Thermodynamic constraints on the biogeochemical cycle of methane in the Black Sea

Due to its permanent vertical stratification, the Black Sea is the world’s largest aquatic methane reservoir, holding an estimated 96 Tg of methane¹. Understanding the biogeochemical processes at work in this unique system is crucial for evaluating the vulnerability of the methane reservoir to environmental perturbations. Additionally, such knowledge is essential for assessing the potential of deep Black Sea waters as a viable option for carbon storage, contributing to strategies aimed at mitigating greenhouse gas emissions.

Below approximately 150 m of water depth, the anoxic waters are enriched with reduced compounds such as dissolved CH₄ and H₂S, and dissolved organic matter. These unique chemical conditions sustain a specialized ecosystem dominated by anaerobic chemotrophic microbes, which rely on these compounds for energy production and play a critical role in the biogeochemical cycling of carbon and sulfur. Specifically, the anaerobic oxidation of methane (AOM) is a critical methane sink regulating the content of methane in the water column. The methanotrophic archaea comprise 3-4% of microbial cells in the water column² and are believed to drive pelagic AOM. While this process typically involves a symbiosis between anaerobic and sulfate-reducing bacteria in marine sediments, AOM mechanisms in the Black Sea water column remain poorly understood.

To better understand the Black Sea’s methane dynamics, a new biogeochemical model of the water column has been developed. This model explores microbial metabolism coupling both thermodynamic and microbiology approaches, shedding light on the processes governing methane oxidation and transfer across water layers. The study also aims to address uncertainties in methane production, oxidation, and storage. By providing updated methane stock estimates and insights into flux dynamics, this research will inform future environmental impact assessments.

¹ Reeburgh, William S., Bess B. Ward, Stephen C. Whalen, Kenneth A. Sandbeck, Katherine A. Kilpatrickt, et Lee J. Kerkhof. 1991. « Black Sea methane geochemistry ». Deep Sea Research Part A. Oceanographic Research Papers, Black Sea Oceanography: Results from the 1988 Black Sea Expedition, 38.
² Durisch-Kaiser E, Klauser L, Wehrli B, et Schubert C. 2005. « Evidence of Intense Archaeal and Bacterial Methanotrophic Activity in the Black Sea Water Column. » Applied and Environmental Microbiology.