Poster: Global Change

Abs # 60: Photoinhibition susceptibility of Okinawan reef building corals

Presenter:	Nakamura, Takashi , takasuken@yahoo.co.jp
Authors	Nakamura, Takashi  (A)   Yamasaki, Hideo  (A)
Affiliations:	(A): University of the Ryukyus

Reef building corals need to cope with dynamic changes in light conditions because of their benthic forms similar to land plants. The corals possess endosymbiotic algae called zooxanthelae which provide essential energy source for the host through photosynthetic activity. In terms of impacts of global warming, coral bleaching phenomena have recently received much attention. It has been suggested that photoinhibition of symbiotic algae is involved in the mechanism for coral bleaching though the primary environmental factor that causes coral bleaching is controversial. Using five coral species inhabiting in Okinawa, here we report coral species-dependent difference in the photoinhibition susceptibility. The maximum quantum yield of PS II (Fv/Fm), an indicator for photoinhibition, was measured with a diving PAM both in laboratory and semi-natural aquarium systems. Photosynthesis of the coral Stylophora pistillata showed the highest sensitivity against strong light. In contrast, Acropora palifera displayed strong-light tolerance among five species tested. The Fv/Fm values measured after a three hours exposure of strong light (1000 mmol) were 0.16 and 0.30 (n = 6) in S. pistillata and A. palifera, respectively. Semi-long term aquarium experiments revealed that the corals can utilize light efficiently only in the morning and evening where the light intensity was less than 200 mmol. In the midday, photoinhibition was observed in all coral species we tested. We suggest that bleaching tolerance would be distinguished by assessing the susceptibility of corals against photoinhibition.