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Groundwater-associated microorganisms are known to play an important role in the biogeochemical C, N and S cycling. Metaproteomics was applied to characterize the diversity and the activity of microbes to identify key species in major biogeochemical processes in the anoxic groundwater of a pristine karstic aquifer located in Hainich, central Germany. Sampling was achieved by pumping 1000 L water from two sites of the upper aquifer assemblage and filtration on 0.3 μm glass filters. In total, 3808 protein groups were identified. Interestingly, the two wells (H4/2 and H5/2) differed not only in microbial density but also in the prevalence of different C, N and S cycling pathways. The well H5/2 was dominated by the anaerobic ammonia-oxidizing (anammox) candidate Brocadiales (31%) while other orders such as Burkholderiales (2%) or Nitrospirales (3%) were less abundant. Otherwise, the well H4/2 featured only …