Complex interactions between Planctomycetota and their viruses in coastal marine sediments

Planctomycetota are widely distributed bacteria that play important roles in global element cycles, yet the diversity and ecological functions of their viruses remain poorly understood. Here, we recovered 326 Planctomycetota metagenome-assembled genomes (MAGs) and 96 associated viruses from coastal sediments of the Bohai Sea. These MAGs expand Planctomycetota diversity to 17 classes, including one novel class (currently assigned as class JAKEIC01 in GTDB Release 232) and 13 previously proposed classes. Most associated viruses belonged to Caudovirales , while 41 could not be assigned to known viral groups. Viral genomes encoded auxiliary metabolic genes involved in carbon and sulfur cycling. Analysis of host-virus interactions revealed diverse bacterial defense systems, including restriction-modification, abortive infection, and CRISPR-Cas. We identified three previously uncharacterized protein families associated with known phage resistance strategies, and their expression conferred antiviral resistance or enhanced neighboring defenses. Viral counter-defense strategies were also detected, indicating active host-virus interactions. Together, these findings provide new insights into Planctomycetota-virus interactions and bacterial immunity in marine sediments.