Algal Betaine Triggers Bacterial Hydrogen Peroxide (H₂O₂) Production that Promotes Algal Demise

Hydrogen peroxide (H₂O₂) plays various roles in the ocean, acting as a signaling molecule at low concentrations and causing oxidative stress when accumulated. While many marine microbes produce H₂O₂, its role in microbial interactions remains unclear. Here, we used transcriptomics, genetics, and metabolomics to study H₂O₂dynamics in the interaction betweenEmiliania huxleyialgae andPhaeobacter inhibensbacteria. We found that H₂O₂levels rise during algal death and that bacterial H₂O₂production triggers this demise. Manipulating H₂O₂levels shifted the outcome of the interaction. We also uncovered a link between H₂O₂and betaine metabolism: aging algae release betaine, which promotes bacterial H₂O₂production and, in turn, accelerates algal death. Genes involved in H₂O₂and betaine metabolism were upregulated in environmental samples from an algal bloom. Together, our findings identify H₂O₂and betaine as key molecules that modulate algal-bacterial interactions, potentially impacting microbial dynamics in marine ecosystems.