Long-term CRISPR array dynamics and stable host-virus co-existence in subsurface fractured shales

Viruses are the most ubiquitous biological entities on earth. Even so, elucidating the impact of viruses on microbial communities and associated ecosystem processes often requires identification of strong host-virus linkages - an undeniable challenge in many ecosystems. Subsurface fractured shales present a unique opportunity to first make these strong linkages and subsequently reveal complex long-term host-virus dynamics and trends in CRISPR array size and frequency. Here, we sampled two replicated sets of fractured shale wells for nearly 800 days (Denver-Julesburg Basin, Colorado, USA). We identified a relatively diverse microbial community with widely encoded CRISPR viral defense systems, which facilitated 2,110 CRISPR-based viral linkages across 90 host MAGs representing 25 phyla. Leveraging these linkages with timeseries data across differing well ages, we observed how patterns of host-virus co-existence develop and converge in this closed ecosystem through time. We observed a transition to smaller CRISPR arrays in older, more established wells, potentially reflecting a natural progression where CRISPR arrays harbor fewer, yet more effective spacers that target viral genes with fewer mutations. Together, our findings shed light on the complexities of host-virus temporal dynamics as well as possible drivers of spacer loss and acquisition within CRISPR arrays of diverse microbial populations through time.