Radiative energy budget reveals high photosynthetic efficiency in symbiont-bearing corals

Abstract

The light field on coral reefs varies in intensity and spectral composition, and is the key regulating factor for phototrophic reef organisms, for example scleractinian corals harbouring microalgal symbionts. However, the actual efficiency of light utilization in corals and the mechanisms affecting the radiative energy budget of corals are underexplored. We present the first balanced light energy budget for a symbiont-bearing coral based on a fine-scale study of the microenvironmental photobiology of the massive coral Montastrea curta. The majority (more than 96%) of the absorbed light energy was dissipated as heat, whereas the proportion of the absorbed light energy used in photosynthesis was approximately 4.0% under an irradiance of 640 mmol photons m 22 s 21. With increasing irradiance, the proportion of heat dissipation increased at the expense of photosynthesis. Despite such low energy efficiency, we found a high photosynthetic efficiency of the microalgal symbionts showing high gross photosynthesis rates and quantum efficiencies (QEs) of approximately 0.1 O 2 photon 21 approaching theoretical limits under moderate irradiance levels. Corals thus appear as highly efficient light collectors with optical properties enabling light distribution over the corallite/tissue microstructural canopy that enables a high photosynthetic QE of their photosynthetic microalgae in hospite.

Non-intrusive Assessment of Photosystem II and Photosystem I in Whole Coral Tissues

“…Photobiological performance and photosynthetic energy budgets of Symbiodinium in hospite have been characterized and reviewed in several studies (Brodersen et al, 2014;Roth, 2014;Warner and Suggett, 2016). However, surprisingly, the available data regarding the fundamental intersystem PSII-PSI electron transport properties FIGURE 6 | Representative FRRf saturation-relaxation fluorescence traces of P. decussata.…”

The engine of the reef: photobiology of the coral–algal symbiosis

“…The light propagation properties in intact corals reduce the effects of self-shading and allow Symbiodinium to maximize light absorption with low investment in pigments (Enríquez et al, 2005; Wangpraseurt et al, 2012, 2014; Marcelino et al, 2013). Symbiodinium in corals can have high gross photosynthetic rates and quantum efficiencies can reach near theoretical limits under moderate irradiances (Rodríguez-Román et al, 2006; Brodersen et al, 2014). Early studies vary widely in reported quantum efficiencies (Dubinsky et al, 1984; Wyman et al, 1987; Lesser et al, 2000), which may have been caused by an underestimation of the absorption cross-section of chlorophyll, differences in light levels during measurements, or differences among corals in light scattering, tissue thickness, and skeletal morphology (for more discussion see Section “Photosynthesis”).…”

“…Early measurements of Φ underestimated the absorption cross-section of chlorophyll because it was measured from freshly isolated Symbiodinium (Dubinsky et al, 1984; Wyman et al, 1987; Lesser et al, 2000) rather than in intact corals where the absorption is two to fivefold higher because of light scattering by the skeleton (Enríquez et al, 2005). Recent studies suggest that corals are efficient energy collectors and that the energy can be utilized close to the theoretical maximum (Rodríguez-Román et al, 2006; Brodersen et al, 2014). Φ varies within the coral (depth within the tissue), in corals collected from distinct light environments (high light vs. shade adapted) and in corals with different degrees of bleaching (Dubinsky et al, 1984; Rodríguez-Román et al, 2006; Brodersen et al, 2014).…”

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.…”