Subsurface Microbial Colonization at Mineral-Filled Veins in 2-Billion-Year-Old Mafic Rock from the Bushveld Igneous Complex, South Africa

Recent advances in subsurface microbiology have demonstrated the habitability of multi-million-year-old igneous rocks, despite the scarce energy supply from rock-water interactions. Given the minimal evolution coupled with exceedingly slow metabolic rates in subsurface ecosystems, spatiotemporally stable igneous rocks can sustain microbes over geological time scales. This study investigated a 2-billion-year-old mafic rock in the Bushveld Igneous Complex, South Africa, where ultradeep drilling is being executed by the International Continental Scientific Drilling Program (ICDP). New procedures were successfully developed to simultaneously detect indigenous and contaminant microbial cells in a drill core sample. Precision rock sectioning coupled with infrared, fluorescence, and electron microscopy imaging of the rock section with submicron resolution revealed microbial colonization in veins filled with clay minerals. The entry and exit of microbial cells in the veins are severely limited by tight packing with clay minerals, the formation of which supplies energy sources for long-term habitability. Further microbiological characterization of drilled rock cores from the Bushveld Igneous Complex will expand the understanding of microbial evolution in deep igneous rocks over 2 billion years.

The terrestrial subsurface is defined by depths greater than 8 m from the ground surface, excluding soil [1], where a significant portion of the Earth’s prokaryotic biomass resides [2,3,4]. The metabolic activities of subsurface microbiomes are exceedingly slow under survival mode [5, 6], leading to an estimated turnover time ranging from several thousand to million years. Consistent with the long turnover time [7], sulfate-reducing bacteria Candidatus Desulforudis audaxviator endemic to the deep subsurface have undergone minimal evolution since 55–165 million years ago [8]. Similarly, minimal evolution over geological time scales has been demonstrated for deep subsurface archaeal lineages called Candidatus Altiarchaeota [9]. Based on the ecological and evolutionary features of subsurface microbiomes, it is hypothesized that microbes can be sustained with minimal evolution for billions of years in a geologically and tectonically stable subsurface environment [10].

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