The geothermal habitats of the Southern and Antarctic regions are unique biodiversity sanctuaries in very remote polar environments. They are located more than 3000 km from the first inhabited areas and are thus very little anthropized. More than 40 terrestrial and submarine geothermal sources with contrasting harsh physico-chemical conditions have been identified in these areas. The microbial communities they host have been the subject of only a few studies and remain largely unknown. These communities are likely shaped by their biogeographic position and the physicochemical parameters of the hot springs (temperature, pH, in situ chemistry) which probably exert a strong selective pressure on indigenous communities. Yet, these microbial communities probably represent a reservoir of diversity, functions and innovation. Like the endemic plant and animal populations of these regions, the microbial communities also require special attention for their protection and conservation. Following the example of what has been done for microbial communities in geothermal areas of Antarctica (ASPAs 140, 175), the aim of this study is to carry out a first comprehensive inventory of bacterial, archaeal, viral, microeukaryotic and fungal communities of the geothermal habitats of the Kerguelen and Saint-Paul islands, which are part of the French Southern and Antarctic Lands, and to predict the functions and adaptations encoded in their metagenomes and expressed in their metatranscriptomes. In this work, the distribution and compositional patterns of microbial communities, as well as the co-occurrence patterns of microorganisms from a wide range of geothermal sources will be studied and compared with each other and with those of similar geothermal environments, to get a first comprehensive snapshot of the microbial composition of these undisturbed communities and its functional potential, and in an attempt to determine the respective share of deterministic and stochastic processes explaining microbial community assembly. Cultivation and isolation attempts will be made for taxa belonging to branches of life yet without cultured representatives, in order to study their physiology and adaptations. Finally, emphasis will also be placed on microbial populations in thermal mineral springs with silica or carbonate deposits as they represent a key to understanding the primitive life forms on Earth and may also provide valuable information on extraterrestrial life forms in analogue environments.