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Figure 1.

Light spectra and Symbiodinium absorption.

A) Normalised absorption spectrum of Symbiodinium, and defined scalar irradiance spectra used in the experiments in which the OL 490 light engine was used to simulate the in situ light exposure (‘coral tissue’, blue) and ‘halogen lamp’ (red) exposure (in µmol photons m−2 s−1 nm−1). The scalar irradiance spectra were normalised to give the same integrated output in µmol photons m−2 s−1 over PAR (400–700 nm). The absorption spectrum of Symbiodinium was normalised to its peak absorption at 465 nm. B) Photosynthetically usable radiation (PUR; see text).

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Figure 2.

Cuvette-based set-up for combined O2 exchange and variable chlorophyll fluorescence measurements on Symbiodinium cultures.

The O2 optode was inserted into the cuvette through a small hole in the gas-tight lid. Actinic light was provided by the OL-490 light engine equipped with a liquid light guide and collimator. The MC-PAM provided measuring light in opposite position to the actinic light and at right angles to the fluorescence detector. The cuvette temperature was controlled at 25°C by an ambient water bath and heater.

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Figure 3.

Effect of spectral light composition on Symbiodinium O2 turnover.

(A) Example of O2 dynamics in a Symbiodinium culture sample under experimental light-dark transitions of increasing irradiance. Black arrows show the onset of a 10 min dark period followed by a 10 min illumination periods at progressively higher irradiances. The slopes during each period of illumination and darkness were used to calculate Symbiodinium net photosynthesis (panel B; PN) and light-enhanced dark respiration (panel C, LEDR), respectively. The sum of PN and LEDR estimated gross photosynthesis (panel D; PG). Experiments were done under 11 experimental irradiance regimes (including darkness). The photon scalar irradiance integrated over 400–700 nm was equal for the two spectral treatments (‘coral tissue’ and ‘halogen lamp’) at each respective irradiance level. Solid lines indicate best fits and dotted lines represent 95% confidence intervals. Symbols and error bars indicate the mean ± SE (n = 3).

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Table 1.

Photosynthetic performance of Symbiodinium under defined broadband spectra simulating in situ coral tissue and halogen lamp spectral composition.

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Figure 4.

Effect of spectral light composition on (A) rETR vs. PAR and (B) ETR vs. PAR.

(C) Relationship between gross photosynthesis (PG) and ETR. (D) Non-photochemical quenching (NPQ). Note that the y-axis scale in (A) and (B) differ. Solid lines indicate best fits and dotted lines represent 95% confidence intervals (r2>0.91). Symbols and error bars represent the mean ± SE (n = 3).

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