Groundwater bacterial communities evolve over time, exhibiting cyclic similarity patterns in response to annual recharge events. Time-series microbiome analysis

The subsurface microbiome is far more diverse than what we thought, playing a crucial role in maintaining the ecological functions in the subsurface. Changes in groundwater chemistry have a deterministic influence on the subsurface microbiome, e.g., through surface recharge events. Yet, how the subsurface microbiome forms and evolves in pristine aquifers through time and space are rarely studied. In this study, groundwater samples were collected from three wells of the Hainich transect on a 4-week basis for 6 years (February 2013 - May 2019), allowing us to explore the dynamics of bacterial communities that corresponded to the temporal changes in the groundwater geochemistry. Using 16S rRNA gene amplicon sequencing, we found that groundwater bacterial communities were not stable, but evolved over time. Groundwater bacterial community shifts coincided with the annual recharge events. The water level difference determines the degree of the changes in bacterial community composition and diversity.