Algal Betaine Triggers Bacterial Hydrogen Peroxide 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. Here, we use transcriptomics, genetics, and metabolomics to study H₂O₂ dynamics in the interaction between Emiliania huxleyi algae and Phaeobacter inhibens bacteria. We find that H₂O₂ levels rise during algal death and that bacterial H₂O₂ production triggers this demise. In co-cultures, but not in axenic algal cultures, chemically reducing H₂O₂ levels prevents algal death, whereas chemically increasing H₂O₂ levels accelerates algal death. We also uncover a link between H₂O₂ and betaine metabolism: aging algae release betaine, which promotes bacterial H₂O₂ production that contributes to algal death. In environmental samples, bacterial genes involved in H₂O₂ and betaine metabolism are highly expressed towards the demise of a natural algal bloom. Together, our findings identify H₂O₂ and betaine as key molecules that modulate algal-bacterial interactions.