Symptoms of Coral Reef Bleaching

The symptoms of coral bleaching are either expulsion of zooxanthellae from the coral host, or loss of pigmentation from the zooxanthellae cells (Venn et al., 2006). Most past investigations have stated that the coral polyps initiate the expulsion of zooxanthellae, but recent studies provide evidence suggesting that the expulsion of zooxanthellae is initiated by the zooxanthellae themselves, rather than the corals (Trapido-Rosenthal, 2005). The studies found the zooxanthellae produced large quantities of nitric oxide synthase before bleaching occurs. This enzyme leads to nitric oxide production in host cells, a reactive molecule that can lead to expulsion of zooxanthellae.

Symptoms of Coral Reef Bleaching (cont)

As previously mentioned, different species of zooxanthellae possess differing resistances to environmental stress, and in turn the symptoms of different species of zooxanthellae to stress also differ. It is thought that the resistance of zooxanthellae is largely genetically programmed and is linked to the species physiology and morphology (McClanahan et al., 2004). The resistance of corals to bleaching and death has also been shown to be affected by the environmental conditions that the corals are exposed to. The threshold conditions that corals can tolerate are thought to be greater if the environmental conditions in that habitat are more variable (McClanahan & Maina, 2003). A coral living in a reef with very small variation in SST is more likely to be affected by a rare warm water event than a coral living in an area with slightly higher variation in SST. McClanahan and Maina (2003) hypothesise that due to climate change, and the likely future trend of an increasing annual temperature range, corals will become more resilient to more extreme rare environmental disturbances such as ENSO events. However they also predict that diversity will decrease as a result, with only the most resistant species of coral surviving, and consequently recolonising areas of dead reef previously occupied by less resilient species, a process generally known as the adaptive bleaching hypothesis (Kinzie III et al., 2001). It has also been shown via molecular studies that there is a certain degree of flexibility within the partnerships that exist between zooxanthellae and their hosts, and that following bleaching recolonisation can occur by more resistant corals (Baker, 2003).

Symptoms of Coral Reef Bleaching (Cont)

Another study provides evidence that corals can acclimatise to increasing temperature by altering the concentrations of different species of zooxanthellae within their tissues (Berkelmans & Van Oppen, 2006), so over time a coral can be populated by less resistant species and then more resistant species following a rise in SST. This ability is only possible in a minority of coral species that are capable of giving residence to more than one zooxanthellae species simultaneously. An increase in tolerance of 1-1.5°C was discovered in the coral species studied. Further research into these issues could help identify the threat posed by bleaching more accurately as if these results are backed up, long term recovery of bleached reefs may be more viable than currently thought.