Research Article: Coral Skeletons Defend against Ultraviolet Radiation

Date Published: November 25, 2009

Publisher: Public Library of Science

Author(s): Ruth Reef, Paulina Kaniewska, Ove Hoegh-Guldberg, Craig Robert White. http://doi.org/10.1371/journal.pone.0007995

Abstract: Many coral reef organisms are photosynthetic or have evolved in tight symbiosis with photosynthetic symbionts. As such, the tissues of reef organisms are often exposed to intense solar radiation in clear tropical waters and have adapted to trap and harness photosynthetically active radiation (PAR). High levels of ultraviolet radiation (UVR) associated with sunlight, however, represent a potential problem in terms of tissue damage.

Partial Text: Photosynthesis is a common pervasive characteristic of shallow tropical marine habitats with organisms being photosynthetic or involved in a tight symbiosis with photosynthetic symbionts. In the latter case, the intimate association of animals such as corals and these primary producers plus the efficient recycling of nutrients underpins their success in the generally nutrient poor waters of the tropics. In this respect, reef-building corals rely greatly on photosynthates produced by their symbiotic photosynthetic dinoflagellate, Symbiodinium[1], which can harnesses the abundant solar energy in the tropics to fix carbon and translocate organic carbon for coral respiration [2]. In return, Symbiodinium gains access to the inorganic nutrients flowing from the catabolic processes of the coral host. The autotrophic energy provided by Symbiodinium to the coral host results in carbon fixation by coral reefs that is six times higher than that in neighbouring oligotrophic waters [3], [4], allowing for the formation of complex reef structures which provide niches for a diverse range of organisms.

While being highly reflective for photosynthetically active radiation, coral skeletons do not reflect UVR to any real extent (Figs. 1A, 1B). Grinding the skeletons to a fine powder did not reduce the average UVR reflectance of skeletal material (paired t-test, p>0.8, n = 10; Fig. 1B), indicating that the UV luminescence is a fundamental property of coral aragonite and not due to structure larger than 100 µm. Skeletons illuminated with UVR (280–360 nm) showed a weak yellow fluorescence (Figs. 2A, 2B). The substrate material (skeleton or polytetrafluoroethylene, PTFE) had a highly significant affect on the UV reflectance (One-Way ANOVA, F(3,8) = 82, p<0.001, Fig. 3A). The level of UVR reflected from PTFE was 4 times higher than that reflected from the Echinopora sp. skeleton (Tukey HSD, p<0.001). The UV reflected from anemones placed on PTFE was significantly higher than that from anemones placed on top of skeletons (Tukey HSD, p = 0.001), indicating that less UVR passed through the tissue of anemones placed on top of the skeleton compared to their counterparts placed on top of the UV reflective PTFE. Reef-building corals face a dilemma akin to that of a “Catch 22” [sensu 32]. While being exposed to full solar radiation is advantageous in terms of their energy budget, it has very negative consequences in terms of increased exposure to harmful UVR. We have shown that coral skeletons have high absorbance in UVR, emitting a weak yellow fluorescence as a result. This is consistent with observations of previous studies [e.g. 22], [33]. By emitting potentially harmful UVR as safe yellow light, coral skeletons dampen the amplification of the UVR while amplifying and increasing harvesting efficiency of the PAR that is necessary for growth and survival [10], [12]. In this respect, coral skeletons reflected PAR at a level that was similar to reflective white PTFE tape. Source: http://doi.org/10.1371/journal.pone.0007995

 

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