LAUSR

Significance Sheltered from radiation and asteroid strikes in Earth’s early history and home to a significant portion of today’s prokaryotic biomass, the deep subsurface could hold keys to understanding the early and continuing evolution of life. However, the processes that shape the distribution of subsurface microbial communities remain poorly understood due to sample inaccessibility. Here, at a deep-underground fractured hard-rock aquifer, we show that fracture activity leads to altered groundwater flow that drives profound changes in fluid-associated microbial communities by physical transport instead of environmental selection. We thereby identify advection induced by geological activity (a notable trigger for fracture activity) as a prominent yet overlooked mechanism shaping subsurface biogeography with potentially profound implications for life’s evolutionary history.