediated metabolic zones of elevated heterotrophy, denitrification, microbial sulfate reduction, and anaerobic methane oxidation were then mapped across the seep structure based on an integrated analysis of porewater geochemistry, bulk organic matter and its carbon isotope compositions, lipidomic diversity and biomarker proxy patterns. Discussion: Increased lipidomic diversity is shown to exist within the seep particularly at boundaries of high lateral geochemical gradients. Biomarker lipid proxies and porewater gradient changes indicate a microbial community dominated by ANME-1 and -2/-3 archaea that is mixed with, but also surrounded by, an envelope of microbial sulfate reduction. Discussion: Spatial changes in the stratified system highlight the complex interplay of micro- and macro-seepage and provide insights into the seep's evolution and impact on microbial dynamics across the carbonate structure.