Abstract

This study aimed to elucidate the mechanism underlying coral bleaching. Galaxea astreata corals were maintained in aquaria either under non-thermal stress (28°C) or under thermal stress (32°C) for 5 days followed by non-thermal stress for 7 days. Changes in the morphology and ultrastructure of the zooxanthellae of these corals were observed using transmission electron microscopy. Results showed that zooxanthella cells under 28°C and 30°C for 24 h displayed normal morphological features. Zooxanthella cells were damaged and their membranes began to fold after exposure to thermal stress for 72 h. These cells became severely damaged and underwent apoptosis upon prolonged exposure (168 h) to thermal stress. When thermal stress was further prolonged to 218 h, necrosis characterized by ruptured cell wall was observed. Recovery process was initiated by gradually reducing the temperature from 32°C to 28°C. The external morphology of Symbiodinium cells recovered at 28°C for 216 h. However, the cytoplasmic organelles needed more time to repair. These results implied that symbiont cells can survive and potentially be reabsorbed for symbiosis after coral bleaching.

Keywords: Symbiodinium; Ultrastructural; Thermal stress; Recovery