Understanding the drivers of white syndrome coral diseases on Indo-Pacific reefs

Coral tissue loss diseases, collectively known as white syndromes (WS) in the Indo- Pacific, have the potential to significantly reduce coral cover and diversity on reefs globally due to their wide geographic distributions, diverse host ranges and the rapid and irreversible damage they cause. To understand and address this emerging threat, the first step is to untangle the complex interactions among susceptible coral hosts, dynamic ocean environments and poorly understood coral microbiomes that lead to disease. This study combines microbiological, genetic, histopathological and ecological approaches to investigate environmental and anthropogenic drivers of WSs and mechanisms underpinning coral holobiont health over multiple spatial scales (i.e. at reefal, population, colony, microbiome and individual pathogen levels).\ud \ud To assess seasonal dynamics in WS diseases away from major anthropogenic stressors, colonies of Acropora hyacinthus at Lizard Island, northern Great Barrier Reef (GBR) were monitored over an 18-month period. Bacterial community structures were assessed on coral samples collected in summer, winter and spring, and host/microbe interactions and cellular responses were visualized (Chapter 2). A deductive approach to disease causation demonstrated that the WS pathogenesis observed was not the result of apoptosis or infection by Vibrio bacteria, ciliates, fungi, cyanobacteria or helminthes. Next generation bacterial sequencing did, however, reveal distinct and temporally consistent bacterial communities at WS lesions fronts (relative to healthy tissues on both healthy and WS-affected corals), which were characterized by a 15-fold increase in Rhodobacteraceae-affiliated bacterial sequences. This case study helps clarify the etiology of WS at Lizard Island and provides potential diagnostic criteria to differentiate etiologically dissimilar forms of WS.\ud \ud To investigate the influence of stressors associated with recreational offshore platforms on coral hosts and their associated microbial communities, water quality and coral health parameters, including immunity and microbiome structure, were evaluated before, during and after a WS disease event adjacent to permanently moored platforms at Hardy Reef, southern GBR (Chapter 3). Over the course of the 8- month study, 31% of Acropora millepora colonies monitored adjacent to reef platforms developed WS, whereas all conspecific corals at a nearby control site remained visually healthy. Interestingly, two months prior to the first macroscopic observation of WS disease signs, bacterial diversity on corals that would develop WS was significantly reduced and bacterial community heterogeneity was significantly elevated relative to those remaining healthy at the same location. Among corals remaining visually healthy throughout the study, immune function and bacterial diversity were significantly reduced at reef platforms relative to the platform-free control site. These results highlight that stressors associated with recreational platforms suppress coral immunocompetence and alter microbiome structure and diversity, which may increase coral susceptibility to disease.\ud \ud The decapod Cymo melanodactylus, which lives amongst the branches of many hard coral species and is capable of consuming coral tissues, often aggregates at WS lesion fronts. Consequently, coral predation by Cymo crabs has recently been proposed as a direct mechanism of WS tissue loss. Manipulative aquarium-based experiments performed on the coral A. hyacinthus at Lizard Island, northern GBR demonstrated that coral predation by C. melanodactylus is not the direct cause of WS tissue loss (i.e. WS lesion progression continued following crab removal) and that Cymo crabs are not a transmission vector for WSs (i.e. healthy corals inoculated with crabs from WS-infected colonies did not develop disease signs) (Chapter 4). Furthermore, threefold more rapid progression of WS lesions on corals when crabs were removed than on those with crabs left on (2.28 ± 0.21 versus 0.74 ± 0.22 cm/day, respectively) indicate that Cymo crabs slow WS disease progression. In choice experiments, C. melanodactylus were strongly attracted to WS lesions (87% migrated to WS fragments versus 3% to healthy fragments). The strong attraction of C. melanodactylus to WS-infected corals and their ability to significantly reduce lesion progression rates suggest that wound debridement by these coral-dwelling crabs might mitigate the effects of WSs on reefs.\ud \ud While many water quality parameters have been proposed to contribute to reef declines, little evidence exists to conclusively link specific parameters with increased disease prevalence in situ. To assess the relationship between sedimentation, turbidity and coral condition, health assessments were conducted at multiple reefs along a dredging-associated sediment plume gradient near Barrow Island, Western Australia (Chapter 5). Reefs exposed to the highest number of days under the sediment plume (296 to 347 days) had two-fold higher levels of disease, largely driven by a 2.5-fold increase in WS, and six-fold higher levels of other signs of compromised coral health (e.g. bleaching and algal overgrowth) relative to reefs with little or no plume exposure (0 to 9 days). Multivariate modeling and ordination, incorporating sediment exposure level, coral community composition and cover, predation and multiple thermal stress indices, confirmed sediment plume exposure level as the main driver of elevated disease and other signs of compromised coral health.\ud \ud While the bacterium Vibrio coralliilyticus was not identified as the etiological agent of acroporid WS cases in the present study, this bacterium has been implicated as the causative agent of WS cases at sites throughout the Indo-Pacific. To allow specific visualization of interactions between this emerging model coral pathogen and the coral host, a mini-Tn7 transposon delivery system was used to chromosomally label a strain of V. coralliilyticus isolated from a WS lesion with a green fluorescent protein gene (GFP) (Chapter 6). Following GFP insertion, biochemical assays and experimental infection trials confirmed no loss of virulence in the labeled bacterium relative to the wild-type strain. Using epifluorescence video microscopy, GFP-labeled V. coralliilyticus could be reliably distinguished from non-labeled bacteria present on coral surfaces, and the pathogen's interactions with the coral host could be visualized in real time. This new tool can now be employed to better document mechanisms of V. coralliilyticus virulence and host/pathogen infection dynamics.\ud \ud Knowledge derived from this study provides new insights into the drivers of WS coral diseases at multiple spatial scales and, more generally, the impacts of environmental stress on the coral host and its associated microbes. While a specific etiological agent of WS was not identified, this body of work demonstrates the importance of both macroscopic and microscopic diversity in coral health and provides deeper insights into the drivers of coral health and disease. Understanding the complex symbioses supporting the health of reef-building corals and the external factors disrupting them is essential to the successful design and implementation of effective management strategies to protect increasingly threatened coral reef ecosystems