Facultative methanotrophs are abundant at terrestrial natural gas seeps.

BACKGROUND: Natural gas contains methane and the gaseous alkanes ethane, propane and butane, which collectively influence atmospheric chemistry and cause global warming. Methane-oxidising bacteria, methanotrophs, are crucial in mitigating emissions of methane as they oxidise most of the methane produced in soils and the subsurface before it reaches the atmosphere. Methanotrophs are usually obligate, i.e. grow only on methane and not on longer chain alkanes. Bacteria that grow on the other gaseous alkanes in natural gas such as propane have also been characterised, but they do not grow on methane. Recently, it was shown that the facultative methanotroph Methylocella silvestris grew on ethane and propane, other components of natural gas, in addition to methane. Therefore, we hypothesised that Methylocella may be prevalent at natural gas seeps and might play a major role in consuming all components of this potent greenhouse gas mixture before it is released to the atmosphere. RESULTS: Environments known to be exposed to biogenic methane emissions or thermogenic natural gas seeps were surveyed for methanotrophs. 16S rRNA gene amplicon sequencing revealed that Methylocella were the most abundant methanotrophs in natural gas seep environments. New Methylocella-specific molecular tools targeting mmoX (encoding the soluble methane monooxygenase) by PCR and Illumina amplicon sequencing were designed and used to investigate various sites. Functional gene-based assays confirmed that Methylocella were present in all of the natural gas seep sites tested here. This might be due to its ability to use methane and other short chain alkane components of natural gas. We also observed the abundance of Methylocella in other environments exposed to biogenic methane, suggesting that Methylocella has been overlooked in the past as previous ecological studies of methanotrophs often used pmoA (encoding the alpha subunit of particulate methane monooxygenase) as a marker gene. CONCLUSION: New biomolecular tools designed in this study have expanded our ability to detect, and our knowledge of the environmental distribution of Methylocella, a unique facultative methanotroph. This study has revealed that Methylocella are particularly abundant at natural gas seeps and may play a significant role in biogeochemical cycling of gaseous hydrocarbons.

Natural geological seepage of hydrocarbon gas in the Appalachian Basin and Midwest USA in relation to shale tectonic fracturing and past industrial hydrocarbon production.

Geological hydrocarbon gas seepage is a major global source of atmospheric methane, ethane and propane as greenhouse gases and photochemical pollutants. Natural gas seepage is generally related to faults and associated fracture intensification domains that provide conduits for natural gas from reservoir rocks to migrate upward and enter the atmosphere. In this study, we compare the case of intense gas seepage stemming directly from source rocks, mostly organic-rich fractured black shales in western New York State (NYS) versus areas with rare seepage in the more southern regions of the Appalachian Basin and the Midwest USA. In addition to thermogenic methane, western NYS shale gas seeps emit ethane and propane with C2+3 gas concentrations reaching up to 35 vol%. Fractures in NYS developed, reactivated and maintained permeability for gas as a result of Quaternary glaciation and post-glacial basin uplift. In contrast, the Appalachian regions farther south and the southern Midwest regions experienced less glacial loading and unloading than in NYS, resulting in less recent natural fracturing, as witnessed by the rarity of seepage on surface outcrops and in caves overlying gas-bearing shales and coals. The historical literature suggests that early western NYS drilling and production of oil and gas diminished shale gas pressure and resulted in declining gas seepage rates. Our survey documented 12 active western NYS natural gas seeps, whereas >32 seeps have been reported or documented since the 17th century. Preliminary tests showed that SCIAMACHY satellite data did not detect atmospheric methane anomalies over western NYS seeps.