Saving coral reefs

In order to survive, reef-building corals must obtain a significant amount of its food from microalgae, called zooxanthellae, that live within their tissues. But when light intensity is too high and under heat stress, the algae absorb more light energy than they can use. Researchers at the University of Liège have reported that when light intensity reaches a certain threshold, a significant number of electrons are directed towards oxygen instead of CO2, leading to the production of free radicals which will, in turn, and through a series of complex processes, lead to cell death, digestion or expulsion of zooxanthellae by the coral.

Near apocalyptic underwater images sometimes appear in the media, including images of pale rather than brightly coloured corals.
These particularly rich ecosystems are especially sensitive to environmental perturbations such as an increase in seawater temperature, pollution by fertilisers and pesticides, etc. ‘Coral reefs rely on the symbiosis between an animal (sceractinian) related to jellyfish and sea anemones and microalgae (Symbiodinium) commonly known as ‘zooxanthellae’, explains Stéphane Roberty, a researcher at the Ecophysiology and Animal Physiology Laboratory at the University of Liège.

Relatively stable environmental conditions are required for corals to develop and prolifer. When these conditions are met the relationship between zooxanthellae and their host works perfectly. The animal provides a shelter, a source of nutrients and adequate light, in exchange, microalgae provide oxygen, sugars and other products derived from photosynthesis.
In other words, one can be said that the coral offers the accommodation and the microalgae the dinner, thus performing a mutually beneficial service.

Corals Agincourt Reef

A protective barrier

Because they produce an exoskeleton, coral reefsform robust three-dimensional structures which are even visible from space (such as the Great Barrier Reef in Australia) and host an extremely important biodiversity; not only are they essential for the animal and plant populations which they shelter, but are also essential to humanity itself. ‘Coral reefs provide numerous services to the human coastal populations’, says the researcher. ‘They are a significant source of food, revenue and employment. Moreover, they prevent erosion of the beaches and protect the coast during storms, hurricanes and tsunamis by reducing the power of the waves. The poor health of some coral reefs in Indonesia and Thailand may have contributed to the catastrophe which afflicted these countries in December 2004.’ For the past 15 to 20 years, a deterioration of the reefs throughout the world has been observed, particularly due to human activities and pollution. But that’s not all: corals suffer from a bleaching phenomenon that can be be local or global. The reason for this is the rising sea surface temperature. ‘The temperature may be higher locally, for example during a particularly hot summer, or may affect areas which are geographically much more vast, such as during the El Niño phenomenon’, continues Stéphane Roberty. He specifies that 1998 was a particularly stressful year for corals, given the high average temperatures recorded. And 2015-16 promises to be just as damaging...

Heat related stress

Stéphane Roberty is particularly interested in the phenomenon of coral bleaching. He had already looked at these marine organisms during his undergraduate studies: ‘I found the subject really fascinating, particularly the symbiosis existing between corals and the microalgae which live within their tissues. I then learned that this symbiosis could be disturbed by the impact of stress upon photosynthesis. This was the theme of my undergraduate degree and it was the first time I was to work with Fabrice Franck’s bioenergy laboratory, which was already studying the effect of various types of stress upon microalgae.’

‘Specifically, we had been studying the photosynthesis of microalgae for several years’, says Franck. ‘With Pierre Cardol (Laboratory of Genetics and Physiology of Microalgae), we had a range of techniques, unique in Belgium, which enabled us to study photosynthesis in great detail. It has to be said that this is a very complex subject. Because Stéphane was interested in microalgae and had some experience with corals, this gave us the possibility to study the symbiosis between the two, using cutting-edge tools.’

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