title: Heterotrophic communities supplied by ancient organic carbon predominate in deep fennoscandian bedrock fluids.
abstract: The deep subsurface hosts diverse life, but the mechanisms that sustain this diversity remain elusive. Here, we studied microbial communities involved in carbon cycling in deep, dark biosphere and identified anaerobic microbial energy production mechanisms from groundwater of Fennoscandian crystalline bedrock sampled from a deep drill hole in Outokumpu, Finland, by using molecular biological analyses. Carbon cycling pathways, such as carbon assimilation, methane production and methane consumption, were studied with cbbM, rbcL, acsB, accC, mcrA and pmoA marker genes, respectively. Energy sources, i.e. the terminal electron accepting processes of sulphate-reducing and nitrate-reducing communities, were assessed with detection of marker genes dsrB and narG, respectively. While organic carbon is scarce in deep subsurface, the main carbon source for microbes has been hypothesized to be inorganic carbon dioxide. However, our results demonstrate that carbon assimilation is performed throughout the Outokumpu deep scientific drill hole water column by mainly heterotrophic microorganisms such as Clostridia. The source of carbon for the heterotrophic microbial metabolism is likely the Outokumpu bedrock, mainly composed of serpentinites and metasediments with black schist interlayers. In addition to organotrophic metabolism, nitrate and sulphate are other possible energy sources. Methanogenic and methanotrophic microorganisms are scarce, but our analyses suggest that the Outokumpu deep biosphere provides niches for these organisms; however, they are not very abundant.
authors.affiliation: VTT Technical Research Centre of Finland, PL1000, 02044, Espoo, Finland, lotta.purkamo@vtt.fi.
authors.forename: Lotta
authors.initials: L
authors.lastname: Purkamo
authors.forename: Malin
authors.initials: M
authors.lastname: Bomberg
authors.forename: Mari
authors.initials: M
authors.lastname: Nyyssönen
authors.forename: Ilmo
authors.initials: I
authors.lastname: Kukkonen
authors.forename: Lasse
authors.initials: L
authors.lastname: Ahonen
authors.forename: Merja
authors.initials: M
authors.lastname: Itävaara
mesh_terms: D001419:Bacteria / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification; D002244:Carbon / Q000737:chemistry*; D057486:Carbon Cycle; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000235:genetics; D005387:Finland; D015723:Gene Library; D005798:Genes, Bacterial; D005819:Genetic Markers; D052836:Heterotrophic Processes; D008697:Methane / Q000378:metabolism; D058458:Nitrogen Cycle; D010802:Phylogeny; D058974:Phylogeography; D016133:Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D012988:Soil Microbiology
chemical_list: D004269:DNA, Bacterial; D005819:Genetic Markers; D002244:Carbon; D008697:Methane