Chemosynthesis increases alkalinity and facilitates stromatolite growth at methane seeps in 731 m water depth within the oxygen minimum zone (OMZ) in the northern Arabian Sea. Microbial fabrics, including mineralized filament bundles resembling the sulfide-oxidizing bacterium Thioploca, mineralized extracellular polymeric substances, and fossilized rod-shaped and filamentous cells, all preserved in 13C-depleted authigenic carbonate, suggest that biofilm calcification resulted from nitrate-driven sulfide oxidation (ND-SO) and sulfate-driven anaerobic oxidation of methane (SD-AOM). Geochemical batch modeling reveals that the collective effects of ND-SO and SD-AOM increase alkalinity more than SD-AOM alone, explaining the preservation of sulfide-oxidizing bacteria in authigenic carbonate. These findings demonstrate the biogenicity of a conical stromatolite associated with OMZ methane seeps and confirm the fact that, apart from photosynthesis-based metabolisms, chemosynthesis-based metabolisms can also account for stromatolite formation.

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