Development of a free radical scavenging probiotic to mitigate coral bleaching

Corals are colonized by symbiotic microorganisms that exert a profound influence on the animals health. One noted symbiont is a single-celled alga (from the family Symbiodiniaceae), which provides the coral with most of its fixed carbon. During thermal stress, hyperactivity of photosynthesis results in a toxic accumulation of reactive oxygen species (ROS). If not scavenged by the antioxidant network, ROS may trigger a signaling cascade ending with the coral host and algal symbiont disassociating; this process is known as bleaching. Our goal was to construct a probiotic comprised of host-associated bacteria able to neutralize free radicals such as ROS. Using the coral model, the anemone Exaiptasia diaphana, and pure bacterial cultures isolated from the model animal, we identified six strains with high free radical scavenging ability belonging to the families Alteromonadaceae, Rhodobacteraceae, Flavobacteriaceae, and Micrococcaceae. In parallel, we established a "negative" probiotic consisting of genetically related strains with poor free radical scavenging capacities. From their whole genome sequences, we explored genes of interest that may contribute to their potential beneficial roles, which may help facilitate the therapeutic application of a bacterial probiotic. In particular, the occurrence of key pathways that are known to influence ROS in each of the strains has been inferred from the genomes sequences and are reported here. IMPORTANCECoral bleaching is tightly linked to the production of reactive oxygen species (ROS), which accumulates to a toxic level in algae-harboring host cells leading to coral-algal dissociation. Interventions targeting ROS accumulation, such as the application of exogenous antioxidants, have shown promise for maintaining the coral-algal partnership. With the feasibility of administering antioxidants directly to corals being low, we aim to develop a probiotic to neutralize toxic ROS during a thermal stress event. This probiotic can be tested with corals or a coral model to assess its efficacy in improving coral resistance to environmental stress.