Abstract
Abstract. Scientific deep drilling of the Koyna pilot borehole into the continental crust up to a depth of 3000 m below the surface at the Deccan Traps, India, provided a unique opportunity to explore microbial life within the deep granitic bedrock of the Archaean Eon. Microbial communities of the returned drilling fluid (fluid returned to the mud tank from the underground during the drilling operation; designated here as DF) sampled during the drilling operation of the Koyna pilot borehole at a depth range of 1681–2908 metres below the surface (m b.s.) were explored to gain a glimpse of the deep biosphere underneath the continental crust. Change of pH to alkalinity, reduced abundance of Si and Al, but enrichment of Fe, Ca and SO42- in the samples from deeper horizons suggested a gradual infusion of elements or ions from the crystalline bedrock, leading to an observed geochemical shift in the DF. Microbial communities of the DFs from deeper horizons showed progressively increased abundance of Firmicutes, Gammaproteobacteria and Actinobacteria as bacterial taxa and members of Euryarchaeota as the major archaeal taxa. Microbial families, well known to strive in strictly anaerobic and extremophilic environments, (e.g. Thermoanaerobacteraceae, Clostridiaceae, Bacillaceae, Carnobacteriaceae, Ruminococcaceae), increased in the samples obtained at a depth range of 2000 to 2908 m b.s. Phylogenetic analysis of common and unique operational taxonomic units (OTUs) of DF samples indicated signatures of extremophilic and deep subsurface relevant bacterial genera (Mongoliitalea, Hydrogenophaga, Marinilactibacillus, Anoxybacillus, Symbiobacterium, Geosporobacter, Thermoanaerobacter). Thermophilic, obligatory anaerobic sulfate-reducing bacterial taxa known to inhabit the deep subsurface were enriched from DF samples using sulfate as a terminal electron acceptor. This report on the geomicrobiology of the DF obtained during drilling of the deep subsurface of the Deccan Traps showed new opportunities to investigate deep life from terrestrial, granite-rock-hosted habitats.
Highlights
The earth’s deep continental crust was found to be of geologically varied morphology with extreme conditions which made it almost impossible for life to survive (Fredrickson and Balkwill, 2006; Colwell and D’Hondt, 2013)
The deep subsurface is among the five main “big habitats” of bacteria and archaea containing about 3 × 1029 cells (Flemming and Wuertz, 2019)
Our results indicated that there was a change in the microbial community structure of DF samples on increasing borehole depth
Summary
The earth’s deep continental crust was found to be of geologically varied morphology with extreme conditions (temperature, pressure, pH etc.) which made it almost impossible for life to survive (Fredrickson and Balkwill, 2006; Colwell and D’Hondt, 2013). For acquiring the sample of more than 300 m depth (especially from crystalline – granite and basalt – rock) a rotary drilling technique has been used which requires the drilling fluid to facilitate the drilling process (Keift et al, 2007; Onstott et al, 1998). Drilling fluid prepared by mixing multiple prepackaged powdered components, including bentonite and a variety of organic constituents with water to lubricate and cool the drill bit, maintain the hydrostatic pressure during the drilling operations so that intact cores can be retrieved and carry the rock cuttings back to the surface (Zhang et al, 2006; Keift, 2010). Specific advantages of bentonite-based fluids included protection of the drilling tools from corrosion, reduced fluid and gas penetration (Gandhi and Sarkar, 2003), and owing to its characteristic “thixotrophic reaction” it hinders the rock material from dropping back into the drilled shaft ( ̇Isçi and Turutoglu, 2011)
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