Finding of 132, 173‐cyclopheophorbide a enol as a degradation product of chlorophyll in shrunk zooxanthellae of the coral Montipora digitata

We examined the morphology and pigment composition of zooxanthellae in corals subjected to normal temperature (27°C) and thermal stress (32°C). We observed several normal and abnormal morphological types of zooxanthellar cells. Normal cells were intact and their chloroplasts were unbroken (healthy); abnormal cells were shrunken and had partially degraded or broken chloroplasts, or they were bleached and without chloroplasts. At 27°C, most healthy zooxanthellar cells were retained in the coral tissue, whereas shrunken zooxanthellae were expelled. Under thermal stress, the abundance of healthy zooxanthellae declined and the proportion of shrunken/abnormal cells increased in coral tissues. The rate of algal cell expulsion was reduced under thermal stress. Within the shrunken cells, we detected the presence of a chl‐like pigment that is not ordinarily found in healthy zooxanthellae. Analysis of the absorption spectrum, absorption maxima, and retention time (by HPLC) indicated that this pigment was 13², 17³‐cyclopheophorbide a enol (cPPB‐aE), which is frequently found in marine and lacustrine sediments, and in protozoans that graze on phytoplankton. The production of cPPB‐aE in shrunken zooxanthellae suggests that the chls have been degraded to cPPB‐aE, a compound that is not fluorescent. The lack of a fluorescence function precludes the formation of reactive oxygen species. We therefore consider the formation of cPPB‐aE in shrunken zooxanthellae to be a mechanism for avoiding oxidative stress.