Effects of cyanide on coral photosynthesis: implications for identifying the cause of coral bleaching and for assessing the environmental effects of cyanide fishing

Modulated chlorophyll fluorescence techniques were used to examine the effe cts of cyanide (NaCN) from cyanide fishing on photosynthesis of the symbiot ic algae (zooxanthellae) located within the tissues of the zooxanthellate h ard coral Plesiastrea versipora. Incubating corals for 3 h in a cyanide con centration of >10(-5) M NaCN under a saturating light intensity (photosynth etically active radiation [PAR] intensity of 250 mu mol quanta m(-2) s(-1)) caused a long-term decrease in the ratio of variable to maximal fluorescen ce (dark-adapted F-v/F-m). The effect of cyanide on dark-adapted F-v/F-m wa s Light dependent; thus F-v/F-m only decreased in corals exposed to 10(-4) M NaCN for 3 h under PAR of 250 mu mol quanta m(-2) s(-1). In corals where dark-adapted F-v/F-m was significantly lowered by cyanide exposure, we obse rved significant loss of zooxanthellae from the tissues. causing the corals to discolour (bleach). To further examine the light-dependent effect of cy anide and its relation to loss of zooxanthellae, corals were exposed to 10- 4 M NaCN or seawater only (control), either in darkness or under 250 mu mol quanta m(-2) s(-1). ill significant decrease in dark-adapted F-v/F-m and l oss of zooxanthellae only occurred in corals exposed to cyanide in the ligh t. These results suggest cyanide causes the dissociation of the symbiosis ( bleaching) by affecting photosynthesis of the zooxanthellae. Quenching anal ysis using the saturation-pulse technique revealed the development of high levels of non-photochemical quenching in cyanide-exposed coral. This result is consistent with the known property of cyanide as an inhibitor of the da rk reactions of the Calvin cycle, specifically as an inhibitor of ribulose- 1,5-bisphosphate carboxylase/oxygenase (Rubisco). Therefore, chronic photoi nhibition and an impairment of photosynthesis of zooxanthellae provides an important 'signal' to examine the environmental effects of cyanide fishing during controlled releases in situ.