| Summary: | Until recently, the only way to describe microbial communities was by cultivation. This is a major obstacle because typically only a small fraction of the microbes can be cultivated by standard techniques. However, the use of rRNA and molecular phylogenetic techniques has allowed us to bypass this limitation. In this thesis, both of these approaches were used to describe the bacterial diversity of a few Icelandic hot springs. One of these hot springs was a sulfide rich hot spring (sulfur-mat), which was of special interest because it had a great biomass above the photosynthetic border. In order to evaluate the abundance and stability of cultivable Thermus spp. as a function of seasonal changes in this hot spring, fifty strains of Thermus were isolated during one and a half-year period. Multilocus enzyme electrophoresis (MEE) was used to identify the Thermus strains and to examine the genetic structure of the population. The majority of the strains were T. oshimai and that population showed temporal variation. Furthermore, estimation of the association index indicated that recombination events were frequent within the population. A similar study of the population structure of the genus Thermus, within and between geothermal areas in Iceland, indicated that there were some geographical variations and some physiological factors like pH that affect the population structure of Thermus in Iceland. A new sulfur-oxidizing Thermus strain was isolated and characterized from this same hot spring. A new Hydrogenobacter strain that was able to grow on thiosulfate but could not utilize hydrogen was also isolated. These metabolic features were previously unknown for these genera. An examination of the microbial diversity of the sulfur-mat, a Chloroflexus-mat and a bacterial mat at 88°C by sequencing of SSU rRNA genes obtained by PCR and cloning from the microbial mats, showed that the population structure was quite different in these three ecosystems. Furthermore, the overall diversity of the sulfur-mat and the 88°C mat was ...
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