Potential sources of microbial colonizers in initial soil ecosystem after retreat of an Alpine glacier. DNA sequences from environmental samples in the Damma glacier

Rapid disintegration of Alpine glaciers has led to the formation of new terrain consisting of mineral debris promptly colonized by microorganisms. Despite the importance of microbial pioneers in triggering the formation of terrestrial ecosystems, their sources (endogenous versus exogenous) and identity remain elusive. We characterized the bacterial and fungal communities in endogenous glacier habitats (ice, sub-, supraglacial sediments and glacier stream leaving the glacier forefront) and atmospheric deposition (snow, rain and aeolian dusts) which potentially establish in recently deglaciated barren soils before and after snow melt (snow-covered soil and barren soil) by 454-pyrosequencing. Atmospheric bacteria and fungi were dominated by plant-epiphytic organisms and differed from endogenous glacier habitats and soils indicating that the atmosphere input of microorganisms is unlikely a major source of microbial pioneers in newly formed soils. We found, however, that bacterial communities of barren soils resembled those of endogenous habitats suggesting that bacterial pioneers mainly come from endogenous habitats related to glacier ecosystem. Conversely, fungal communities differed between habitats suggesting a lower dispersal capability than bacteria. Interestingly, psychrophilic black yeasts characterized snow and supraglacial sediments whereas these habitats were not spatially connected. This suggests that environmental filtering selects particular fungal communities in cold habitats. Atmospheric deposition nonetheless provided non-negligible sources of C and N, such as dissolved organic C, nitrate and ammonium. Overall, we found that microbial colonizers triggering soil development in Alpine environments mainly originate from endogenous glacier habitats while atmospheric deposition contributes to the establishment of microbial communities by providing available C and N sources.