| Summary: | 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.
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