| Summary: | PhD Theses Thermophilic bacteria can be found in large numbers (108 m????2) as endospores (thermospores) in Arctic sediments and soils around Svalbard (Norway), where they cannot grow. It is hypothesised that thermospores originate from warmer marine or terrestrial geothermal environments, and are dispersed passively as dormant spores to colder places. Being subject to limited local selective pressure, thermospores can be used as models for the debatable role of dispersal in microbial biogeography, which has rarely, if ever, been tracked directly for a microbe. Thus, this PhD project aimed to use the local distribution and diversity of thermospores around Svalbard to trace their dispersal in time and space, including their potential sources. Here, I characterised the dispersal of thermospores in time using sediment depth cores (ch.2) and in space along fjord transects (ch.3). To investigate the possibility of dispersal from multiple sources, I compared the community composition of thermospores in terrestrial soils with that of marine sediments in Svalbard (ch.4). Finally, I used metagenomics sequencing to investigate the functional potential and taxonomic composition of the thermophilic communities without primer biases (ch.5). The ndings suggest that despite their high dispersal potential, thermospore distribution is impacted by dispersal limitation in both space and time and by di erences in dispersal history among phylotypes. Their spatial distribution pattern suggests the presence of dispersal barriers and at least one marine source located West of Svalbard. However, additional sources are evidenced by the markedly di erent soil and sediment community compositions, the fragmented spatial distribution of within-OTU diversity in two prominent thermospore phylotypes, and di erences in inferred dispersal history of di erent phylotypes. These results highlight the importance of dispersal limitations in microbial biogeography, laying to rest the widely held notion of ubiquitous distribution for microbes, and ...
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