Apoptosis and Coral Bleaching

A review of: Tchernov D, Kvitt H, Haramaty L, Bibby TS, Gorbunov MY, Rosenfeld H, Falkowski PG (2011) Apoptosis and the selective survival of host animals following thermal bleaching in zooxanthellate corals. Proceedings of the National Academy of Sciences, 108 (24): 9905-9909.

Coral bleaching is a global phenomenon that occurs when environmental factors such as increases in temperature and/or solar irradiance causes the expulsion, digestion or loss of pigmentation of zooxanthellae which reside in the endoderm of symbiotic cnidarians. Periodic bleaching episodes can cause cnidarian mortality and have significantly reduced coral abundance in tropical reef ecosystems, however the molecular and physiological mechanisms that lead to death is not fully understood.

Apoptosis or programmed cell death has been suggested to be the pathway that leads to the breakdown of symbiosis and ultimately to the death of the host. Apoptosis is characterised by morphological changes to cells which include plasma membrane blebbing, cytoplasmic shrinkage, chromatin condensation and DNA degradation, brought about by the activity of a number of intracellular proteins in which caspases play a central role. Caspases can be up regulated by reactive oxygen species (ROS) predominantly produced from the photosynthetic activity of the symbiont and/or the mitochondrial activity of the host. Tchernov et al (2011) set out to investigate the relationship between elevated temperature and irradiance, as well as the concomitant increase in ROS production and caspase activity in a variety of symbiotic corals under controlled laboratory conditions and in situ natural conditions.

Seriatopora hystrix and Stylophora pistillata specimens were grown for several months at 26 °C and were transferred to aquaria with a temperature of 32 °C, to test for thermal stress under controlled conditions. Both corals eventually bleached but S. pistillata exhibited apoptotic morphology along with an increase in caspase activity while S. hystrix retained morphological integrity and had a decrease in caspase activity. In field experiments, Pocillopora damicornis and Montipora capitata were exposed to full natural light, low light, ambient temperature and elevated temperature conditions. An increase in caspase activity for both corals was detected at elevated temperatures in both light conditions however caspase activity was more enhanced with full sunlight exposure. Moreover, bleaching and apoptosis was curtailed upon the introduction of caspase inhibitors thus confirming the role of apoptosis in coral bleaching events.

To determine the role and source of ROS production, aposymbiotic and symbiotic M. capitata colonies were kept at 32 °C while ROS levels were measured after 11 hours. In symbiotic corals ROS production and caspase activity was higher than in aposymbiotic corals. Hydroxyl anion radicals were subsequently introduced to the symbiotic and aposymbiotic colonies kept at 26 °C under low light and 32 °C under high light, to elucidate whether ROS induces caspase activity. There was a significant increase in caspase activity observed in both colonies, although caspase activity was detected earlier in the aposymbiotic colonies while the highest caspase activity was detected in symbiotic colonies in both temperature and light regimes.

The results of the study provide insights into the physiological developments associated with bleaching and mortality processes. Different zooxanthellae clades have different lipid compositions which can impart resistance to environmental stressors that cause the uncoupling of thylakoid membranes and an associated increase in ROS production. It is hypothesised that under low light conditions, the symbiont is able to support the antioxidant defence of the host, whereas an increase in ROS production beyond a threshold can lead to the activation of apoptotic processes. Due to the different response to stress between S. pistillata and S. hystrix, thermally induced bleaching and mortality may be a host and symbiont species specific process which might explain why some corals bleach and recover, some bleach and die while some do not bleach at all. Interestingly, caspase inhibitors were able to prevent the death of the host even under high ROS production, therefore it can be speculated that therapy involving caspase inhibitors and/or anti apoptotic proteins like Bcl-2, may provide future strategies for preserving reef systems under threat from environmental perturbation.