The complexities of coral disease are starting to be unravelled with the key revelation that a similar mechanism that causes cholera in humans may be causing White Syndrome (WS) in coral.
Meir Sussman, a postgraduate student at James Cook University (JCU), working with AIMS scientist Dr David Bourne, coral biologist Dr Bette Willis at the ARC Centre of Excellence for Coral Reef Studies, and colleagues from the University of Groningen in the Netherlands and the Palau Coral Reef Center, have published a paper* showing for the first time how bacterial WS kills coral.
A bacterial zinc-metalloprotease enzyme has been revealed as central in the WS disease process. The enzyme carries out a two-pronged attack, first causing whitening of coral tissue as symbiotic algae are targeted, and subsequently causing coral tissue lesions. This two-stage process leads to the distinctive appearance of bands of white coral skeleton typical of the disease.
The enzyme disturbs the ability of the symbiotic algae living in coral to carry out photosynthesis and breaks down the symbiosis between the coral and the algae, leading to death of the coral. The bleaching caused by WS is distinct from that caused by thermal stress. Unlike bleached corals which can recover from short-term temperature stress, WS causes the infected coral to die, though lesions may stop progressing if the coral can mount an immune response.
The research team published pioneering work last year that uncovered the bacterial cause of WS, specifically certain members of a common family of aquatic bacteria known as Vibrios. Another member of the Vibrio family causes cholera in humans.
While there are many kinds of Vibrio bacterial species, only a small group carrying the gene for the zinc-metalloprotease enzyme can cause WS. This enzyme is a powerful weapon as it disrupts basic processes in target organisms at a cellular level. This mechanism of attack against cells is similar to the one used by the Vibrio bacterial species that causes cholera.
“This study is the first to investigate the clinical effect of the enzyme zinc-metalloprotease on corals,” Dr Bourne said. More work needed to be done, he said, to determine the exact process by which the enzyme affects the way the algae photosynthesise within coral and the extent to which the temperature of the surrounding water plays a role in helping the enzyme do its work.
Coral diseases are of increasing concern to marine scientists, particularly in the light of other sources of stress for corals such as warmer seawater. “Coral diseases have been detected along the length of the Great Barrier Reef, worryingly so in healthy reefs with high coral cover,” Professor Willis said.
In other parts of the world, notably the Caribbean, coral disease has been a major factor contributing to the decline of coral reefs, which in some places have undergone ecological “phase shifts” from coral to algal-dominated ecosystems.
*The paper, written by Meir Sussman, Jos Mieog, Jason Doyle, Steven Victor, Bette Willis and David Bourne, titled “Vibrio Zinc-Metalloprotease Causes Photoinactivation of Coral Endosymbionts and Coral Tissue Lesions”, has been published by the international, peer-reviewed, open-access, online publication PLoS (Public Library of Science) ONE. Go to: journal article
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