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

Journal article, 2011

Six cores were drilled and retrieved from 186-m 12 depth in the Äspö Hard Rock Laboratory (HRL) tunnel to 13 investigate whether indigenous biofilms develop on fracture 14 surfaces in groundwater-conducting aquifers in granitic 15 rock. A clone library was constructed from fracture surface 16 material (FSM), for community composition analysis. 17 Quantitative polymerase chain reaction (qPCR) was applied Q1 18 to quantify gene copies using the 16S rRNA gene for 19 domain Bacteria and the adenosine-phosphosulfate reduc- Q1 20 tase gene (apsA) for sulfate-reducing bacteria (SRB). 21 Results were compared with three groundwater systems 22 with biofilms in laminar flow reactors (LFRs) at 450-m 23 depth in the Äspö HRL. The total number of cells, counted 24 microscopically, was approximately 2×105 cells·cm–2 in the 25 LFR systems, consistent with the obtained qPCR 16S rRNA 26 gene copies. qPCR analysis reported ∼1×102 up to ∼1×104 27 gene copies·cm–2 on the FSM from the drill cores. In the 28 FSM biofilms, 33% of the sequenced clones were related to 29 the iron-reducing bacterium Stenotrophomonas maltophilia, 30 while in the LFR biofilms, 41% of the sequenced clones 31 were affiliated with the genera Desulfovibrio, Desulforho- 32 palus, Desulfomicrobium, and Desulfobulbus. The commu- 33 nity composition of the FSM biofilms differed from the drill water community, excluding drill water contamination. 34 This work reports significant numbers of microorganisms 35 on natural hard rock aquifer fracture surfaces with site- 36 specific community compositions. The probability that 37 biofilms are generally present in groundwater-conducting 38 aquifers in deep granitic rock is consequently great.