Archaeal Communities in Deep Terrestrial Subsurface Underneath the Deccan Traps, India

Area of Science:

• Deep biosphere microbiology
• Extremophile archaeal ecology
• Igneous rock geochemistry

Background:

• The deep, aphotic, oligotrophic, and hot igneous crust represents a largely unexplored habitat.
• Understanding microbial life in such extreme environments is crucial for comprehending Earth's biogeochemical cycles.
• Archaeal communities are key players in subsurface ecosystems, but their roles in deep igneous provinces remain poorly understood.

Purpose of the Study:

• To explore archaeal community structure and potential functions within ancient basalt and granitic basement rocks.
• To investigate the influence of rock geochemistry and physical constraints on archaeal colonization in deep igneous provinces.
• To elucidate the potential role of these archaeal communities in biogeochemical cycles and energy flow within the deep biosphere.

Main Methods:

• Archaeal 16S rRNA gene amplicon sequencing was performed on environmental DNA extracted from rock core samples.
• Rock core samples were collected from three distinct horizons: basaltic (BS), transition (TZ), and granitic (GR).
• Quantitative PCR was used to estimate archaeal cell abundance per gram of rock.

Main Results:

• Archaeal communities in basaltic (BS) and granitic (GR) horizons were distinct, with no common operational taxonomic units (OTUs) found across samples, indicating limited dispersal.
• BS rocks, richer in organic carbon and iron(III) oxide (Fe₂O₃), harbored Euryarchaeota, Thaumarchaeota, and Crenarchaeota, including sulfur-respiring and iron-oxidizing taxa.
• GR horizon showed higher proportions of Thaumarchaeota and specific Euryarchaeota affiliated with methanogens and methane oxidizers, suggesting niche-specific adaptations to organic carbon-lean conditions.

Conclusions:

• Rock geochemistry, particularly organic carbon, Fe₂O₃ availability, and physical constraints (temperature, pressure), significantly shapes niche-specific colonization of extremophilic archaeal communities.
• Diverse extremophilic archaeal groups, capable of sulfur, iron, and methane metabolism, form intricate associations within the deep igneous crust.
• These findings provide new insights into the potential role of archaea in biogeochemical cycles and energy flow within the deep biosphere.