Microbial and geochemical architecture of an active Scotian Slope cold seep

Overview

Site 2A-1 on the Scotian Slope represents an active cold seep at approximately 2,500 m water depth characterized by biogenic methane venting through a carbonate mound colonized by chemosynthetic mussels. The methane originates from subsurface sources approximately 1 km below the seafloor within basin bedrock positioned above a salt diapir. This study examines the integrated microbial and geochemical architecture of the seep system through spatial analysis of a 600 m transect.

Methods and approach

Six push cores were collected across a 600 m transect spanning the seep structure. Porewater ion concentrations and lipidomic profiles were analyzed, with 24 lipid classes identified and quantified across the transect, predominantly of archaeal origin. Intact polar lipids, core lipids, core lipid degradation products, and photosynthetic pigments were resolved and compiled as two-dimensional heatmaps to examine vertical and lateral subsurface variation. Metabolic zonation was mapped through integrated analysis of porewater geochemistry, bulk organic matter composition, stable carbon isotope ratios, lipidomic diversity patterns, and lipid biomarker proxies.

Key Findings

Lipidomic diversity increased within the seep particularly at boundaries of steep lateral geochemical gradients. Biomarker lipid proxies and porewater geochemical gradients indicated a microbial community dominated by ANME-1 and ANME-2/−3 archaea mixed with and surrounded by an envelope of microbial sulfate reduction. Discrete metabolic zones corresponding to elevated heterotrophy, denitrification, microbial sulfate reduction, and anaerobic methane oxidation were spatially delineated across the seep structure. The subsurface architecture demonstrated vertical and lateral stratification reflecting distinct microbial metabolic domains.

Implications

The spatial complexity of microbial zonation at Site 2A-1 reflects the interplay between micro- and macro-seepage that structure chemical gradients and microbial community distribution. The dominance of ANME archaea mixed with and surrounded by sulfate reducers suggests established syntrophic relationships in the anaerobic methane oxidation pathway. Integration of lipidomic diversity with geochemical gradients provides resolution of metabolic processes operating at different spatial scales within the seep system. These findings contribute to understanding of how seep architecture and fluid dynamics influence microbial dynamics and ecosystem function at marine hydrocarbon seeps. The spatial changes in the stratified system provide insights into the seep's evolution and its impact on microbial dynamics across the carbonate structure.