Determining the Contributions of Microorganisms to H2O2 Degradation and its Implications for the Success of Prochlorococcus in the Oligotrophic Ocean

Cyanobacteria of the genus Prochlorococcus are one of the most abundant microorganisms throughout the oligotrophic ocean. Prochlorococcus, while being highly adapted to thrive in nutrient limited marine environments, are completely dependent upon other members of the microbial community for the detoxification of radical oxygen species (ROS), specifically hydrogen peroxide (H₂O₂). Prior research has demonstrated the ability of certain heterotrophic “helper” organisms to benefit the growth of Prochlorococcus both in the natural environment and in laboratory culture by mitigating concentrations of H₂O₂ in their surrounding environment. Organisms that perform this function are crucial to the success of Prochlorococcus, as H₂O₂ generated daily in marine surface waters reaches concentrations lethal to the cyanobacterium. While this “helping” phenotype has been observed with a small number of heterotrophic organisms, the diversity of microorganisms able to carry out this function in the oligotrophic ocean remains largely unknown. In order to address this knowledge gap, we performed both mono- and coculture experiments to determine the ability of strains of the marine Synechococcus, picoeukaryotic phytoplankton, and oligotrophic heterotrophs to degrade ecologically relevant concentrations of H₂O₂ and provide a benefit to Prochlorococcus. H₂O₂ exposure was carried out by either direct addition or artificial production by HEPES buffer over a diel. These laboratory studies improve our understanding of the contributions of microorganisms participating in H₂O₂ degradation and highlight the interdependencies of marine microbial communities.