Abstract
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.
Highlights
Over the past decades, many fundamental discoveries, especially the hyperthermophily concept, have enabled us to expand life boundaries, with prokaryotic life occurring up to ◦ C in Geosciences 2019, 9, 3; doi:10.3390/geosciences9010003 www.mdpi.com/journal/geosciencesGeosciences 2019, 9, 3 deep-sea hydrothermal vents [1] and evidence of deep-sea chemolithoautotrophic methanogenic cell proliferation and activity at ◦ C when grown at high hydrostatic pressure [2]
Detrital fraction consisted of rounded grains of dominant monocrystalline quartz with minor polycrystalline quartz, K-feldspar, and lithic clasts
Detrital fraction consisted of rounded grains of dominant monocrystalline quartz with minor polycrystalline quartz and K-feldspar
Summary
Many fundamental discoveries, especially the hyperthermophily concept, have enabled us to expand life boundaries, with prokaryotic life occurring up to ◦ C in Geosciences 2019, 9, 3; doi:10.3390/geosciences9010003 www.mdpi.com/journal/geosciencesGeosciences 2019, 9, 3 deep-sea hydrothermal vents [1] and evidence of deep-sea chemolithoautotrophic methanogenic cell proliferation and activity at ◦ C when grown at high hydrostatic pressure [2]. The combination of hydrostatic pressure, temperature, salinity, and anoxia are major subsurface parameters that drive life boundaries, but hydrogeological and geochemical aspects should be considered. They include rock porosity and permeability [8], water activity [9], electron acceptors, energy and carbon source availability [3,10], or pH values, which are strongly influenced by dissolved gas in the deep subsurface [11]. The liquid/solid ratio decreases with depth, granting only little space for life in ultra-deep environments [12] In this respect, a decrease of the microbial biomass has been observed with depth in numerous studies [13,14]. Biomass in the continental subsurface, estimated at 2 to 6 × 1029 cells, account for a significant fraction of the total prokaryote biomass on Earth [7]
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