AsianScientist (Jan. 9, 2015) – Researchers have proposed mechanisms explaining what might cause coral bleaching, the loss of symbiotic zooxanthellae that results the the collapse of coral reef ecosystems. Their results have been published in PLOS One.
The symbiosis between corals and zooxanthellae (dinoflagellate genus Symbiodinium) form the foundation of coral reef biology. Researchers from Hiroshima University including Associate Professor Kazuhiko Koike and Ms. Lisa Fujise demonstrated that corals more actively digest and expel damaged symbiotic zooxanthellae under conditions of thermal stress, and that this is likely to be a mechanism that helps corals to cope with environmental change.
They showed that zooxanthellae expelled at 27°C (non-thermal stress conditions) were part of a regulatory mechanism required to maintain zooxanthellal density and a stable carbon concentration. At 30°C (moderate thermal stress), Symbiodinium were damaged and corals selectively digested the damaged cells or immediately expelled them without digestion by exocytosis. The authors suggest that this is most likely an adaptive mechanism in response to moderate thermal stress to avoid the accumulation of damaged cells.
However, under thermal stress, the accumulation of damaged cells may exceed the increased rate of expulsion of digested zooxanthella. More photosynthetically damaged zooxanthellae were observed upon prolonged exposure to thermal stress and were released by corals without digestion, therefore preventing their accumulation.
Although the expulsion response may be an adaptive strategy to moderate stress and ensure survival, the accumulation of damaged Symbiodinium may occur when this response cannot cope with the magnitude or duration of environmental stress. The authors propose that this might be a possible mechanism underlying coral bleaching during prolonged moderate thermal stress.
The article can be found at: Fujise et al. (2015) Moderate Thermal Stress Causes Active and Immediate Expulsion of Photosynthetically Damaged Zooxanthellae (Symbiodinium) from Corals.
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Source: Hiroshima University; Photo: Matt Kieffer/Flickr/CC.
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