Metaproteomics of an natural aquifer (65 m below surface) from 1000 L filtered groundwater Comprehensive taxonomic and functional proteomic map revealed Candidatus Brocadiales dominating the hot spot well H5-2. Candidatus Brocadiales is responsible for the major C, N and S biogeochemical cycles. Important role of chemolithoautotrophy in the deep aquifer biogeochemical cycling Anaerobic ammonium oxidation (Anammox) is highly present in the aquifer.