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First published online January 3, 2006
Journal of Experimental Biology 209, 273-283 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01983

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Diel `tuning' of coral metabolism: physiological responses to light cues

O. Levy^1,*, Y. Achituv¹, Y. Z. Yacobi², Z. Dubinsky¹ and N. Stambler¹

¹ Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel
² Israel Oceanographic and Limnological Research, Kinneret Limnological Laboratory, Migdal 14950, Israel

^* Author for correspondence at present address: The University of Queensland, Centre for Marine Studies, Cooper Rd, Gehrmann Building (Bldg. 60), St Lucia, Brisbane, QLD 4072, Australia (e-mail: o.levy{at}uq.edu.au)

Accepted 14 November 2005

Hermatypic-zooxanthellate corals track the diel patterns of the main environmental parameters - temperature, UV and visible light - by acclimation processes that include biochemical responses. The diel course of solar radiation is followed by photosynthesis rates and thereby elicits simultaneous changes in tissue oxygen tension due to the shift in photosynthesis/respiration balance. The recurrent patterns of sunlight are reflected in fluorescence yields, photosynthetic pigment content and activity of the two protective enzymes superoxide dismutase (SOD) and catalase (CAT), enzymes that are among the universal defenses against free radical damage in living tissue. All of these were investigated in three scleractinian corals: Favia favus, Plerogyra sinuosa and Goniopora lobata. The activity of SOD and CAT in the animal host followed the course of solar radiation, increased with the rates of photosynthetic oxygen production and was correlated with a decrease in the maximum quantum yield of photochemistry in Photosystem II (PSII) (F'/F_m'). SOD and CAT activity in the symbiotic algae also exhibited a light intensity correlated pattern, albeit a less pronounced one. The observed rise of the free-radical-scavenger enzymes, with a time scale of minutes to several hours, is an important protective mechanism for the existence and remarkable success of the unique cnidarian-dinoflagellate associations, in which photosynthetic oxygen production takes place within animal cells. This represents a facet of the precarious act of balancing the photosynthetic production of oxygen by the algal symbionts with their destructive action on all living cells, especially those of the animal host.

Key words: SOD, catalase, Photosystem II, zooxanthellae, photosynthesis, Favia favus, Plerogyra sinuosa, Goniopora lobata

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