Alteromonas macleodii breaches the diatom frustule triggering protoplast emergence and programmed necrosis-like death...

Diatom-bacteria interactions drive biogeochemical fluxes at the base of the marine food-web. Although bacteria are increasingly recognised as important drivers of algal mortality, the diversity of algicidal strategies remains understudied. Here we show that Alteromonas macleodii, a recurrent member of Thalassiosira rotula's microbiome, breaches the diatom's silica frustule, inducing >95% of cells to emerge from their shells within 72 h, exposing protoplasts to lysis and exploitation. Whilst protoplast formation was induced by diverse bacteria, A. macleodii uniquely caused population-wide collapse, used here to characterise this diatom stress response. Dual transcriptomics indicated the presence of a programmed necrosis-like cell death pathway in diatoms underpinning T. rotula's terminal morphological change, alongside upregulation of bacterial motility, chemotaxis and opportunistic metabolism. These findings reveal a potent algicidal strategy, recasting A. macleodii as an antagonist that exploits diatom mortality for its own benefit, removes the ballast provided by their frustules and thus reshapes oceanic carbon cycling.