Abstract
BackgroundMany 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.Methodology/Principal FindingsBy measuring UVR and PAR reflectance from intact and ground bare coral skeletons we show that the property of calcium carbonate skeletons to absorb downwelling UVR to a significant extent, while reflecting PAR back to the overlying tissue, has biological advantages. We placed cnidarians on top of bare skeletons and a UVR reflective substrate and showed that under ambient UVR levels, UVR transmitted through the tissues of cnidarians placed on top of bare skeletons were four times lower compared to their counterparts placed on a UVR reflective white substrate. In accordance with the lower levels of UVR measured in cnidarians on top of coral skeletons, a similar drop in UVR damage to their DNA was detected. The skeletons emitted absorbed UVR as yellow fluorescence, which allows for safe dissipation of the otherwise harmful radiation.Conclusions/SignificanceOur study presents a novel defensive role for coral skeletons and reveals that the strong UVR absorbance by the skeleton can contribute to the ability of corals, and potentially other calcifiers, to thrive under UVR levels that are detrimental to most marine life.
Highlights
Photosynthesis is a common pervasive characteristic of shallow tropical marine habitats with organisms being photosynthetic or involved in a tight symbiosis with photosynthetic symbionts
Conclusions/Significance: Our study presents a novel defensive role for coral skeletons and reveals that the strong ultraviolet radiation (UVR) absorbance by the skeleton can contribute to the ability of corals, and potentially other calcifiers, to thrive under UVR levels that are detrimental to most marine life
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
Summary
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. As a result of these evolutionary pressures, corals have evolved into highly efficient light-harvesting organisms [12] They can utilise light six times more efficiently than plants [10] due to multiple scattering of photons within the skeleton and the tissuewater interface [13], thereby increasing photonic path lengths and subsequently the chance of interception by a photosystem [13]. High levels of ultraviolet radiation (UVR) associated with sunlight, represent a potential problem in terms of tissue damage
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