Abstract
Alpine karst aquifers are important groundwater resources for the provision of drinking water all around the world. Yet, due to difficult accessibility and long-standing methodological limitations, the microbiology of these systems has long been understudied. The aim of the present study was to investigate the structure and dynamics of bacterial communities in spring water of an alpine limestone karst aquifer (LKAS2) under different hydrological conditions (base vs. event flow). The study was based on high-throughput 16S rRNA gene amplicon sequencing, study design and sample selection were guided by hydrology and pollution microbiology data. Spanning more than 27 months, our analyses revealed a taxonomically highly stable bacterial community, comprising high proportions of yet uncultivated bacteria in the suspended bacterial community fraction. Only the three candidate phyla Parcubacteria (OD1), Gracilibacteria (GN02), Doudnabacteria (SM2F11) together with Proteobacteria and Bacteroidetes contributed between 70.0 and 88.4% of total reads throughout the investigation period. A core-community of 300 OTUs consistently contributed between 37.6 and 56.3% of total reads, further supporting the hypothesis of a high temporal stability in the bacterial community in the spring water. Nonetheless, a detectable response in the bacterial community structure of the spring water was discernible during a high-discharge event. Sequence reads affiliated to the class Flavobacteriia clearly increased from a mean proportion of 2.3% during baseflow to a maximum of 12.7% during the early phase of the studied high-discharge event, suggesting direct impacts from changing hydrological conditions on the bacterial community structure in the spring water. This was further supported by an increase in species richness (Chao1) at higher discharge. The combination of these observations allowed the identification and characterization of three different discharge classes (Q1–Q3). In conclusion, we found a taxonomically stable bacterial community prevailing in spring waters from an alpine karst aquifer over the entire study period of more than 2 years. Clear response to changing discharge conditions could be detected for particular bacterial groups, whereas the most responsive group – bacteria affiliated to the class of Flavobacteriia – might harbor potential as a valuable natural indicator of “system disturbances” in karst aquifers.
Highlights
In many regions of the world, drinking water abstraction relies on raw water from alpine or mountainous karst aquifers
Community dissimilarity analysis based on the Bray–Curtis dissimilarity-index measure and Non-metric multidimensional scaling (NMDS) scaling revealed a clear spatial separation of bacterial communities between BF and EV samples (Figure 2; PERMANOVA R2 = 0.14, p-value 0.001)
Bacteria affiliated to the class of Flavobacteriia showed highest response and might harbor potential as a valuable natural indicator for “system disturbance” in karst aquifers
Summary
In many regions of the world, drinking water abstraction relies on raw water from alpine or mountainous karst aquifers. Around 50% of the entire population of Austria (Europe) is supplied from alpine karst aquifers – including its capital Vienna with ∼1.8 million citizens (Kralik, 2001; Stadler et al, 2008) This holds major challenges regarding the management of these vulnerable water resources and their sustainable use. A high potential for chemolithoautotrophic processes has been suggested based on a strong contribution of dissolved inorganic carbon to the build-up of groundwater microbial biomass (Nowak et al, 2017; Schwab et al, 2017) These studies further support the perception of a generally high importance of chemolithoautotrophic processes in groundwater aquifers as already previously proposed for a shallow alluvial groundwater aquifer featuring the potential for anaerobic ammonium oxidation (Jewell et al, 2016)
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