“What microbes are where, and how do they live there” is now an essential question to understand life on Earth, even when comparing seemingly similar ecosystems in different locations. Soil bacterial populations are known to play important roles in biogeochemical cycles, soil maintenance, climatic effects and agriculture. I used pyrosequencing of PCR amplified 16S rDNA from total extracted DNA in order to reveal the bacterial populations living in four different unusual and oligotrophic environments: A. brackish sediments and soils from the Camargue region in Southern France, B. the rhizosphere of rice (Oryza sativa) growing in the Camargue region in Southern France, C. Eroded surface soils from the Padza de Dapani in Mayotte and D. The surface and rhizosphere soil from pioneer plants in the Jizan desert in Saudi Arabia. A. Saline areas are widely distributed on Earth’s and are represented by both saline lakes and saline soils. We examined the bacterial composition of estuary sediments, brackish and sandy soil samples from the Camargue region (Rhône delta in southern France) sampled in two consecutive years. Members belonging to the Proteobacteria, Bacteroidetes, Chloroflexi, Firmicutes, Acidobacteria and Actinobacteria phyla were found principally in saline sediment and soil samples. We found that members from these phyla were associated principally to halophilic bacteria, sulphate reducing bacteria (SRB), nitrate reducing bacteria and coliforms, and that their varying proportions were likely affected by salinity and geographical location. B. Bacterial populations associated with the rhizosphere of plants are known to play essential roles in biogeochemical cycles, plant nutrition and disease biocontrol. We examined the bacterial populations of the rhizosphere of rice (Oryza sativa) growing in the Camargue region in 2013 and 2014. The most abundant bacterial populations were found to be members belonging to the Proteobacteria, Acidobacteria, Chloroflexi and Gemmatimonadetes phyla. The genera members belong these phyla were found to participate in soil biogeochemical processes such as nitrification, denitrification, 196 oxidation, as well as act as biocontrol agents. The bacterial populations were found to significantly vary by geographical location as well by year of collection. C. We examined the surface soils from “Padza de Dapani” on the island of Mayotte off the east coast of Africa, as this region is not a true (hot) desert, but resembles one due to extensive soil erosion. In the acidic, oligotrophic and mineralized soil samples from Mayotte, members of the Actinobacteria, Proteobacteria and Acidobacteria phyla dominated the bacterial populations. Interestingly, members of the genera Acinetobacter, Arthrobacter, Burkholderia and Bacillus were found to be predominant in our samples, as is also observed in hot (Asian) deserts and may play roles in soil mineral weathering, thus helping to understand desertification processes. D. Earth’s arid regions comprise >30% of the continental surface and the oligotrophic soils are subjected to harsh environmental factors such as low average annual rainfall, high UV exposure and large temperature fluctuations. We examined the bacterial populations present in the rhizosphere of pioneer plants and surface soils in the Jizan desert of Saudi Arabia. The most abundant bacterial phyla belonged to the Bacteroidetes, Proteobacteria and Firmicutes phyla that were different between the rhizosphere of plant versus these from surface sand, with the exception of the plant “Panicum Turgidum”, which contain in its rhizosphere high proportions (~70%) of members belonging to the Flavobacterium genus.