COMMUNITY STRUCTURE AND BIOGEOGRAPHY OF BACTERIA ON TWO ALPINE                 DEBRIS-COVERED GLACIERS
High-elevation cold environments are considered ideal places to test                 hypotheses both about the mechanisms of bacterial colonization and succession, and                 about bacterial biogeography. Debris-covered glaciers (glaciers whose ablation                 tongue is mainly covered by a continuous layer of rock debris) represent extreme                 environments that have never been investigated in this respect so far. We                 constructed 38 metagenomic libraries of the V5-V6 hypervariable regions of the gene                 16S rRNA of bacterial strains collected in July and in September 2009 at different                 distance from the terminus on two debris-covered glaciers (Miage and Belvedere) in                 the Italian alpsAlps. The dominant taxa (Actinobacteridae, Sphingobacteriales and                 Burkholderiales) were not specialized primary producers or N-fixing organisms, thus                 indicating that, even at the initial stages of soil development, C and N are                 probably provided mostly by allochthonous deposition of organic matter. Multivariate                 statistical analysis showed that organic carbon content of the debris and distance                 from the glacier terminus significantly explained variation in bacterial                 communities, thus suggesting that the bacterial communities changed during downwards                 debris transport on the glacier surface. We also investigated whether phypotyopes of                 the taxon Polaromonas, which is ubiquitous in cold environments, do present a                 biogeographical distribution by combining the RNA sequences retrieved in the present                 study with others available in literature. We found that Porlaromonas sequences                 showed a significant divergence by distance, with sequences collected at sites less                 than 100 km apart being more similar than those collected at larger distance. This                 evidence contrasts the Baas-Becking hypothesis that “Everything is everywhere, the                 environment selects”.