MEPS 177:83-91 (1999)  -  doi:10.3354/meps177083

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

Ross J. Jones*, Ove Hoegh-Guldberg

School of Biological Sciences, The University of Sydney, Sydney, New South Wales 2006, Australia

ABSTRACT: Modulated chlorophyll fluorescence techniques were used to examine the effects of cyanide (NaCN) from cyanide fishing on photosynthesis of the symbiotic algae (zooxanthellae) located within the tissues of the zooxanthellate hard coral Plesiastrea versipora. Incubating corals for 3 h in a cyanide concentration of >10-5 M NaCN under a saturating light intensity (photosynthetically active radiation [PAR] intensity of 250 µmol quanta m-2 s-1) caused a long-term decrease in the ratio of variable to maximal fluorescence (dark-adapted Fv/Fm). The effect of cyanide on dark-adapted Fv/Fm was light dependent; thus Fv/Fm only decreased in corals exposed to 10-4 M NaCN for 3 h under PAR of 250 µmol quanta m-2 s-1. In corals where dark-adapted Fv/Fm was significantly lowered by cyanide exposure, we observed significant loss of zooxanthellae from the tissues, causing the corals to discolour (bleach). To further examine the light-dependent effect of cyanide 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 µmol quanta m-2 s-1. A significant decrease in dark-adapted Fv/Fm and loss of zooxanthellae only occurred in corals exposed to cyanide in the light. These results suggest cyanide causes the dissociation of the symbiosis (bleaching) by affecting photosynthesis of the zooxanthellae. Quenching analysis 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 dark reactions of the Calvin cycle, specifically as an inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Therefore, chronic photoinhibition and an impairment of photosynthesis of zooxanthellae provides an important 'signal' to examine the environmental effects of cyanide fishing during controlled releases in situ.

KEY WORDS: Bleaching · Cyanide · Zooxanthellae · Coral · Chlorophyll fluorescence

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