Biofilm-associated microbial indicators of stony coral tissue loss disease transmissibility across carbonate substrates, sediments, and seawater filters

Stony coral tissue loss disease (SCTLD) has driven unprecedented population declines of Western Atlantic corals for more than a decade, yet the causative agent(s) remains unknown. Efforts to identify consistent SCTLD-associated microbes from coral metagenomes have been challenged by strong spatial and host variability. Here, we describe a series of controlled laboratory disease transmission experiments using passive vectors (reef sediments, coral skeletons, and 0.22µm seawater filters) as modes of disease transport. Disease transmission was successful using coral skeletons and seawater filters, however reef sediments failed to initiate a disease response. We then employed a comparative, genome-resolved approach to identify potential disease reservoirs on the passive fomites. High quality metagenomes were generated, and these metagenomes were used to map reads from a previously published SCTLD metagenome-assembled genome (MAG) coral dataset to identify commonalities between the passive vectors and diseased coral samples. Differentially abundant prokaryotic MAGs correlated to biofilm development on dead skeletons, suggesting saprophytic/opportunistic microbial proliferation. Interestingly, common differentially abundant MAGs identified in this study and the previously published SCTLD coral dataset showed inverse abundance patterns, suggesting context-dependent microbial responses to SCTLD between fomites and coral hosts. The comparative framework used identifies substrate-derived MAGs shared with SCTLD-associated coral samples and provides support that biofilms may play a role in the transmissibility and reservoir potential of SCTLD. Together, the results support a reservoir-to-lesion framework wherein variable, early biofilm-associated taxa may facilitate transmissibility and help explain the high variability in microbial associates reported across SCTLD studies.