Research > Coral Disease
Coral reefs are amongst the most complex and diverse ecosystems on the planet. During the past two decades there has been a growing concern over the worldwide degradation of coral reef ecosystems, and outbreaks of coral diseases have dramatically increased in recent years. This increase includes the appearance of new, previously uncharacterised diseases, and the re-emergence of more virulent forms of known diseases, and an increase taxonomic range of corals affected. The Coral Reef Ecosystems Lab has been at the forefront of coral disease research on the Great Barrier Reef, and has collaborated with research projects in the Red Sea, Caribbean, Mediterranean and across the Indo-Pacific. Our research topics have included:
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Coral disease diagnostics: apoptosis as a primary cell death mechanism in coral disease
Our research group was the first to investigate the cellular processes of 'white syndrome' on the Great Barrier Reef. Histological studies of white syndrome affected tabular Acropora using flourescent in-situ hybridisation (FISH) revealed an absence of microbial communities associated with white syndrome lesions. Further research using in-situ end labelling (ISEL) provided the first evidence of apoptotic cell death in the host tissues, with no evidence of necrosis, despite widespread reference to this in the coral disease literature (Ainsworth et al 2007). Further research into the impacts of coral diseases in the red sea (Ainsworth et al 2008) has led to a the differentiation between 'white diseases' (dense bacterial communities associated with lesions of symbiont loss and/or extensive necrosis of tissues) from 'white syndrome' (extensive programmed cell death/apoptosis associated with the lesion and the adjacent tissues).
Positive ISEL staining for apoptosis in Acroporid corals from the Great Barrier Reef. |
Absence of apoptosis and intra-cellular bacteria in histological samples of bleached Oculina patagonica from the Mediterranean coastline |
White syndrome affecting Acropora cytherea from Heron Reef showing rapid tissue loss and exposed white skeleton. |
Bacterial bleaching and Oculina patagonica
The idea that mass coral bleaching is caused by bacteria bleaching as opposed to temperature stress has been quite a contentious issue over the past decade.In 2005, students and researchers from the Coral Reef Ecosystems Lab spent 6 months in Israel exploring the microbial ecology of Oculina patagonica in the Mediterranean Sea, which has resulted in a recent publication in the Nature journal ISME. Our team made the surprising discovery that Occulina patagonica off the Medditeranean coast of Israel (the same place where the specimens were collected for the numerous studies by Rosenberg and colleagues) bleached without any Vibrio being present.The team from my laboratory documented the natural microbial community of the Oculina patagonica during a survey of the annual bleaching event. Our research determined that the microbial community shifts are complex, and resulted in significant shifts in the endolithic population associated with the bleaching of the host tissue. In contrast to previous studies by Rosenberg et al, Vibrio could not be detected within the host tissues, despite continued active bleaching and degradation of the symbiosis. This was not consistent with Vibrio being the cause of coral bleaching in Oculina patagonica.
This study is the first to determine microbial communities associated with the bleaching of O.patagonica under field conditions, which are in sharp contrast to previous laboratory “infections” of this temperate coral. The complete absence of intra-cellular bacteria (including V.shiloi) from corals collected in the wild is a surprising finding, and raises considerable doubt as to the generality and basis of the bacterial bleaching hypothesis.
Multiple colonies of the encrusting coral Oculina patagonica at Sdot Yam, Israel. |
Partial bleaching of Oculina patagonica at Sdot Yam, Israel during the summer months. |
Healthy colony of Oculina patagonica at Sdot Yam, Israel |
Photosynthetic impact of coral diseases
Within the literature, a growing body of evidence suggested that coral disease-like syndromes are likely impact primarily on the zooxanthellae, leading to secondary impacts and resultant necrosis in the coral host tissue. In collaboration with Karin Ulstrup and Peter Ralph from the University of Technology Sydney, our research group was the first to investigate the impacts of coral diseases on photosynthetic activity using chlorophyll a fluorescence techniques. We demonstrated that in disease-like syndromes associated with active tissue loss there was minimal impact to the photochemical function of the zooxanthellae. In contrast to aquaria based experiments showing that Vibrio bacteria were responsible for inducing bleaching and secondary tissue lysis in corals affected by white syndrome, our in-situ research into white syndrome at Heron Reef shows that under natural circumstances, white syndrome is a host based response, with no impact to the zooxanthellae.
Progression of white syndrome in a colony of tabular Acropora sp. showing blooms of phototrophic endoliths in recent exposed tissues. |
Transmission electron microscopy images of healthy and white syndrome affected tissues. |
Chlorophyll a fluorescence images of Acropora formosa affected by white syndrome. |