Characterisation of White Patch Syndrome, a putative disease affecting corals in the genus Porites on the Great Barrier Reef

Davy, Joanne (2007). Characterisation of White Patch Syndrome, a putative disease affecting corals in the genus Porites on the Great Barrier Reef PhD Thesis, Centre for Marine Studies , University of Queensland.

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Author Davy, Joanne
Thesis Title Characterisation of White Patch Syndrome, a putative disease affecting corals in the genus Porites on the Great Barrier Reef
School, Centre or Institute Centre for Marine Studies
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Professor Ian Hoegh-Guldberg
Abstract/Summary Coral reefs around the globe are increasingly threatened by a number of natural and anthropogenic factors, and coral cover on many reefs has already been greatly reduced due to mortality associated with bleaching events and coral disease outbreaks. Understanding the potential effects of disease is, in most instances, hampered by a lack of knowledge. The pathogens causing the majority of coral diseases have yet to be identified and little is known about the effects of environmental factors on disease initiation and progression. In general, reefs in the Caribbean and Western Atlantic have suffered the greatest losses due to coral disease and the majority of research has focused on these regions. However, reports of disease outbreaks in the Indo-Pacific have increased in recent years and the need to understand the potential risks to reefs in this region is no less important. The aim of this research was to characterise a newly discovered disease-like state affecting corals in the genus Porites at Heron and Wistari reefs, at the southern end of the Great Barrier Reef (GBR). Results of the study revealed that this disease is dissimilar to other coral diseases in several respects. Named White Patch Syndrome (WPS), it manifests as discrete bleached foci on colonies of the common massive corals Porites australiensis, P. lobata and P. lutea. The results of a long-term monitoring programme of tagged corals indicated a discrete and dynamic (yet persistent) pattern of lesion formation that could suggest a chronic, recurring infection, but which importantly was not observed to result in colony mortality. The erratic pattern of lesion progression and regression was not linked to seawater temperature fluctuations, and colonies in close proximity to each other showed markedly different patterns of disease occurrence. Analysis of physiological variables, such as host protein content (i.e. tissue biomass), Symbiodinium density and Symbiodinium photosynthetic capacity was performed to clarify how the host and symbionts differed in the lesion tissue as compared to the healthy tissue surrounding it. Symbiont density was found to be reduced by approximately two thirds in lesion tissue when compared to the visibly healthy tissue, but symbiont cells remaining within WPS lesions photosynthesised efficiently and no host tissue loss was evident. Furthermore, there was no difference in the sub-clade of Symbiodinium sp. contained within lesion or visibly healthy tissue. Microscopic examination of diseased, visibly healthy and healthy control tissues, using a range of microscopic techniques, was carried out to further clarify if and how both partners in the symbiosis were affected by WPS. A bacterial role in the disease was not evident using histopathology or fluorescence in situ hybridisation (FISH). However, transmission electron microscopy (TEM) revealed the presence of virus-like particles (VLPs) in both visibly healthy and diseased tissue. TEM also revealed that both apoptosis and necrosis were occurring at a low level in both the host and symbiont. TEM analysis of VLPs associated with the coral surface microlayer (CSM) of visibly healthy and diseased tissues showed that while a single viral morphotype was present within the coral tissue more than 14 morphotypes were identifiable in the CSM. No discernible difference between the viral communities of the CSM of diseased and visibly healthy tissues was apparent, however. A methodology combining density gradient centrifugation and TEM was established to isolate the DNA from VLPs in the tissues of Porites corals. Preliminary phylogenetic analysis of the protein sequence of a fragment of the ribonucleoside reductase (small sub-unit) gene (rr2) indicated that the virus may be unique, but further characterisation is needed to confirm this. Important aspects of a previously undocumented disease affecting a major genus of reefbuilding coral on the GBR were determined through the course of this study. Most notably, the disease does not exhibit temporal changes that are related to seawater temperature, which is in contrast to the majority of other coral diseases, nor does it result in coral colony mortality. The discovery of a putative virus within tissues of Porites colonies affected by WPS could be indicative of a chronic viral infection, and this may or may not be related to the appearance of the bleached foci. The discovery of a coral tissue-associated VLP represents the first such discovery for wild corals. Further research is now required to complete the molecular characterisation of the Porites virus and to establish what role, if any, it plays in WPS.

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Created: Fri, 21 Nov 2008, 16:34:13 EST