The dark and the bright: New insights into the light field of coral symbionts
by UTS press
26 November 2012
Microscopic light sensor inserted into coral tissue at a scale from 1cm to 500um. Image courtesy of D.Wangpraseurt. The highly productive and diverse nature of coral reefs is due, in large part, to the remarkable relationship between the coral host and the microalgal photosymbionts, known as zooxanthellae. Coral bleaching occurs when this relationship breaks down. The surrounding light field is an important player in triggering this phenomenon and can enhance the effect of other stress factors on corals such as elevated sea water temperature linked to global warming. However, until now researchers did not know about the actual light field that surrounds these microalgae inside living tissues. By inserting microscopic light sensors (with a thickness comparable to a human hair) into coral tissue, researchers within the UTS Plant Functional Biology and Climate Change Cluster (C3) Aquatic Processes Group found that on a scale of a few tenths of a mm into the coral tissue, light levels decrease by over one order of magnitude. These light gradients are comparable to the reduction in irradiance that takes place between oceanic surface waters and approximately 25m depth. “The optical properties of corals are quite astonishing and are to some extent similar to that of terrestrial leaves. Understanding the physics of how light travels through coral tissue is very important for biologists who study coral physiology,” said lead author and C3 PhD candidate Daniel Wangpraseurt. “Our results show that zooxanthellae populations live in an extremely heterogeneous light environment. Lower tissue layers within one single polyp harbour optical microniches that provide effective shelter from stressful radiation. In contrast, upper tissue layers are subject to two to three times higher than ambient light levels”. Daniel said that the findings were very important for understanding the stress response to climate change as well as the genotypic and phenotypic plasticity of coral symbionts. “The results also have implications for the understanding of coral bleaching patterns as they show that thick-tissued corals harbour sheltered light environments for resident zooxanthellae. This means that these corals are likely less susceptible to excess radiation, which in combination with high temperature readily leads to bleaching. We are now looking more into the physiology and stress response of symbionts in relation to the new microenvironmental insight,” he said. This study combined micro-incision with scalar irradiance profiling. Further investigations led by Prof Michael Kuhl are now using secondary-ion mass spectroscopy on a nanometer scale (Nano-SIMS) together with colleague Dr Mathieu Pernice to look at the physiology of individual symbiont cells in relation to these light gradients. Publication details: Light gradients and optical microniches in coral tissues.Wangpraseurt, D., Larkum, A. W. D., Ralph, P. J, and Kühl, M, Frontiers in Microbiology. 3:316. doi: 10.3389/fmicb.2012.00316
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