Development of alternative in situ treatments for stony coral tissue loss disease

Florida’s coral reefs are currently experiencing a multi-year disease-related mortality event known as stony coral tissue loss disease (SCTLD) that has resulted in massive die- offs in multiple coral species. Over 20 species of coral, including both Endangered Species Act-listed and the primary reef-building species, have displayed tissue loss lesions which often result in whole colony mortality. The best available information indicates that the disease outbreak is continuing to spread into the Dry Tortugas and throughout the Caribbean with devastating consequences to these reefs. We have learned a lot about SCTLD since it was first observed, but many fundamental questions remain about the causes and environmental drivers of disease. We know that antibiotic treatment with amoxicillin can stop many disease lesions from progressing and that coinfections with the pathogen Vibrio coralliilyticus can cause lesions to progress more rapidly, indicating that bacteria can be important in SCTLD etiology. We also know that probiotics have offered an alternative treatment for SCTLD in aquaria trials. Therefore, we have worked to find new probiotic strains from a variety of different coral species to increase the likelihood of slowing or stopping SCTLD along the reef. In the past few years, we have isolated over 1,000 new diverse bacterial strains from multiple coral species, approximately 200 of which are promising candidates that inhibit potential bacterial pathogens and could be tested on corals to determine their success as probiotics. Further, we have tested several of these new strains on diseased corals in aquaria trials, advancing our investigation of the strains that are successful. After testing in aquaria at the Smithsonian Marine Station, we have brought two of these strains onto Florida’s Coral Reef where we have developed two methods to apply the probiotic bacteria to corals. Our probiotic bagging treatment appears to be the most successful by slowing the advancement of the disease on corals where the disease is progressing. This study follows up on these past efforts to test the effectiveness of the probiotic bag method directly with antibiotic paste treatment and untreated controls in the Florida Keys. Additionally, we examined whether a combined antibiotic plus probiotic treatment was more effective than antibiotics alone on difficult to treat corals.