Adaption mechanism and ecological role of CPR bacteria in brackish-saline groundwater

Candidate phyla radiation (CPR) constitutes a substantial fraction of bacterial diversity, yet their survival strategies and biogeochemical roles in brackish-saline groundwater remain unknown. By reconstructing 399 CPR metagenome-assembled genomes (MAGs) and 2007 non-CPR MAGs, we found that CPR, affiliated with 44 previously proposed phyla and 8 putative novel phyla, played crucial roles in maintaining the microbial stability and complexity in groundwater. Metabolic reconstructions revealed that CPR participated in diverse processes, including carbon, nitrogen, and sulfur cycles. Adaption of CPR to high-salinity conditions could be attributed to abundant genes associated with heat shock proteins, osmoprotectants, and sulfur reduction, as well as their cooperation with Co-CPR (non-CPR bacteria co-occurred with CPR) for metabolic support and resource exchange. Our study enhanced the understanding of CPR biodiversity in high-salinity groundwater, highlighting the collaborative roles of self-adaptive CPR bacteria and their reciprocal partners in coping with salinity stress, maintaining ecological stability, and mediating biogeochemical cycling.