| Summary: | Surtsey is a volcanic island located on the south-east offshore extension of the Icelandic rift zone. It was formed during successive eruptions from the seafloor in 1963-1967 and has been officially protected and studied ever since. It represents an exceptional natural setting for studying colonization and succession of life on land. Also for subsurface microbial communities associated with newly formed basaltic tuff deposits in a seawater hydrothermal system that is still active and at temperatures approaching the presumed thermal limit for functional life. During an international drilling operation at Surtsey in 2017, drill core samples at successive depths as well as associated hot fluids and surface fumes from fumaroles were collected for microbial investigations. This thesis presents the first and most comprehensive research of the Surtsey subsurface biosphere. Multiple approaches were combined on the rare and unique samples to increase the knowledge of microbial communities inhabiting the oceanic subsurface and of the processes that sustain such life. These included molecular analyses of environmental DNA through 16S rRNA gene amplicon and metagenome sequencing, isolation and characterization of bacterial strains and microscopic investigations. Based on the DNA concentration, the microbial cell numbers present in the drill cores were estimated to range from about 5×104 to 1×106 cells per gram of sample. The Surtsey subsurface is therefore a low biomass environment, making the samples extremely sensitive to external contamination. It is nevertheless a diverse habitat that hosts bacterial and archaeal clades, including extremophiles, that have been previously detected in other terrestrial and marine environments. Yet, many clades belonged to unknown lineages. Predictive functional analyses based on taxonomic identifications revealed that the Surtsey subsurface biosphere is composed of heterotrophic microorganisms as well as chemoautotrophs involved in the sulfur, nitrogen, and methane cycles. However, these ...
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