The research of this thesis focussed on the investigation of the microbial diversity in soda lakes, giving a special attention to the micro-organisms involved in the sulphur cycle. The present PhD was part of a bigger project aiming to develop a biological process for the removal of hydrogen sulphide under halo-alkaliphilic conditions. In this new process, sulphide is mainly oxidized to elemental sulphur by sulphur-oxidizing bacteria (SOB). However, a small percentage of unwanted sulphate is still produced, which is reduced back to sulphide by sulphate-reducing bacteria (SRB). Since this process operates under halo-alkaliphilic conditions, which are naturally occurring in soda lakes, a better understanding of the sulphur microbial diversity in such environments was required. Soda lakes are extreme environments, characterized by an extremely high pH (up to 11.5) and moderately to extremely high salinities (up to 500 g/L). They are a specific type of saline lakes and are spread all over the world, generally confined to arid and semi-arid regions. We especially researched soda lakes from the Kulunda Steppe, which are located in the south-east of Siberia (Russia). These lakes are still less investigated, due to their difficulty to be reached. So-called halo-alkaliphilic organisms inhabit these lakes, which raise the interest of the bioindustry for their capability to stand such harsh conditions. In this research we focussed on the halo-alkaliphilic sulphur bacteria. Special attention was given to the genetic diversity and biogeography of a specific group of halo-alkaliphilic chemolithoautotrophic SOB, i.e. Thioalkalivibrio, and to the diversity, activity and dynamics of the SRB communities in different soda lakes of the Kulunda Steppe, also in relation to the increase of the salt concentration.