Abundance and Diversity of Biofilms in Natural and Artificial Aquifers of the Äspö Hard Rock Laboratory, Sweden

Abstract

Six cores were drilled and retrieved from 186-m depth in the Äspö Hard Rock Laboratory (HRL) tunnel to investigate whether indigenous biofilms develop on fracture surfaces in groundwater-conducting aquifers in granitic rock. A clone library was constructed from fracture surface material (FSM), for community composition analysis. Quantitative polymerase chain reaction (qPCR) was applied to quantify gene copies using the 16S rRNA gene for domain Bacteria and the adenosine-phosphosulfate reductase gene (apsA) for sulfate-reducing bacteria (SRB). Results were compared with three groundwater systems with biofilms in laminar flow reactors (LFRs) at 450-m depth in the Äspö HRL. The total number of cells, counted microscopically, was approximately 2 × 10(5) cells cm(-2) in the LFR systems, consistent with the obtained qPCR 16S rRNA gene copies. qPCR analysis reported ∼1 × 10(2) up to ∼1 × 10(4) gene copies cm(-2) on the FSM from the drill cores. In the FSM biofilms, 33% of the sequenced clones were related to the iron-reducing bacterium Stenotrophomonas maltophilia, while in the LFR biofilms, 41% of the sequenced clones were affiliated with the genera Desulfovibrio, Desulforhopalus, Desulfomicrobium, and Desulfobulbus. The community composition of the FSM biofilms differed from the drill water community, excluding drill water contamination. This work reports significant numbers of microorganisms on natural hard rock aquifer fracture surfaces with site-specific community compositions. The probability that biofilms are generally present in groundwater-conducting aquifers in deep granitic rock is consequently great.