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”.