Algal Betaine Triggers Bacterial Hydrogen Peroxide Production that Promotes Algal Demise

ABSTRACT 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.