Photosynthetic responses of corals Mussismilia harttii (Verrill, 1867) from turbid waters to changes in temperature and presence/absence of light

Responses of corals to climate change stresses are species and locality specific. As light is an important component of temperature-induced stresses, we experimentally tested the responses of a turbid water coral, Mussismilia harttii, to changes in temperature in the presence and absence of light. Chlorophyll fluorescence parameters were measured using a diving-PAM. Experiments were carried out at distinct temperatures. Polyps were kept in the dark or were continuously exposed to 300 µmol photons m^-2.s^-1 irradiance. No visible bleaching of coral samples was seen in temperatures between 26.5 and 35.0 °C, but most polyps at higher temperatures showed signs of tissue necrosis. There was a reduction of F_v/F_m as temperature increased, with a further drop in the presence of light, indicating a synergistic effect of these factors. We suggest that the photodamage to M. harttii endosymbionts triggered by temperatures of 33.0 °C and 35.0 °C results from a decline of the repair process, as well as the effect of light on the PSII. Recovery data for polyps kept in 31.0 °C showed that this temperature (depending on time of exposure) seems to be borderline; temperatures higher than 31.0 °C lead to long-term damage or death of M. harttii.