Alpine headwaters fed by permafrost thaw and glacial melt host different bacterial communities: possible ecological implication for the future

Effects of atmospheric warming are particularly pronounced on mountain regions of the world, including the European Alps. As the thawing rate of mountain permafrost ice is lower than for glacier ice, a progressive shift from glacial to periglacial dynamics is predicted for Alpine landscapes during the 21 st century in relation to deglaciation. Despite the growing literature on the impacts of deglaciation on Alpine hydrology and ecosystems, chemical and biological features of waters emerging from Alpine rock glaciers (i.e. permafrost landforms composed by a mixture of ice and debris) have been poorly investigated so far, while knowledge on microbial biodiversity of glacial headwaters is still sparse. A set of glacier- and permafrost-fed headwaters was investigated in the Central Italian Alps in late summer 2016, aiming at exploring bacterial community composition and diversity in epilithic biofilm and surface sediments in relation to water origin andchemical properties. Rock glacier-fed waters showed enhanced conductivity and trace element concentrations, and their highly diverse bacterial assemblages significantly differed from those detected in glacial streams, although no bacteria taxa exclusive for rockglacier-fed were detected.Bacterial taxonomical composition appeared to be related to water and sample type, as well as to water chemical properties, especially including concentrations of nutrients and trace metals. The results outline the need of further research aimed at understanding the role of environmental features in modulating bacterial diversity of Alpine headwaters affected by deglaciation.