Algal Betaine Triggers Bacterial Hydrogen Peroxide (H ₂ O ₂ ) Production that Promotes Algal Demise

Hydrogen peroxide (H ₂ O ₂ ) 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 H ₂ O ₂ , its role in microbial interactions remains unclear. Here, we used transcriptomics, genetics, and metabolomics to study H ₂ O ₂ dynamics in the interaction between Emiliania huxleyi algae and Phaeobacter inhibens bacteria. We found that H ₂ O ₂ levels rise during algal death and that bacterial H ₂ O ₂ production triggers this demise. Manipulating H ₂ O ₂ levels shifted the outcome of the interaction. We also uncovered a link between H ₂ O ₂ and betaine metabolism: aging algae release betaine, which promotes bacterial H ₂ O ₂ production and, in turn, accelerates algal death. Genes involved in H ₂ O ₂ and betaine metabolism were upregulated in environmental samples from an algal bloom. Together, our findings identify H ₂ O ₂ and betaine as key molecules that modulate algal-bacterial interactions, potentially impacting microbial dynamics in marine ecosystems.