Damage and recovery of Photosystem II during a manipulative field experiment on solar bleaching in the coral Goniastrea aspera

The solar radiation regime on differently oriented surfaces of the shallow-water coral Goniastrea aspera is markedly different. Field experiments involving rotation of small (20 cm diameter) hemispherical colonies exposed hitherto protected surfaces of the colonies to high levels of photosynthetically active radiation (PAR) whilst reducing PAR on the opposite surface. Photodamage and recovery, respectively, were recorded in the symbiotic algae using non-invasive chlorophyll fluorescence techniques. Measurements of photochemical efficiency (Fv/Fm) indicated that the Photosystem II (PSII) reaction centres of algae in previously protected surfaces suffered 'chronic' photoinhibition, probably on a timescale of 3 h, as a result of exposure to elevated solar PAR. Furthermore, evidence from this study corroborates earlier findings that this photodamage is a function of high PAR irradiance for short periods rather than a similar dose accumulated more slowly. Simultaneously, algae on the opposite surfaces were observed to recover their photochemical efficiency, but this recovery was at a much slower rate involving several days at reduced PAR. The photodamage was sufficiently severe to lead to the formation of visible lesions (bleached areas). Observations of coral polyp retraction and measurements of the protective xanthophyll pigments in the algae showed behavioural and photoprotective mechanisms being deployed to mitigate the photodamage.