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 µmol photons m(-2) s(-1). 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 O2 photon(-1) 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.

Keywords: bio-optics; light energy budget; light utilization; microsensors; photosynthetic efficiency; thermal boundary layer.

Figure 1.

Experimental setup. (a) Schematic of the experimental setup visualizing the relative position of light source, microsensors and coral fragment. (b) A scalar irradiance microsensor inserted into the coenosarc tissue of a M. curta coral. (c) Conceptual diagram showing the fate of light energy (abbreviations explained in text).

Figure 2.

Photon scalar irradiance and spectral attenuation in coral tissue. Photon scalar irradiance spectra measured at two different downwelling photon irradiances of (a) 640 and (b) 1280 µmol photons m⁻² s⁻¹. The dashed line is the incident downwelling spectral irradiance. Spectral attenuation coefficient of scalar irradiance K₀(λ) (mm⁻¹) measured at two different downwelling photon irradiances of (c) 640 and (d) 1280 µmol photons m⁻² s⁻¹. Legends show the measurement depth below the tissue surface (0 mm = coral tissue surface). (Spectra represent mean values; n = 3.)

Figure 3.

Vertical distribution of photon scalar irradiance (400–700 nm), E₀(PAR), in coral tissue under an incident downwelling irradiance of 640 µmol photons m⁻² s⁻¹ (black symbols) and 1280 µmol photons m⁻² s⁻¹ (red symbols) (n = 3, R² of exponential fits (solid lines) were 0.99 and 0.98, respectively). y = 0 indicates the position of the tissue surface.

Figure 4.

(a) Vertical microprofiles of temperature (°C) through the TBL under five different downwelling photon irradiances (160, 320, 640, 1280 and 2400 µmol photons m⁻² s⁻¹) and a flow velocity of 0.4 cm s⁻¹. Symbols and error bars indicate mean ± s.d.; n = 3. (b) Measured temperature gradients (°C) between the ambient seawater and the coral tissue surface measured at three vector irradiances (116, 234 and 445 J m⁻² s⁻¹). R² of linear fit = 0.98. Error bars are ± s.d. (n = 3).

Figure 5.

Vertical microprofiles of volumetric gross photosynthesis rates (in nmol O₂ cm⁻³ s⁻¹) (black bars; bottom x-axis) and the corresponding O₂ concentrations (in µM) (line and symbols; top x-axis) measured under four different downwelling photon irradiances (160, 320, 640 and 1280 µmol photons m⁻² s⁻¹). Error bars indicate ± s.d. (n = 3).

Figure 6.

Light energy budget in % of the incident irradiance calculated for two downwelling photon irradiances (640 and 1280 µmol photons m⁻² s⁻¹) and at two different flow velocities (0.4 and 0.8 cm s⁻¹). Blue bars are the fraction of energy backscattered by the coral surface, red bars are the fraction of energy dissipated as heat and black bars are fraction of energy conserved by photosynthesis. Note the break in the y-axis.

Figure 7.

Vertical microprofiles of the local photosynthetic quantum efficiency (QE) (η) in the studied coral (in units of mol O₂ produced per mol photon absorbed) under three different incident photon irradiances noted as colour bar legends (in µmol photons m⁻² s⁻¹). y = 0 indicates the position of the tissue surface.