Selenite reduces algal reactive oxygen species (ROS) accumulation and enhances algal resistance to a bacterial pathogen

Oxidative stress arises when cells fail to maintain redox balance. Among marine microalgae, oxidative stress is a hallmark of interaction with pathogenic bacteria. In co-culture of Emiliania huxleyi algae with pathogenic bacteria of the species Phaeobacter inhibens, algal intracellular levels of reactive oxygen species (ROS) increase prior to bacterial-induced population collapse. Here, we tested whether antioxidant amendments can alter the pathogenic outcome. Screening several environmentally relevant antioxidants revealed that low-nanomolar levels of selenite (Se (IV)) prevented algal death in co-culture. Time-resolved ROS measurements showed that selenite-treated co-cultures maintained lower intracellular ROS levels during exponential growth, while bacterial growth dynamics were comparable with and without selenite. Together, these results show that selenite availability shifts the outcome of the E. huxleyi-P. inhibens interaction and link host oxidative stress to resilience in co-culture.