Microbial community dynamics in alpine karst aquifers

Karst aquifers are particular types of groundwater ecosystems, showing unique characteristics in terms of hydrogeology, biogeochemistry and vulnerability. In contrast to porous media aquifers, karst aquifers with their discrete rock fractures and large conduits exhibits distinct and very dynamic discharge characteristics from their respective catchment areas. Karst aquifers are thus particular susceptible to environmental impacts and changes. A lot of effort is put into the development of protection strategies in order to maintain high water quality; but thorough understanding of karst-aquatic environments is a complex subject and demands for multiple scientific disciplines. In alpine areas like Austria, groundwater resources from alpine karst aquifers play a fundamental role for high quality public water supply. As an important example, Vienna, the capital of Austria (about 1.6 million inhabitants), is almost exclusively supplied (>90%) by water from alpine karst aquifers. So far, investigations on microbial communities in alpine karst ecosystems have focused almost exclusively on a small fraction of microbes, namely on microbial fecal indicators. This lack of of knowledge is sharply contrasting the general considered importance of microbes for ecosystem prossesses such as energy- and matter flux. The proposed research project will bridge those gaps by addressing the following issues: i) The general characterisation of the abundance and dynamics of total prokaryotic-, protozoan-, and viral number direct counts from spring water of representative alpine karst aquifers, ii) the determination of the genetic eubacterial community structure and dynamics of the dominant spring water populations, iii) the assessment of the discrimination ability between the eubacterial autochthonous- and transient microbial endokarst communities, iv) and the comparative analysis between the genetic community structure of the attached- (i.e. the biofilm) versus the suspended eubacterial communities. In the proposed study it is hypothesized that knowledge based on autochthonous microbial communities (AMEC) and transient microbial endokarst communitis (TMEC) will help to i) brigde the gap between different karst compartments and the catchment area and thus extending the concept of carst systems, ii) to sumplement exisitng vulnerabilty assessment approaches, and iii) to provide the scientific basis for a total microbial community based spring water quality assessment for drinking water such as for bottled water production.

During this research project spring water of two alpine karst aquifers differing in hydrogeology but of nearby catchments were investigated for their bacterial population dynamics. DKAS 1 represents a dolomitic-limestone karst aquifer spring showing high average water residence time and relative constant flow. LKAS 2 constitutes a typical limestone karst aquifer spring with a dynamic hydrological regime and discharge. DKAS 1 yielded constantly lower cell counts and biomasses (median of 1510 6 cells L-1 and 0.22 g C L-1 ) as the LKAS 2 (median of 6310 6 cells L-1 and 1.1 g C L-1 ) and distribution of morphotypes and mean cell volumes were also different between the considered systems, indicating the influence of hydrogeology on microbial spring water quality. Molecular bacterial V3 16S-rDNA profiles revealed remarkable constancy within each spring water throughout the investigation period. Time course analysis of a flood event in LKAS 2 further supported the trend of the temporal constancy of the microbial community. Except for one case, retrieval of partial and full length 16S rDNA gene sequences from the relative constant DKAS 1 revealed similarities to presently known sequences between 80% up to 96%, supporting the discreteness of the microbial populations. The gathered results provide first evidence for the presence of autochthonous microbial endokarst communities (AMEC). Recovery of AMEC may be considered of relevance for the understanding of alpine karst aquifer biogeochemistry and ecology, which is of interest as many alpine and mountainous karst springs are important water resources throughout the world. The investigation of the relevance of AMEC for the aquifer self purification potential in terms of chemical or health related microbial pollution, for karstification processes in the aquifer itself and respective biofilm formation, or for the bio-stability of spring water as used for water supply will be investigated in future ongoing research projects.