La Caldera” is a high mountain lake located in Sierra Nevada, Spain. For its position and
characteristics it is considered as an important “sensor” for global changes. Water samples were collected
during the ice-free season in order to observe changes in the bacterial community structure. Experiments were carried out complementing culture and culture-independent methods. Strong variations of the bacterial
communities were recorded along the sampling period in particular regarding number and phylogenetic
relationships of the microorganisms detected. At the season beginning, bacterial counts were rather high but
strains belonged only to β- and γ-Proteobacteria. By contrast, in the middle of the season, possibly due to
external inputs, total number of bacteria decreased but other phylogenetic groups, such as α–proteobacteria, Actinobacteria and Firmicute appeared. On the whole, Proteobacteria were the dominant microbiota. Bacterial diversity and presence of most abundant species were analysed by PCR-TGGE. Sequencing of TGGE bandsconfirmed that prevalent Bacteria populations were evolutively close to Proteobacteria with predominance of α- and β-Proteobacteria. Analysis of diversity and functional organization suggested that “La Caldera” bacterial microbiota, typical of oligotrophic and/or extreme environments with low habitability, was characterized by a community organized in an adequate distribution of dominant and resilient species. Although rather specialized, the community presented a sufficient functionality and flexibility to react to changing and stressing conditions. However, along the ice-free season, the community evolved showing more specialized populations with less adaptation abilities. This is the first work complementing cultural and cultural-independent methods for the “La Caldera” lake.