Diverse H2O2-scavenging phenotypes of coral-associated bacteria reveal candidates for bleaching mitigation

There is growing interest in manipulating coral-associated bacteria to enhance coral thermal bleaching tolerance. Reactive oxygen species (ROS), particularly hydrogen peroxide (H2O2), are major drivers of coral bleaching, therefore, coral-associated bacteria capable of scavenging H2O2 may help reduce bleaching. Bacterial candidates are commonly selected through genome inference and occasionally phenotypic assays, but how well these approaches predict H2O2-scavenging capacity is unknown. We screened 29 bacterial strains isolated from the scleractinian corals Acropora loripes and Galaxea fascicularis for genome-inferred H2O2-scavenging pathways and in vitro activity using an ABTS radical-scavenging assay, H2O2 tolerance (the highest H2O2 concentration that allows bacterial growth), H2O2 removal from the medium, and qualitative catalase activity. Although all strains encoded at least two complete H2O2-scavenging pathways, genomic prediction did not consistently align with in vitro scavenging activity. Growth under H2O2 exposure was strongly coupled with H2O2 scavenging, but these traits showed no correlation with ABTS inhibition or catalase activity inferred from bubble formation. The top-ranked phenotypes, based on H2O2 tolerance and scavenging, were observed in Roseovarius, Roseibium, Ruegeria, and Muricauda strains, which tolerated 5-50 mM H2O2. These genera are common associates of microalgae, including coral endosymbionts, which are key sources of excess ROS during thermal stress. Among these, Roseovarius and Muricauda also showed high ABTS inhibition (>97%), indicating broader antioxidant capacity that may be advantageous within the complex oxidative environment of thermally stressed corals. Strains combining strong H2O2 scavenging with broad antioxidant activity may therefore represent the most promising candidates for subsequent validation within the coral holobiont.