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.