Biogeochemistry and microbial ecology of shallow marine sediments from methane seeps, Katakolo Bay, Western Greece

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Βαζιουράκης, Κωνσταντίνος-Μάριος
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Anaerobic oxidation of methane (AOM) and other hydrocarbons prevents methane emission from marine sediments into the atmosphere. This study tries to illuminate aspects of AOM investigating shallow marine core sediments from Katakolo bay (W. Greece), one of the vastest thermogenic gas seepage fields in Europe. Belonging to the central Ionian geotectonic zone of the external units of the Hellenides, Katakolo has attracted the interest of scientists due to the vast seepage occurring in the harbor and can be considered as a unique natural laboratory in studying O2 versus gas seepage by long term monitoring. In the terms of this master dissertation, a number of multidisciplinary methodologies was implemented in order to shed light on the key biogeochemical process and the microbial ecology of nearshore (less than eight meter water depth) organic carbon depleted silty sands. We combined pore-water geochemical measurements, bulk geochemical and mineralogical analyses with 16S rRNA sequencing-based community analysis for both Archaea and Bacteria and fluorescence in situ hybridization (CARD-FISH) to investigate and visualize the microbial communities in this environment. Although the sediments are highly disturbed by natural and anthropogenic activities at the port area, several geochemical proxies indicated the presence of recent and paleo-redox changes along the sediment cores. Congruent with the existence of a seep microbiome, a distinct microbial community shift was observed in deeper sediment horizons (25-35cm below seafloor). The archaeal community from surface sediments was characterized by Bathyarchaeia and the recently discovered, Woesarchaeota, whose metabolic potential is possible consistent with methanogenesis in hypoxic surface marine sediments. By contrast, in the deeper layer with methane as the dominant hydrocarbon, strictly anaerobic methane oxidation is likely the main process controlled by the governance of the clade of ANME-2c in conjunction with the presence of ethane and butane oxidizers. In the same depth, bacterial communities are characterized by the presence of Parcubacteria (Candidatus Moranbacteria). The latter coincides with the anoxic environment of the sediments but it differs from the typical governance of the sulfate reducing bacteria that were expected. Slight decrease of sulfate, formation of free sulfide and the presence of aggregates consisted of ANME-2c indicate the predominance of sulfate-coupled AOM in these anoxic shallow marine sediments.
Biogeochemistry, Microbial ecology, Methane seeps, Katakolo bay