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. 2016 May 26;6:26599. doi: 10.1038/srep26599

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Copyright © 2016, Macmillan Publishers Limited

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Three cases of FP density are shown, (a) low FP, (b) medium FP and (c) high FP. The vertical coral tissue structure shows the oral tissue layer with the epidermis (e) the connecting tissue layer (mesoglea; m), gastrodermal host cells (g) containing Symbiodinium photosymbionts (yellow circles; s). The scheme ignores the lower tissue layer (aboral tissue) and the skeleton (see text) and assumes that (i) fluorescent pigments (blue-green triangles; FP) are situated above the symbionts, (ii) that skeleton scattering is low, and (iii) that tissue background scattering is constant for (a–c). Incident light (black arrow) is either reflected (R), absorbed and subsequently dissipated as heat (ΔT), or transmitted and lost from the system (T). (a) Light reaches FPs, causing scattering and enhanced chance of symbiont photon absorption and heating. Part of the light reaches lower layers (dotted black arrow) where symbionts absorb light and dissipate heat. For low FP content the scattering effect is relatively small and transmission through the coral is high. (b) As FP content increases, light scattering is enhanced, causing multiple scattering between FP granules. This strongly enhances net heating of the coral tissue system. Penetration depth and transmission is somewhat reduced. Reflectivity is enhanced because of diffuse backscattering by FP. (c) At high densities of FP, strong diffuse scattering enhances reflectivity and heating by symbionts located in proximity to the FPs. However, strong scattering follows reduced penetration depth and thus reduced chance of photon absorption and heating for deeper tissue layers (red cross on dotted black arrow). Net heating is high but caused by localized scattering of a small part within the tissue. As light scattering increases further, heating will eventually decrease and most of the light is lost as diffuse reflectance. The model was supported by Monte Carlo simulations of photon transport (Fig. S4).