Targeted culturing of putative pathogens associated with stony coral tissue loss disease

Stony coral tissue loss disease (SCTLD) has impacted Florida’s Coral Reef for nearly a decade. Despite years of research, a causative agent has not been identified. Identification of potential pathogens requires cultivation in the lab to demonstrate damage to the host. Previous attempts to cultivate potential SCTLD pathogens have not produced obvious or consistent pathogenic strains. We propose that anaerobic bacteria may be an overlooked source of potential pathogens in coral disease. Characterization of coral microbiomes through both taxonomic and functional surveys have implicated bacteria in the class Clostridia as potential pathogens in SCTLD. Clostridia would have been missed with standard cultivation methods because they are strict anaerobes, growing only in the absence of oxygen. In the coral host, Clostridia may inhabit anoxic niches deep within coral tissues where the disease is thought to begin. Thus, we used a targeted approach to cultivate anaerobic bacteria like Clostridia and to test for virulence in both corals and cultures of Durisdinium algal symbionts. We isolated and sequenced the genomes of more than 20 anaerobic strains from SCTLD-affected corals, including Orbicella faveolata exhibiting fast lesion progression. We identified four strains of Clostridia from the genera Wukongibacter and Vallitalea. Anaerobic bacteria were tested for virulence against both corals and cultures of the algal symbiont Durisdinium, but no signs of disease were observed. While we were not able to initiate disease with these strains under the conditions tested, ongoing analysis of the genome content of these bacteria may reveal clues about their potential virulence and aid our understanding of the disease.