Selenium protects Emiliania huxleyi algae from a bacterial pathogen by modulating oxidative stress

Oxidative stress arises when cells fail to maintain redox balance, leading to the accumulation of reactive oxygen species (ROS) that can damage proteins, lipids, and DNA, causing cells to function poorly or die. In marine algae, oxidative stress is a hallmark of bacterial pathogenicity and usually appears before algae die. During the interaction of Emiliania huxleyi algae with Phaeobacter inhibens pathogenic bacteria, algae experience elevated levels of ROS that precede and likely drive cell death. Here, we tested whether antioxidants could improve algal survival by alleviating oxidative stress. Among several environmentally relevant antioxidants, we found that nanomolar concentrations of the trace metal selenium, in the form of H₂SeO₃, completely prevented algal death in co-cultures with P. inhibens. Measurements over time showed that selenium significantly lowered ROS levels in algal cells, reducing oxidative stress. This effect did not interfere with bacterial growth, suggesting that selenium acts by helping algae, not by harming bacteria. Our findings demonstrate that oxidative stress plays a central role in bacterial pathogenicity towards algae, and that selenium can protect algae by reducing this stress.