“Subsurface Microbial Colonization at Mineral-Filled Veins in 2-Billion-Year-Old Mafic Rock from the Bushveld Igneous Complex, South Africa,” focuses on the examination of microbial life in ancient igneous rocks from the Bushveld Igneous Complex in South Africa. The study investigates the possibility of long-term microbial survival in stable subsurface environments over geological timescales.

The research team, led by Yohey Suzuki, selected the Bushveld Igneous Complex due to its age (2.05 billion years old), minimal geological alteration, and location within the tectonically stable Kaapvaal Craton. These factors suggest that the complex could provide a stable habitat for microbial life over an extended period.

The researchers developed new procedures to simultaneously detect indigenous and contaminant microbial cells in drill core samples. This involved precision rock sectioning, coupled with infrared, fluorescence, and electron microscopy imaging of the rock section with submicron resolution. The analysis of the rock samples revealed evidence of microbial colonization in veins filled with clay minerals.

The presence of clay minerals in the veins is significant because clay minerals can provide energy sources for microbial life. The formation of clay minerals through the interaction of water with iron in the rocks can produce hydrogen, a potential energy source for certain microbes. Furthermore, clay minerals can adsorb organic matter, which could also serve as a food source for microbes.

The researchers concluded that the tight packing of clay minerals within the veins limited the entry and exit of microbial cells, effectively isolating them from the surrounding environment. This isolation, coupled with the potential energy sources provided by the clay minerals, could have enabled the long-term survival of these microbes over billions of years.
Methodology.

The Potential Lifespan of Microbes in Ancient Rocks

The suggestion that these organism are of ancient origin and possibly present since the Archaen raise some obvious questions about the life cycle of such organism and their life span. The presence of clay minerals is important because they can provide energy sources for microbial life. Additionally, the tight packing of clay minerals within the veins could limit the entry and exit of microbial cells, isolating them from the surrounding environment and contributing to their long-term survival.

While the specific lifespan of these microbes is not explicitly stated by the authors, several factors suggest the possibility of extremely long lifespans.

• Metabolic Rates: Subsurface microbes typically exhibit exceedingly slow metabolic rates, existing in a “survival mode”. This slow metabolism can lead to estimated turnover times ranging from thousands to millions of years.
• Minimal Evolution: Some deep subsurface microbial lineages have demonstrated minimal evolution over geological timescales. For example, the sulfate-reducing bacteria Candidatus Desulforudis audaxviator found in deep subsurface environments has shown minimal evolution since 55–165 million years ago.
• Stable Environments: Geologically and tectonically stable subsurface environments, like the Bushveld Igneous Complex, can provide long-term habitat stability, potentially sustaining microbial life for billions of years.

The combination of slow metabolic rates, minimal evolution, and stable habitats creates conditions that could support the persistence of microbial life over extremely long periods. While further research is necessary to determine the actual lifespan of these specific microbes, the authors present compelling evidence suggesting the potential for microbial survival over geological timescales.

Methodology

The methodology of this study incorporated various techniques for sample collection, contamination control, and microbial detection. Here is a breakdown of the key steps involved:

Drilling and On-Site Core Handling:

• Rotary core barrel (RCB) drilling was conducted using a diamond drill bit.
• The drilling fluid used was locally sourced water mixed with a chemical additive (AMC CAP 21) and fluorescent microspheres (DayGlo Color Corp., pigment SPL-594NXC).
• The fluorescent microspheres, with blue fluorescence under ultraviolet (UV) excitation and a size range of 0.25 to 0.45 μm, were added to the drilling fluid to monitor contamination.
• After retrieval, the core sample was cleaned with filtered water, lightly flamed to reduce contamination, and fractured to expose the interior.

Visualization of Fluorescence Microspheres:

• The rock core fragments and drilling fluid were examined under UV light to detect the blue fluorescence of the microspheres.
• A precision diamond band saw was used to cut a section of the rock, perpendicular to the fracture, for microscopic observation of microsphere penetration.

Optical-Photothermal Infrared (O-PTIR) Spectroscopy:

• A 3-mm-thick rock section was analyzed using O-PTIR spectroscopy with a spatial resolution of 0.5 μm.
• The technique utilized a 532-nm laser probe beam and a tunable quantum cascade laser (QCL) pump beam to obtain intensity maps and spectra over the mid-IR range.
• Intensity maps were generated at specific wave numbers (1000, 1530, and 1640 cm−1) to identify the presence of silicate minerals and microbial cells.

Visualization of Microbial Cells:

• The drilling fluid sample was fixed, stained with SYBR Green I, and filtered for fluorescence microscopy observation of microbial cells.
• The rock section, after O-PTIR analysis, was fixed with formaldehyde, stained with SYBR Green I, and observed under a fluorescence microscope to visualize microbial cells.

Scanning Electron Microscopic (SEM) Characterization:

• The thin section was examined using SEM–EDS (energy-dispersive X-ray spectrometer) to determine the elemental composition of the vein-fill minerals.
• The accelerating voltage for SEM was 20 kV.

This multi-faceted approach enabled the researchers to assess contamination levels, identify microbial cells, and characterize the mineralogy of the veins within the ancient rock samples.