Abstract
Scientific deep drilling at Koyna, western India provides a unique opportunity to explore microbial life within deep biosphere hosted by ~65 Myr old Deccan basalt and Archaean granitic basement. Characteristic low organic carbon content, mafic/felsic nature but distinct trend in sulfate and nitrate concentrations demarcates the basaltic and granitic zones as distinct ecological habitats. Quantitative PCR indicates a depth independent distribution of microorganisms predominated by bacteria. Abundance of dsrB and mcrA genes are relatively higher (at least one order of magnitude) in basalt compared to granite. Bacterial communities are dominated by Alpha-, Beta-, Gammaproteobacteria, Actinobacteria and Firmicutes, whereas Euryarchaeota is the major archaeal group. Strong correlation among the abundance of autotrophic and heterotrophic taxa is noted. Bacteria known for nitrite, sulfur and hydrogen oxidation represent the autotrophs. Fermentative, nitrate/sulfate reducing and methane metabolising microorganisms represent the heterotrophs. Lack of shared operational taxonomic units and distinct clustering of major taxa indicate possible community isolation. Shotgun metagenomics corroborate that chemolithoautotrophic assimilation of carbon coupled with fermentation and anaerobic respiration drive this deep biosphere. This first report on the geomicrobiology of the subsurface of Deccan traps provides an unprecedented opportunity to understand microbial composition and function in the terrestrial, igneous rock-hosted, deep biosphere.
Highlights
Microbial life that resides within the deep continental subsurface represents one of the largest and most diverse biospheres on this planet[1]
Based on the tested parameters we find distinct and characteristic geochemical nature of the granitic and basaltic horizons
Variations in oxide concentrations with respect to depth in basalts and granites are consistent with the fact that granites are felsic whereas basalts are mafic in nature
Summary
Microbial life that resides within the deep continental subsurface represents one of the largest and most diverse biospheres on this planet[1]. Endolithic microbial populations in deep igneous provinces have been investigated in basalt hosted oceanic crusts[22,23,24,25,26,27], subsurface crustal environments with high temperature basalts[28], subsurface gabbros and mantle type rock[29,30] and crustal fluids[31] Most of these studies reveal the dominance of bacterial members affiliated to Proteobacteria (Alpha-, Gamma- and Delta- sub divisions), Actinobacteria, Firmicutes, Bacteroidetes and Chloroflexi known for sulfate reduction, nitrate reduction, fermentation, and chemolithotrophic metabolism[25,26,29,32,33,34]. Scientific deep drilling in the Koyna-Warna region of Deccan traps, western India provides a unique opportunity to investigate microbial life within deep terrestrial igneous rocks (basalts and granite-gneiss basement)[39,40]. Rocks appear to be intermixed with both granitic and basaltic compositions
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