Breaching the diatom frustule: Alteromonas macleodii triggers protoplast emergence and programmed necrotic-like cell death in the oceanic diatom Thalassiosira rotula

Diatom-bacteria interactions drive biogeochemical fluxes at the base of the marine food-web. While the role of bacteria in recycling organic matter is well established, their potential to actively induce algal death remains poorly understood. Here we examined the interaction between the bloom-forming diatom Thalassiosira rotula and a known member of its microbiome, Alteromonas macleodii. Microscopy revealed that A. macleodii breached the diatoms silica frustule, inducing >95% of diatom cells to emerge from their shells within 72 hours, exposing protoplasts to lysis and exploitation. This algicidal interaction is species-specific and contact-dependent. Dual transcriptomics revealed programmed necrotic-like cell death underpins T. rotulas terminal morphological change, alongside bacterial upregulation of motility, chemotaxis, and coordinated metabolism to exploit the diatom host. These findings indicate the presence of a programmed necrosis-like pathway in diatoms, akin to that known in mammals. They also reveal a novel algicidal strategy, recasting A. macleodii as a specialized antagonist capable of influencing bloom termination and removing the ballast provided by diatom frustules, thereby reshaping carbon cycling in the ocean.