Coral bleaching - understanding it a bit more

Corals are structure building invertebrates which create a habitat for many organisms. Within the corals live photosynthetically active symbionts like dinoflagellates (Symbiodinium) which provide up to 85 % of the host’s nutrition. Stress factors like high temperature, salinity stress or high light intensity trigger the loss or death of Symbiodinium (coral bleaching) resulting in coral death. Many studies investigated the effect of high temperature stress (32 °C or more) on corals and their symbionts. The study conducted by Fujise et al. (2014) assesses the effect on expulsion of Symbiodinium of moderate and more realistic thermal stress. 

Two coral species (Acropora selago and Acropora muricata) were collected at 2 m depth in Japan. Measurements were performed at normal conditions (27 °C), at transition conditions (27 – 30 °C) and at moderate thermal stress conditions (30 °C). To determine differences between Symbiodinium in hospite and expelled Symbiodinium the PSII maximum quantum yield (Fv/Fm) was measured by using a PAM fluorometer. Additionally Fujise et al. used transmitted light and fluorescence microscopy to count the expelled symbionts. 

Fujise et al. found that the expulsion rates in both A. selago and A. muricata were constant at normal conditions, under transition conditions and under moderate thermal stress conditions. The amount of expelled normal Symbiodinium cells did not increase at 30 °C, but the expelled degraded cells increased in both coral species. The in hospite cells have constant Fv/Fm values despite the increasing temperature. The Fv/Fm values of normal expelled cells decreased with an increase in temperature. 

Combining previous findings Fujise et al. suggest that the expulsion of degraded and normal Symbiodinium cells is a normal regulation mechanism to maintain the density of symbionts in the coral and to preserve a constant amount of fixed organic matter within the host by releasing surfeit fixed carbon. They suggest that corals digest more degraded cells under stress conditions and also releases more degraded Symbiodinium under moderate thermal stress conditions. More normal but photosynthetically damaged cells are expelled under stress conditions and a decrease of the Fv/Fm values of the expelled cells at 30 °C was detected. Furthermore, elevated temperature leads to reactive oxygen species (ROS) which have a negative influence on the photosynthesis and the symbiont in general and seem to lead to an increase of expulsion rates of Symbiodinium. Normal but photosynthetically damaged symbionts are expelled to avoid an accumulation of ROS. 

In conclusion Fujise et al. suggest that there are two different mechanisms in expulsion. One seems to be the normal regulation mechanism of the symbiont density within the hosts tissue. The other one can be an adaptive mechanism as a response to moderate thermal stress. So coral bleaching can occur even after long moderate thermal stress and might be a result of an accumulation of damaged Symbiodinium in the tissue.

All in all this study gives a very important insight in the mechanisms of coral bleaching and we can infer that even with moderate but prolonged thermal stress corals will have big issues to adapt and the world is about to lose important habitats. Consequently it would be interesting to conduct similar studies with different coral species to see if they react the same way or if they have developed other mechanisms for dealing with thermal stress maybe without expelling their symbionts. 

Reviewed paper:

Fujise, L., Yamashita, H., Suzuki, G., Sasaki, K., Liao, L. M., & Koike, K. (2014). Moderate thermal stress causes active and immediate expulsion of photosynthetically damaged zooxanthellae (Symbiodinium) from corals. PloS one, 9(12), e114321.