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Open AccessArticle

Ultradeep Microbial Communities at 4.4 km within Crystalline Bedrock: Implications for Habitability in a Planetary Context

1
School of Earth and Environmental Sciences, University of St Andrews, St Andrews KY16 9AL, UK
2
Geological Survey of Finland, 02151 Espoo, Finland
3
Department of Geosciences and Geography, University of Helsinki, 00014 Helsinki, Finland
4
VTT Technical Research Centre of Finland, 02044 VTT, Finland
*
Author to whom correspondence should be addressed.
Received: 1 November 2019 / Revised: 31 December 2019 / Accepted: 1 January 2020 / Published: 4 January 2020
(This article belongs to the Special Issue Planetary Exploration of Habitable Environments)
The deep bedrock surroundings are an analog for extraterrestrial habitats for life. In this study, we investigated microbial life within anoxic ultradeep boreholes in Precambrian bedrock, including the adaptation to environmental conditions and lifestyle of these organisms. Samples were collected from Pyhäsalmi mine environment in central Finland and from geothermal drilling wells in Otaniemi, Espoo, in southern Finland. Microbial communities inhabiting the up to 4.4 km deep bedrock were characterized with phylogenetic marker gene (16S rRNA genes and fungal ITS region) amplicon and DNA and cDNA metagenomic sequencing. Functional marker genes (dsrB, mcrA, narG) were quantified with qPCR. Results showed that although crystalline bedrock provides very limited substrates for life, the microbial communities are diverse. Gammaproteobacterial phylotypes were most dominant in both studied sites. Alkanindiges -affiliating OTU was dominating in Pyhäsalmi fluids, while different depths of Otaniemi samples were dominated by Pseudomonas. One of the most common OTUs detected from Otaniemi could only be classified to phylum level, highlighting the uncharacterized nature of the deep biosphere in bedrock. Chemoheterotrophy, fermentation and nitrogen cycling are potentially significant metabolisms in these ultradeep environments. To conclude, this study provides information on microbial ecology of low biomass, carbon-depleted and energy-deprived deep subsurface environment. This information is useful in the prospect of finding life in other planetary bodies. View Full-Text
Keywords: deep subsurface; deep biosphere; bedrock; microbial ecology; extraterrestrial habitat analog; environmental microbiology; microbial community; low biomass deep subsurface; deep biosphere; bedrock; microbial ecology; extraterrestrial habitat analog; environmental microbiology; microbial community; low biomass
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Purkamo, L.; Kietäväinen, R.; Nuppunen-Puputti, M.; Bomberg, M.; Cousins, C. Ultradeep Microbial Communities at 4.4 km within Crystalline Bedrock: Implications for Habitability in a Planetary Context. Life 2020, 10, 2.

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