Reef-building corals are acutely threatened by ocean warming which calls for active interventions that reduce further coral bleaching and mortality. Corals associate with a wide diversity of bacteria which exert a profound influence on coral health. Early studies have therefore suggested coral probiotics to enhance coral resilience to bleaching. Knowledge of specific bacterial functions that may be beneficial for corals under thermal stress is scant. However, given that the accumulation of toxic reactive oxygen species (ROS) and reactive nitrogen species (RNS) are key drivers of bleaching, ROS and/or RNS neutralization may be important bacterial traits.
To identify potential coral probiotic bacteria with ROS- and RNS-scavenging properties, we firstly determined the bacterial community structure associated with the coral Galaxea fascicularis by 16S rRNA gene metabarcoding. Pure cultured representatives of the bacterial community were isolated and their sequenced genomes were screened for ROS- and RNS-scavenging functions. We isolated 91 bacterial strains and sequenced and assembled the genomes of 82 strains affiliated with 37 genera. The genome data contained a diverse range of genes encoding different ROS-scavenging functions including the production of the antioxidants mannitol, glutathione or zeaxanthine. Isolates containing such genes belonged to the genera Ruegeria, Muricauda and Roseibium, which have been suggested to possess ROS-scavenging properties beyond coral-associated contexts. For other isolates, e.g., Tenacibaculum, the discovery of ROS-scavenging genes is novel. Genes encoding functions involved in RNS-scavenging were found in many G. fascicularis-associated bacteria as well, and for most genera (e.g. Leisingera) this is novel. We are currently phenotypically determining the ROS- and RNS-scavenging properties of the isolates. Our work provides important insights into the ROS- and RNS-scavenging potential of coral-associated bacteria which may be beneficial for their hosts to withstand coral bleaching. Identifying and characterising bacteria that are beneficial for corals is critical for the development of successful probiotics that enable long-term coral climate resilience.