Piezo-tolerant natural gas-producing microbes under accumulating pCO(2)

BACKGROUND: It is known that a part of natural gas is produced by biogenic degradation of organic matter, but the microbial pathways resulting in the formation of pressurized gas fields remain unknown. Autogeneration of biogas pressure of up to 20 bar has been shown to improve the quality of biogas to the level of biogenic natural gas as the fraction of CO(2) decreased. Still, the pCO(2) is higher compared to atmospheric digestion and this may affect the process in several ways. In this work, we investigated the effect of elevated pCO(2) of up to 0.5 MPa on Gibbs free energy, microbial community composition and substrate utilization kinetics in autogenerative high-pressure digestion. RESULTS: In this study, biogas pressure (up to 2.0 MPa) was batch-wise autogenerated for 268 days at 303 K in an 8-L bioreactor, resulting in a population dominated by archaeal Methanosaeta concilii, Methanobacterium formicicum and Mtb. beijingense and bacterial Kosmotoga-like (31% of total bacterial species), Propioniferax-like (25%) and Treponema-like (12%) species. Related microorganisms have also been detected in gas, oil and abandoned coal-bed reservoirs, where elevated pressure prevails. After 107 days autogeneration of biogas pressure up to 0.50 MPa of pCO(2), propionate accumulated whilst CH(4) formation declined. Alongside the Propioniferax-like organism, a putative propionate producer, increased in relative abundance in the period of propionate accumulation. Complementary experiments showed that specific propionate conversion rates decreased linearly from 30.3 mg g(−1) VS(added) day(−1) by more than 90% to 2.2 mg g(−1) VS(added) day(−1) after elevating pCO(2) from 0.10 to 0.50 MPa. Neither thermodynamic limitations, especially due to elevated pH(2), nor pH inhibition could sufficiently explain this phenomenon. The reduced propionate conversion could therefore be attributed to reversible CO(2)-toxicity. CONCLUSIONS: The results of this study suggest a generic role of the detected bacterial and archaeal species in biogenic methane formation at elevated pressure. The propionate conversion rate and subsequent methane production rate were inhibited by up to 90% by the accumulating pCO(2) up to 0.5 MPa in the pressure reactor, which opens opportunities for steering carboxylate production using reversible CO(2)-toxicity in mixed-culture microbial electrosynthesis and fermentation. [Figure: see text] ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13068-016-0634-7) contains supplementary material, which is available to authorized users.