Gas Seepage Pockmark Microbiomes Suggest the Presence of Sedimentary Coal Seams in the Öxarfjörður Graben of NE-Iceland

Natural gas seepage pockmarks are found off and onshore in the Öxarfjörður graben, NE Iceland. The bacterial communities of two onshore seepage sites were analysed by amplicon sequencing of 16S rDNA, along with determining the geochemical characteristics, hydrocarbon content and the carbon isotope composition of the sites. While one site was found to be characterised by biogenic origin of methane gas, with carbon isotope ratio δ13C [‰] = −63.2, high content of organic matter and complex hydrocarbons, the other site showed a mixed origin of the methane gas (δ13C [‰] = −26.6) with geothermal characteristics and lower organic matter content. While both sites harboured Proteobacteria as the most abundant bacterial phyla, the Deltaproteobacteria were more abundant at the geothermal site, and the Alphaproteobacteria at the biogenic site. The Dehalococcoidia class of the Chloroflexi phylum was abundant at the geothermal site while the Anaerolineae class was more abundant at the biogenic site. Bacterial strains from the seepage pockmarks were isolated on a variety of selective media targeting bacteria with bioremediation potential. A total of 106 strains were isolated and characterised, including representatives from the phyla Proteobacteria, Bacterioidetes, Firmicutes , and Actinobacteria . This article describes the first microbial study on gas seepage pockmarks in Iceland.