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).