PAM Chlorophyll Fluorometry: a New in situ Technique for Stress Assessment in Scleractinian Corals, used to Examine the Effects of Cyanide from Cyanide Fishing

Sodium cyanide is being used on reefs in the Asia–Pacific region to capture live fish for the aquarium industry, and to supply a rapidly growing, restaurant-based demand. The effects of cyanide on reef biota have not been fully explored. To investigate its effect on hard corals, we exposed small branch tips of Stylophora pistillata and Acropora aspera to cyanide concentrations estimated to occur during cyanide fishing. Pulse amplitude modulation (PAM) chlorophyll fluorescence techniques were used to examine photoinhibition and photosynthetic electron transport in the symbiotic algae (zooxanthellae) in the tissues of the corals. These measurements were made in situ and in real time using a recently developed submersible PAM fluorometer. In S. pistillata, exposure to cyanide resulted in an almost complete cessation in photosynthetic electron transport rate. Both species displayed marked decreases in the ratio of variable fluorescence (Fv) to maximal fluorescence (Fm) (dark-adapted Fv/Fm), following exposure to cyanide, signifying a decrease in photochemical efficiency. Dark-adapted Fv/Fm recovered to normal levels in 6 d, although intense tissue discolouration, a phenomenon well-recognised as coral ‘bleaching’ was observed during this period. Bleaching was caused by loss of zooxanthellae from the coral tissues, a well-recognised sub-lethal stress response of corals. Using the technique of chlorophyll fluorescence quenching analysis, corals exposed to cyanide did not show light activation of Calvin cycle enzymes and developed high levels of non-photochemical quenching (qN), signifying the photoprotective dissipation of excess light as heat. These features are symptomatic of the known properties of cyanide as an inhibitor of enzymes of the Calvin cycle. The results of this in situ study show that an impairment of zooxanthellar photosynthesis is the site of cyanide-mediated toxicity, and is the cue that causes corals to release their symbiotic zooxanthellae following cyanide exposure. This study demonstrates the efficacy of PAM fluorometry as a new tool for in situ stress assessment in zooxanthellate scleractinian corals.