Monte Carlo Modeling of Photon Propagation Reveals Highly Scattering Coral Tissue

Abstract

Corals are very efficient at using solar radiation, with photosynthetic quantum efficiencies approaching theoretical limits. Here, we investigated potential mechanisms underlying such outstanding photosynthetic performance through extracting inherent optical properties of the living coral tissue and skeleton in a massive faviid coral. Using Monte Carlo simulations developed for medical tissue optics it is shown that for the investigated faviid coral, the coral tissue was a strongly light scattering matrix with a reduced scattering coefficient of μs’ = 10 cm-1 (at 636 nm). In contrast, the scattering coefficient of the coral skeleton was μs’ = 3.4 cm-1, which facilitated the efficient propagation of light to otherwise shaded coral tissue layers, thus supporting photosynthesis in lower tissues. Our study provides a quantification of coral tissue optical properties in a massive faviid coral and suggests a novel light harvesting strategy, where tissue and skeletal optics act in concert to optimize the illumination of the photosynthesizing algal symbionts embedded within the living coral tissue.

In vivo Microscale Measurements of Light and Photosynthesis during Coral Bleaching: Evidence for the Optical Feedback Loop?

“…It is possible that diffusely backscattered photons from the skeleton are trapped by the tissue (see detailed discussion in Wangpraseurt et al, 2014a) and/or that the scattering coefficient is higher for coral tissue than for coral skeleton, as is the case for Favites sp. (Wangpraseurt et al, 2016a). …”

“…For a small cup-shaped polyp, with low tissue absorption and scattering, the greatest effect of corallite architecture on light scattering is expected in close proximity to the skeletal surface at the center of the corallite (Teran et al, 2010; Wangpraseurt et al, 2016a). Our measurements for P. damicornis support these predictions since the highest rate of light enhancement was measured within aboral polyp tissues ( R 2 = 0.45, Figure 5C ).…”

“…Favites sp. are characterized by thick, light scattering tissues (Wangpraseurt et al, 2014a, 2016a) with host pigments such as GFP (Salih et al, 2000; Lyndby et al, 2016) and mycosporine-like amino acids (Dunlap and Shick, 1998; Lesser and Farrell, 2004) that often remain present during bleaching (Smith et al, 2013). Although, P. damicornis does also have a GFP-like pigment (Takabayashi and Hoegh-Guldberg, 1995), the GFP-like pigments in Favites sp.…”

“…Such irradiance enhancement is modulated by the unique optical properties of coral tissue and skeleton (Enriquez et al, 2005; Teran et al, 2010; Kahng et al, 2012; Marcelino et al, 2013; Wangpraseurt et al, 2016a) and can improve photosynthesis under low light conditions (Brodersen et al, 2014; Wangpraseurt et al, 2014a) or lead to light stress under high irradiance (Marcelino et al, 2013; Swain et al, 2016). The loss of Symbiodinium spp.…”

Changes in the Number of Symbionts and Symbiodinium Cell Pigmentation Modulate Differentially Coral Light Absorption and Photosynthetic Performance

“…Micro-scale approaches are useful tools for the characterization of the internal light fields of the symbionts (Kühl et al, 1995;Wangpraseurt et al, 2012Wangpraseurt et al, , 2014Wangpraseurt et al, , 2016Brodersen et al, 2014). This approach has documented the presence of light gradients (Wangpraseurt et al, 2012) and lateral light transfer within coral tissues , which apparently are more pronounced in corals with thicker tissues.…”