Scientists Probe Enigma of Coral Death

The evidence suggests reef systems are becoming more brittle, as a result of bleaching and disease.

With corals currently dying en masse in Indonesian waters and a significant risk of large scale coral bleaching deaths on Australian reefs in 2009, scientists are racing the clock to unravel the causes of these devastating global losses.

The puzzling thing is that surrounding waters need only increase in temperature by one or two degrees Celsius for the corals to expel their symbiotic algae, a process known as bleaching, and risk death by starvation, says the ARC Centre of Excellence for Coral Reef Studies’ Dr Andrew Baird.

'A degree or two is well within the range of tolerance for most sea life – but for some reason, as yet unknown, symbiotic organisms like corals become stressed and lose their algae, resulting in large scale mortality which can devastate reefs.'

Researchers believe that the answer may lie in the chemical ‘conversation’ which takes places between the symbiotic algae and their host corals, but whether the breakdown of the relationship is triggered by the algae or by the coral remains a mystery.

Solving this mystery could constitute a vital step forward in understanding how to manage and protect coral reefs through the mid-century onslaught of warm water that is expected to take place due to global warming, Dr Baird says.

'Coral reefs are under severe threat from many sources, including mass coral mortality events caused largely by sea surface temperature (SST) anomalies associated with global warming,' Dr Baird and colleagues write in a new paper in the journal Trends in Ecology and Evolution.

Recently, the scientific focus has been on the symbiotic algae – and whether corals can survive by switching to more heat-tolerant types of algae. However, there are many features of coral bleaching, such as dramatic differences in susceptibility to stress among species that can not be explained by algal types. Furthermore, the coral has many ways to influence its fate. For example, coral produces molecular sunscreens that prevent the photosynthetic apparatus of the algae from being damaged. Similarly, the host produces anti-oxidants that can mop up toxic molecules produced by the breakdown of photosynthesis. Finally, corals that can change their diet from carbon produced by the algae to carbon captured by the host in the form of planktonic prey are better able to recover from heat stress.

Most importantly, the team argue, we shouldn’t forget that corals have been in symbiosis with photosynthetic organisms for up to 200 million years, a selective pressure that has modified both host and algae. Consequently, there needs to be more focus on studying the symbiosis as if it were a single organism: 'Clearly both partners are involved in determining the organisms’ response to stress and, therefore, understanding the threat climate change poses requires the organism to be treated as a whole.'

The evidence suggests reef systems are becoming more brittle, as a result of bleaching and disease.
In particular, they say, there is a lack of population ecology studies of corals meaning that the response of even the most basic vital rates in corals to thermal stress, such as growth, mortality and fecundity, are poorly understood. Through such studies much more precise predictions can be made about the probable impacts of climate change on coral reefs, they say.

'Coral bleaching is one of the most devastating manifestations of climate change. The last big event in Australia’s waters in 2002 affected almost half of the Great Barrier Reef – an immense area. Every year now, bleaching events are being reported somewhere in the world.

'If we are not to lose the corals, then we need urgently to understand what is going on here – and what, if anything, we can do about it,' Dr Baird says.

‘Coral bleaching: the role of the host’ by Andrew H. Baird of CoECRS and James Cook University, Ranjeet Bhagooli of Mauritius University, Peter Ralph of the University of Technology Sydney and Shunichi Takahashi of the Australian National University appears in Trends Ecol Evol 24: 16-20 doi:10.1016/j.tree.2008.09.005

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