Deep Microbial Colonization in 2-Billion-Year-Old Ultramafic Rock from the Bushveld Complex

Abstract

Archean cratons may provide stable microbial habitats in the deep subsurface, as evidenced by the discovery of billion-year-old crustal fluids1,2. However, the long-term habitability of these cratonic environments is uncertain, as polymetamorphic evolution in most cratons typically destroys microbial habitats through mineral reactions and porosity loss3,4. Preservation of deep microbial habitats is more likely where mantle-derived magma intruded the craton after metamorphic overprinting4. Here we report the discovery of dense microbial colonization at 814 m depth within the 2.05-billion-year-old, unmetamorphosed Bushveld Igneous Complex intrusion, South Africa5. Using advanced contamination-control protocols6,7 and synchrotron-based X-ray spectroscopy, we identified indigenous microbial cells localized at the rims of phlogopite, a hydrous phyllosilicate mineral. Our study reveals that microbial colonization is associated with Fe(III) derived from the structure of phlogopite, where the dehydrogenation likely oxidizes Fe(II) to Fe(III) coupled to H2 generation8. Despite the absence of fracture-driven fluid ingress in the unfractured rock matrix, aqueous alteration evidenced at the rims by potassium removal indicates a self-sustaining habitat driven by an internal redox gradient9. These findings demonstrate that aqueous alteration of ultramafic rocks can sustain isolated microbial life over geological timescales, significantly expanding the potential for long-term habitability on both Earth and Mars4,10.