Abstract
Dark Spot Syndrome (DSS) is one of the most common diseases of boulder corals in the Caribbean. It presents as sunken brown lesions in coral tissue, which can spread quickly over coral colonies. With this study, we tested the hypothesis that similar to other coral diseases, DSS is a dysbiosis characterized by global shifts in the coral microbiome. Because Black Band Disease (BBD) was sometimes found following DSS lesions, we also tested the hypothesis that DSS is a precursor of BBD. To track disease initiation and progression 24 coral colonies were tagged. Of them five Orbicella annularis corals and three O. faveolata corals exhibited DSS lesions at tagging. Microbiota of lesions and apparently healthy tissues from DSS-affected corals over the course of 18 months were collected. Final visual assessment showed that five of eight corals incurred substantial tissue loss while two corals remained stable and one appeared to recover from DSS lesions. Illumina sequencing of the V6 region of bacterial 16S rRNA genes demonstrated no significant differences in bacterial community composition associated with healthy tissue or DSS lesions. The epimicrobiomes of both healthy tissue and DSS lesions contained high relative abundances of Operational Taxonomic Units assigned to Halomonas, an unclassified gammaproteobacterial genus, Moritella, an unclassified Rhodobacteraceae genus, Renibacterium, Pseudomonas, and Acinetobacter. The relative abundance of bacterial taxa was not significantly different between samples when grouped by tissue type (healthy tissue vs. DSS lesion), coral species, collection month, or the overall outcome of DSS-affected corals (substantial tissue loss vs. stable/recovered). Two of the tagged corals with substantial tissue loss also developed BBD during the 18-month sampling period. The bacterial community of the BBD layer was distinct from both healthy tissue and DSS lesions, with high relative abundances of the presumed BBD pathogen Roseofilum reptotaenium and an unclassified Bacteroidales genus, similar to previous results. Roseofilum was detected in all samples from this study, with the highest relative abundance in healthy tissue from DSS-affected corals sampled in August, suggesting that while DSS is not a precursor to BBD, DSS-affected corals are in a weakened state and therefore more susceptible to additional infections.
Highlights
It is clear that many, if not all, animals and plants depend on their microbiota for nutrient acquisition and/or assimilation, responses to environmental stressors, resistance to pathogens, and overall health (Bosch and McFall-Ngai, 2011)
Excluding the two Black Band Disease (BBD) samples, no significant differences were detected in the abundance of bacterial orders, families, or genera when samples were grouped by health condition (Orbicella Dark Spot Syndrome (DSS) lesion or healthy tissue), sample date, or the outcome of the infections (ANOVA with Benjamini–Hochberg False Discovery Rate, q-value filter >0.05)
While DSS has been previously reported in boulder corals (GilAgudelo and Garzon-Ferreira, 2001), this is the first study to characterize the bacterial community associated with DSSaffected Orbicella corals
Summary
It is clear that many, if not all, animals and plants depend on their microbiota for nutrient acquisition and/or assimilation, responses to environmental stressors, resistance to pathogens, and overall health (Bosch and McFall-Ngai, 2011). Field studies report that up to 3% of S. siderea colonies in relatively pristine reefs in the Bahamas and up to 80% of corals in more impacted reefs have been affected by this disease (Cervino et al, 2001; Borger, 2003; Gochfeld et al, 2006; Voss and Richardson, 2006; Porter et al, 2011). A significant (13– 56%) decrease in the number of intracellular algal symbionts (Symbiodinium spp.) and their cell division rates have been reported in DSS-affected Siderastrea and Stephanocoenia coral tissue (Cervino et al, 2001). Despite the high prevalence of DSS in Caribbean corals, total colony mortality due to DSS was generally low; impacted colonies experienced up to 50% net tissue loss (Porter et al, 2011)
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