Methane dynamics in a microbial community of the Black Sea traced by stable carbon isotopes in vitro

Microbial mats thriving at gas seeps within anoxic bottom water were sampled during a Black Sea expedition in summer 2001. These mats contain associations of archaea which belong to the ANME-1 and, to a lesser extent, to the ANME-2 cluster, both accompanied by sulphate-reducing bacteria of the Desulfosarcina/Desulfococcus-group and perform the anaerobic oxidation of methane (AOM). Transferring living mat into the laboratory allowed for an in vitro investigation of its methane turnover. We incubated aliquots of these AOM performing mats over a time period of 242 days tracking concentrations and 13C/12C ratios of the methane. The data obtained showed a decrease in concentration accompanied by a relative enrichment of 12C in the residual methane. These results indicate that the mats performed, in the presence of sulphate, both methane oxidation and methane production (methanogenesis). Rate calculations using information from additional experiments and field data gave a ratio between methane oxidation and methanogenesis of about 2:1. The parallel investigation of concentrations and signatures of stable carbon isotopes of methane revealed methane dynamics within these mats exceeding considerably what becomes apparent from only recognising the methane concentrations.