Biofilm prokaryotic diversity in Alpine streams
Windows of Opportunity (WOs), periods of milder environmental conditions supporting the seasonal development of benthic microorganisms (mainly primary producers) in alpine glacial streams, are currently being defined based on changes in biofilm biomass. However, very little is known about the seasonal development of benthic prokaryotic diversity of alpine headwaters of origin other than glacial in the present context of glacier retreat.    A two year (2017-2018) metabarcoding of epilithic and sediment prokaryotes was conducted in alpine headwaters fed by glacial melt (kryal), rock glacial thawing, or groundwater (krenal) in two catchments of the Central-Eastern European Alps (Italy), aiming at better understanding prokaryotic distribution in different water types and substrates, and, especially, at testing the hypothesis that environmental WOs enhances prokaryotic a-diversity besides biomass in both glacial and non glacial streams.     The study confirmed kryal and krenal streams as endmembers of epilithic and sediment prokaryotic a- and b-diversity, with rock glacial streams sharing a large portion of taxa with both the two other water types. Alpha-diversity appeared to respond to ecological WOs, but, contrary to expectations, seasonality was less pronounced in turbit kryal than in clear non glacial streams, likely in relation to the changing hydrological influences of small glaciers over the downstream water. Indirect effects of climate change over habitat conditions of high altitude headwaters may also be responsible for the higher prokaryotic a-diversity that was found in early summer in comparison to late summer/autumn in non glacial streams. Our findings, while confirming high altitude streams as heavily threatened by climate change, underscore the still neglected role of rock glacier runoffs as climate refugia for the most stenothermic aquatic organism. This advocates the necessity to define and test strategies for protecting such climate refugia for preserving, restoring and connecting cold alpine aquatic biodiversity in the context of the progressing global warming.