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.
Shaping of the Present-Day Deep Biosphere at Chicxulub by the Impact Catastrophe That Ended the Cretaceous