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Open AccessArticle

Microbial and Geochemical Investigation down to 2000 m Deep Triassic Rock (Meuse/Haute Marne, France)

1
BRGM, Geomicrobiology and Environmental Monitoring Unit, F-45060 Orléans CEDEX 02, France
2
Bordeaux University, Centre d’Etudes Nucleaires de Bordeaux Gradignan, UMR5797, F-33170 Gradignan, France
3
CNRS-IN2P3, Centre d’Etudes Nucleaires de Bordeaux Gradignan, UMR5797, F-33170 Gradignan, France
4
Aix Marseille Université, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, 13288 Marseille, France
5
Université de Toulon, CNRS/INSU, 83957 La Garde, France
*
Author to whom correspondence should be addressed.
Geosciences 2019, 9(1), 3; https://doi.org/10.3390/geosciences9010003
Received: 31 August 2018 / Revised: 7 December 2018 / Accepted: 13 December 2018 / Published: 20 December 2018
In 2008, as part of a feasibility study for radioactive waste disposal in deep geological formations, the French National Radioactive Waste Management Agency (ANDRA) drilled several boreholes in the transposition zone in order to define the potential variations in the properties of the Callovo–Oxfordian claystone formation. This consisted of a rare opportunity to investigate the deep continental biosphere that is still poorly known. Four rock cores, from 1709, 1804, 1865, and 1935 m below land surface, were collected from Lower and Middle Triassic formations in the Paris Basin (France) to investigate their microbial and geochemical composition. Rock leachates showed high salinities ranging from 100 to 365 g·L−1 NaCl, current temperatures averaging 65 °C, no detectable organic matter, and very fine porosity. Microbial composition was studied using a dual cultural and molecular approach. While the broad-spectrum cultural media that was used to activate microbial communities was unsuccessful, the genetic investigation of the dominant 16S rRNA gene sequences revealed eight bacterial genera considered as truly indigenous to the Triassic cores. Retrieved taxa were affiliated to aerobic and facultative anaerobic taxon, mostly unknown to grow in very saline media, except for one taxon related to Halomonas. They included Firmicutes and α-, β-, and γ-Proteobacteria members that are known from many subsurface environments and deep terrestrial and marine ecosystems. As suggested by geochemical analyses of rocks and rock leachates, part of the indigenous bacterial community may originate from a cold paleo-recharge of the Trias aquifer with water originating from ice melting. Thus, retrieved DNA would be fossil DNA. As previously put forward to explain the lack of evidence of microbial life in deep sandstone, another hypothesis is a possible paleo-sterilisation that is based on the poly-extremophilic character of the confined Triassic sandstones, which present high salinity and temperature. View Full-Text
Keywords: subsurface; Trias sandstone; microbial communities subsurface; Trias sandstone; microbial communities
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Leblanc, V.; Hellal, J.; Fardeau, M.-L.; Khelaifia, S.; Sergeant, C.; Garrido, F.; Ollivier, B.; Joulian, C. Microbial and Geochemical Investigation down to 2000 m Deep Triassic Rock (Meuse/Haute Marne, France). Geosciences 2019, 9, 3.

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