Photoacclimation dynamics in coral holobionts responding to thermal and irradiance changes correlate with photon pressure per symbiont

Abstract

AbstractPhotoacclimation is a key homeostatic process that enables photosymbiotic reef corals to flourish within highly variable light environments. However, the temporal dynamics of photoacclimation remain poorly characterized. We studied photoacclimation at several time points over 24 d in two scleractinian coral taxa (mounding Porites spp. and branching Acropora muricata). Coral specimens preacclimated at 24°C and 171 μmol quanta m−2 s−1 were exposed to three light levels (−32%, +27%, and +65% relative to that of the acclimation period) at two temperatures (control, 24°C; mildly stressful, 29°C). Intra‐tissue light environment—specifically, photon pressure per symbiont—increased dramatically with each additional light dose and again with increasing temperature in Porites spp.; net photosynthesis (Pnet) rates continued to change for at least 2 weeks following light increase but not light reduction, and vice versa for symbiont densities. Intra‐tissue light environments were substantially dimmer and more homogeneous among all light treatments in A. muricata, which continued to increase its light‐use efficiency and Pnet through time in most or all treatments. Consequently, prolonged change in Fv/Fm at elevated temperature in Porites spp. but not A. muricata may have been caused by differing photodamage loads. Moderation of light levels by conditions within the coral tissue seems to affect the temporal dynamics of the photoacclimation response, and this finding emphasizes new avenues for photoacclimation research.