Triggers of Coral Reef Bleaching

There are several environmental conditions that can initiate the process of coral bleaching. These include:

• Temperature. This is generally seen as the principle trigger of bleaching, and is the main consequence of ENSO (Douglas, 2003). Temperatures that are sustained above, or below, the threshold of the coral or zooxanthellae, can lead to bleaching.

• High exposure to solar irradiance. This is another major trigger which is also linked to ENSO. Both the photosynthetically active wavelengths of the spectrum and the ultraviolet band are both linked to bleaching.

• A rapid fall in the nutrient levels of sea water. This can occur after storms and flooding, the ENSO can again be involved in causing such events.

• Subaerial exposure. This occurs when sea levels fall below the corals exposing them to air. Exposure to the atmosphere leads to other triggers in the form of increased irradiance, and increased or decreased temperature.

• High levels of inorganic nutrients. Pollution is the primary cause of this, such as run-off from inland and oil spills. Substances such as copper, oil and cadmium are all potential triggers.

• Salinity. This can affect coral growth but can also trigger bleaching, via both hypo and hyper salinity.

• Falling ocean pH. This makes the process of calcification more difficult within the corals, due to the increasing acidity. This in turn reduces the availability of carbonate ions which are incorporated into their framework. This may not directly lead to bleaching but a weakened framework may leave corals vulnerable to other environmental conditions.

These triggers are not independent of one another, bleaching may occur due to a combination of two or more of these triggers (Porter et al., 1999; Smith & Buddemeier, 1992).

Mechanisms of coral reef bleaching

To date, the mechanisms involved in coral bleaching are not fully understood, with many possible theories to explain the phenomenon.

One involves damage to Photosystem II (PSII) in the chloroplasts in algae from the Symbiodinium species (Warner et al., 1999). Without an efficiently functioning PSII, photosynthesis cannot occur and as a result will not be able to supply their symbiotic corals with nutrients. This may lead to expulsion by the corals. Warner et al. (1999) studied a number of species of zooxanthellae that had been naturally affected by significant bleaching in Florida in 1999, after SST rose to above 30˚C for a prolonged period, as well as experimentally bleaching corals. They found that an increase in SST leads to heat stress within the zooxanthellae, damaging the D1 protein that is key to the PSII reaction. At 32˚C the rate of degradation of the D1 protein was higher than its resynthesis, leading to the loss of PSII function. As a result, a reduction in photosynthesis was recorded before algae densities fell within the corals. The events leading up to the loss of D1 are not known however.

Warner et al. (1999) also found that corals more resistant to bleaching have a greater ability for the maintenance of PSII, with the resynthesis of the D1 protein occurring at a rate that matches its destruction. Another study demonstrated that corals with high protein turnover rates are better adapted for acclimatisation via the synthesis of heat shock proteins and regulatory enzymes (Gates & Edmunds, 1999).

Mechanisms of coral reef bleaching (continued)

Photosystem II can also be damaged by sustained high temperature via a reduction in the efficiency of the Calvin cycle to fix carbon. This is thought to be due to the degradation of enzymes involved in the Calvin cycle, such as possibly the unstable nature of the Rubisco enzyme found in zooxanthellae, unlike that found in most plants (Jones et al., 1998). A reduction in the operating efficiency of the electron transport chain between PSII and PSI also appears to take place (Jones et al., 1998; Fitt & Warner, 1995). Jones et al. (1998) also found that zooxanthellae located in areas of coral with high levels of irradiance also had an impaired PSII with lower photochemical efficiency, or quantum yield. This in turn leads to a decrease in oxygen production. A combination of high temperature and high irradiance led to the greatest decrease in PSII quantum yield, but temperature was found to be the primary determinant. The results of Jones et al. (1998) differ slightly to those of Warner et al. (1999) in that Jones et al. (1998) suggest that damage to PSII is the secondary effect of environmental stress, as a result of damage to the Calvin cycle and a reduction in electron flow, not the primary effect. Further research into the molecular mechanisms involved in bleaching is certainly needed to fully understand the process.

Mechanisms of coral reef bleaching (continued)

The fundamental results of Jones et al. (1998) and Warner et al. (1999) are backed up by Takahashi et al. (2008). Their investigations found that photobleaching occurred in the CS-73 strain of Symbiodinium at temperatures over 31˚C, with inhibition of PSII taking place. They also found that loss of photosynthetic pigment can occur as a result of heat stress combined with irradiance, which is due to the loss of antennae proteins that contain large concentrations of photosynthetic pigments to harvest large quantities of light. The molecular mechanisms leading up to this loss of antennae are not fully known, but it is likely due to the degradation and inhibition of protein synthesis of structural proteins within the antennae, such as acpPC. Another strain of zooxanthellae studied, the OTcH-1 strain, showed no loss of pigment after exposure to the same temperatures, indicating differences in the resistance to environmental stress between species of zooxanthellae (Takahashi et al., 2008).