| Summary: | Natural gas seepage is the emission of gaseous hydrocarbon from the subsurface of the Earth, commonly found in sea sediments and along tectonic plate boundaries. Natural gas seepage pockmarks are found offshore and onshore in the Öxarfjörður graben. Previous measurements of hydrocarbon content and carbon isotope ratio of methane, suggested that the gas originates from coal bearing strata in the graben. No previous studies have been made on natural gas seepage microbiota in Iceland, but natural gas seeps are likely to harbour hydrocarbon-utilising microbes that can be extremely valuable for bioremediation of polluted environments. This thesis describes a study that aims towards characterizing the microbiota of two distinct natural gas seepage pockmark sites in Öxarfjörður. Geochemical analysis of the sites included chemical analysis of the water submerging the pockmarks, hydrocarbon content and carbon isotope composition of methane. Analysis of the bacterial community structure was performed with amplicon sequencing of variable regions V3-V4 in the 16S rRNA gene from environmental samples of the gas seepage pockmarks. Bacterial strains from the pockmarks were isolated on selective and differential media to screen for bacterial strains with bioremediative potential. The carbon isotope composition and hydrocarbon content at the two study sites revealed biogenic generation of methane with more complex hydrocarbon composition and thermogenic origin with less organic matter, respectively. Both sites had Proteobacteria as the most abundant bacterial phyla, where 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. Bacteroidetes were more abundant at the biogenic site. A total of 106 strains were isolated to purity and characterised, including representatives from the abundant phyla Proteobacteria, ...
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