Bacterial dynamics in an alpine karst spring in response to prevailing hydrology

Alpine karst aquifers are important groundwater resources for the provision of drinking water and multiple other ecosystem services around the world. Yet, due to difficult accessibility and long-standing methodological limitations, the microbiology of these systems has long been understudied. Here, we present results from a long-term monitoring study on the bacterial spring water community dynamics in the permanently water-saturated, phreatic zone of an alpine limestone karst aquifer (LKAS2) based on high-throughput-DNA-sequencing. Our analyses revealed a stable and abundant community in the suspended bacterial community fraction as previously suggested by Farnleitner et al. (2005). The communities were dominated by bacteria affiliated to candidate phylum Parcubacteria (OD1), Proteobacteria and candidate phylum Gracilibacteria (GN02). The high abundance as well as the hypothesized symbiotic lifestyle of most known Parcubacteria (OD1) based on available information from metagenomics studies suggests a high importance of a biofilm-associated lifestyles in these aquatic systems. The comparative study of the bacterial communities occurring during an isolated high-discharge event triggered by a summer precipitation event with those dominating during baseflow conditions further supported a generally high stability in the bacterial spring water community. Yet, a clear increase of Flavobacteriia-affiliated sequences especially during the early phase of the studied high-discharge event clearly proved the existence of direct impacts of changing hydrological conditions on the bacterial spring water community in this particular aquifer, which was further supported by an incline of species richness at higher discharge. Combining all these findings further allowed the characterization of three different 'discharge classes' for this particular aquifer. Summing up, these results significantly expand our limited knowledge on the microbiology in alpine karst aquifers, and allow the further development of the previously proposed concept of a stable autochthonous microbial endokarst community (AMEC) in these aquifers.