Microbial succession in methane seeps: a conceptual model

Abstract: methane seeps are important hotspots of biodiversity where chemosynthetic microbial communities are its keystone components. It is poorly understood how the succession of these microbial communities¿ proceeds through the life of a methane seep because its monitoring is expensive and demands a broad temporal window. In the present manuscript, a theoretical model of microbial succession processes in methane seep ecosystems is proposed. The model focuses on successions through the seep `life cycle¿, and is based on the composition, metabolisms, and spatial distribution of the typically observed archaea and bacterial at seeps. The model was subjectively built, based on case studies of methane seeps of approximately 50 kyr or less. In the first stage of the microbial succession model (`stage of birth¿, 0-30 kyr), subsurface methanogenic archaea and oil-oxidizing bacteria convert hydrocarbons into methane from upward-flowing crude oil. In the second stage (`stage of youth¿, 31-40 kyr), methanotrophic consortia colonize shallow seep sediments, where the abundant methane and sulfide gases are oxidized, leading to the formation of microbial mats and symbioses with metazoans. In the third stage (`stage of maturity¿, 41-45 kyr), necromass degradation dominates the seep sediments, followed by the fourth stage of microbial succession (`stage of senescence¿, 46-48 kyr), whereby authogenic carbonate zones triggered by the high alkalinity associated with AOM eventually `caps¿ the seep. In the last stage, (`stage of extinction¿, 49-50 kyr), the thick autogenic carbonate layers reduce methane fluxes to levels too low to support the ecoysystem and cause the seep ecosystem to be extinguished. This model is based on the assumption that hydrocarbons are the main source of energy in seep ecosystems, and observations that chemosynthesis-based ecosystems are replaced by heterotrophic-based ecosystems when the methane flux is restricted due to production of authigenic carbonate rocks that slowly form at the seep cycle due to AOM. By `shutting off the gas¿, these carbonates capping the methane end up reducing chemosynthesis-supported biodiversity hotspots at the seafloor.