Articles | Volume 13, issue 10
Biogeosciences, 13, 3091–3108, 2016
Biogeosciences, 13, 3091–3108, 2016

Research article 30 May 2016

Research article | 30 May 2016

Microbial co-occurrence patterns in deep Precambrian bedrock fracture fluids

Lotta Purkamo1, Malin Bomberg1, Riikka Kietäväinen2, Heikki Salavirta1, Mari Nyyssönen1, Maija Nuppunen-Puputti1, Lasse Ahonen2, Ilmo Kukkonen2,a, and Merja Itävaara1 Lotta Purkamo et al.
  • 1VTT Technical Research Centre of Finland Ltd., Espoo, Finland
  • 2Geological Survey of Finland (GTK), Espoo, Finland
  • anow at: Department of Physics, University of Helsinki, Helsinki, Finland

Abstract. The bacterial and archaeal community composition and the possible carbon assimilation processes and energy sources of microbial communities in oligotrophic, deep, crystalline bedrock fractures is yet to be resolved. In this study, intrinsic microbial communities from groundwater of six fracture zones from 180 to 2300 m depths in Outokumpu bedrock were characterized using high-throughput amplicon sequencing and metagenomic prediction. Comamonadaceae-, Anaerobrancaceae- and Pseudomonadaceae-related operational taxonomic units (OTUs) form the core community in deep crystalline bedrock fractures in Outokumpu. Archaeal communities were mainly composed of Methanobacteriaceae-affiliating OTUs. The predicted bacterial metagenomes showed that pathways involved in fatty acid and amino sugar metabolism were common. In addition, relative abundance of genes coding the enzymes of autotrophic carbon fixation pathways in predicted metagenomes was low. This indicates that heterotrophic carbon assimilation is more important for microbial communities of the fracture zones. Network analysis based on co-occurrence of OTUs revealed possible “keystone” genera of the microbial communities belonging to Burkholderiales and Clostridiales. Bacterial communities in fractures resemble those found in oligotrophic, hydrogen-enriched environments. Serpentinization reactions of ophiolitic rocks in Outokumpu assemblage may provide a source of energy and organic carbon compounds for the microbial communities in the fractures. Sulfate reducers and methanogens form a minority of the total microbial communities, but OTUs forming these minor groups are similar to those found in other deep Precambrian terrestrial bedrock environments.

Short summary
The microbial communities of up to 2.3 km depth of Precambrian crystalline bedrock fractures share features with serpenization-driven microbial communities in alkaline springs and subsurface aquifers. This study suggests that phylotypes belonging to Burkholderiales and Clostridia are possible "keystone microbial species" in Outokumpu deep biosphere. Many of the keystone species belong to the rare biosphere with low abundance but a wide range of carbon substrates and a capacity for H2 oxidation.
Final-revised paper