# Minimal changes in microbial abundances and diversity over 7 years of emplacement for modules of compacted bentonite exposed to natural groundwater

## TL;DR

This study shows that microbial communities in bentonite samples remained stable over 7 years in deep geological repository-like conditions.

## Contribution

The study provides 7-year empirical data on microbial stability in bentonite under DGR-like subsurface conditions.

## Key findings

- Microbial abundances in inner bentonite layers showed no significant changes over 7 years.
- Potential changes in microbial community composition were suggested by PLFA data.
- Results support the long-term stability of bentonite-based materials in deep geological repositories.

## Abstract

The multi-national in situ Materials Corrosion Test (MaCoTe), being conducted at the Grimsel Test Site in Switzerland, assesses the stability of bentonite as it may be utilized within deep geological repositories (DGRs), which are proposed for the safe, long-term disposal of used nuclear fuel. This experiment provides an opportunity for long-term assessments of changes in microbial communities associated with compacted subsurface bentonite samples exposed to a natural groundwater. Leveraging samples from MaCoTe, herein, we report temporal data for the abundance and community composition of microorganisms associated with compacted bentonite samples emplaced over 7 years under in situ subsurface conditions. Phospholipid fatty acid analysis, 16S rRNA gene quantification and sequencing, cultivation, and natural organic matter analyses all indicated no significant changes for microbial community abundances associated with inner layers of bentonite samples over the 7 years. While microbial abundances did not change in the inner layers, the PLFA data suggest potential changes in microbial community composition and could also indicate prolonged microbial turnover rates. Overall, the results support microbial stability in compacted bentonite exposed to DGR-like conditions for at least 7 years.

Long-term assessments of changes in microbial activity in compacted low-biomass bentonite systems analogous to deep geological repositories (DGRs) are critical to test conditions for stable engineered bentonite barrier components. This study assesses long-term temporal changes in microbial communities of compacted bentonites exposed to natural groundwater. It offers 7-year data that indicate stability of bentonite-based materials intended for use in engineered barrier systems of a DGR for the safe, long-term disposal of used nuclear fuel, with wider implications for microbial persistence in a deep subsurface environment.