Molecular and cellular insights into the parasitic infection of bloom-forming marine diatoms by Pirsonia diadema

Microbial host–parasite interactions shape global biogeochemical cycles, yet their cellular and evolutionary mechanisms remain poorly understood. Here we developed a model pathosystem to explore the interaction between the eukaryotic microparasite, Pirsonia diadema , and its host, the bloom-forming diatom, Coscinodiscus . Culture conditions recapitulated Pirsonia ’s rapid life cycle in vitro, and live-cell imaging enabled the quantitative analysis of infection dynamics and host mortality. Genome sequencing and time-resolved dual RNA sequencing of Pirsonia revealed unique genetic innovations relative to their oomycete relatives and an upregulation of an expanded multi-gene family of integrin-like proteins in Pirsonia zoospores during infection. Targeted drug perturbations combined with ultrastructure expansion microscopy confirm stage specific roles for Pirsonia ’s cytoskeleton, including actin-dependent host attachment and formation of intracellular parasite-induced feeding structures and microtubule-associated processes in zoospore differentiation. Together, these findings reveal mechanisms underpinning Pirsonia infection and provide insights into the evolution of strategies that underpin parasitic shunts in marine trophic networks.