Combining tangential flow filtration and size fractionation of mesocosm water as a method for the investigation of waterborne coral diseases

Investigating microbial size classes associated with the transmission of stony coral tissue loss disease (SCTLD)

Effective treatment and prevention of any disease necessitates knowledge of the causative agent, yet the causative agents of most coral diseases remain unknown, in part due to the difficulty of distinguishing the pathogenic microbe(s) among the complex microbial backdrop of coral hosts. Stony coral tissue loss disease (SCTLD) is a particularly destructive disease of unknown etiology, capable of transmitting through the water column and killing entire colonies within a matter of weeks. Here we used a previously described method to (i) isolate diseased and apparently healthy coral colonies within individual mesocosms containing filtered seawater with low microbial background levels; (ii) incubate for several days to enrich the water with coral-shed microbes; (iii) use tangential-flow filtration to concentrate the microbial community in the mesocosm water; and then (iv) filter the resulting concentrate through a sequential series of different pore-sized filters. To investigate the size class of microorganism(s) associated with SCTLD transmission, we used 0.8 µm pore size filters to capture microeukaryotes and expelled zooxanthellae, 0.22 µm pore size filters to capture bacteria and large viruses, and 0.025 µm pore size filters to capture smaller viruses. In an attempt to further refine which size fraction(s) contained the transmissible element of SCTLD, we then applied these filters to healthy "receiver" coral fragments and monitored them for the onset of SCTLD signs over three separate experimental runs. However, several factors outside of our control confounded the transmission results, rendering them inconclusive. As the bulk of prior studies of SCTLD in coral tissues have primarily investigated the associated bacterial community, we chose to characterize the prokaryotic community associated with all mesocosm 0.22 µm pore size filters using Illumina sequencing of the V4 region of the 16S rRNA gene. We identified overlaps with prior SCTLD studies, including the presence of numerous previously identified SCTLD bioindicators within our mesocosms. The identification in our mesocosms of specific bacterial amplicon sequence variants that also appear across prior studies spanning different collection years, geographic regions, source material, and coral species, suggests that bacteria may play some role in the disease.

A meta-analysis of the stony coral tissue loss disease microbiome finds key bacteria in unaffected and lesion tissue in diseased colonies

Stony coral tissue loss disease (SCTLD) has been causing significant whole colony mortality on reefs in Florida and the Caribbean. The cause of SCTLD remains unknown, with the limited concurrence of SCTLD-associated bacteria among studies. We conducted a meta-analysis of 16S ribosomal RNA gene datasets generated by 16 field and laboratory SCTLD studies to find consistent bacteria associated with SCTLD across disease zones (vulnerable, endemic, and epidemic), coral species, coral compartments (mucus, tissue, and skeleton), and colony health states (apparently healthy colony tissue (AH), and unaffected (DU) and lesion (DL) tissue from diseased colonies). We also evaluated bacteria in seawater and sediment, which may be sources of SCTLD transmission. Although AH colonies in endemic and epidemic zones harbor bacteria associated with SCTLD lesions, and aquaria and field samples had distinct microbial compositions, there were still clear differences in the microbial composition among AH, DU, and DL in the combined dataset. Alpha-diversity between AH and DL was not different; however, DU showed increased alpha-diversity compared to AH, indicating that, prior to lesion formation, corals may undergo a disturbance to the microbiome. This disturbance may be driven by Flavobacteriales, which were especially enriched in DU. In DL, Rhodobacterales and Peptostreptococcales-Tissierellales were prominent in structuring microbial interactions. We also predict an enrichment of an alpha-toxin in DL samples which is typically found in Clostridia. We provide a consensus of SCTLD-associated bacteria prior to and during lesion formation and identify how these taxa vary across studies, coral species, coral compartments, seawater, and sediment.

Transmission of stony coral tissue loss disease (SCTLD) in simulated ballast water confirms the potential for ship-born spread

Stony coral tissue loss disease (SCTLD) remains an unprecedented epizootic disease, representing a substantial threat to the persistence and health of coral reef ecosystems in the Tropical Western Atlantic since its first observation near Miami, Florida in 2014. In addition to transport between adjacent reefs indicative of waterborne pathogen(s) dispersing on ocean currents, it has spread throughout the Caribbean to geographically- and oceanographically-isolated reefs, in a manner suggestive of ship and ballast water transmission. Here we evaluate the potential for waterborne transmission of SCTLD including via simulated ballast water, and test the efficacy of commonly-used UV radiation treatment of ballast water. Two species of reef-building corals (Orbicella faveolata and Pseudodiploria strigosa) were subjected to (1) disease-exposed or UV-treated disease-exposed water, and (2) a ballast hold time series of disease-exposed water in two carefully-controlled experiments to evaluate transmission. Our experiments demonstrated transmission of SCTLD through water, rather than direct contact between diseased and healthy corals. While UV treatment of disease-exposed water led to a 50% reduction in the number of corals exhibiting disease signs in both species, the statistical risk of transmission and volume of water needed to elicit SCTLD lesions remained similar to untreated disease-exposed water. The ballast hold time (24 h vs. 120 h) did not have a significant effect on the onset of visible disease signs for either species, though there appeared to be some evidence of a concentration effect for P. strigosa as lesions were only observed after the 120 h ballast hold time. Results from both experiments suggest that the SCTLD pathogens can persist in both untreated and UV-treated ballast water and remain pathogenic. Ballast water may indeed pose a threat to the continued spread and persistence of SCTLD, warranting further investigation of additional ballast water treatments and pathogen detection methods.