ORIGINAL ARTICLE Sponge and coral zooxanthellae in heat and light: preliminary results of photochemical efficiency monitored with pulse amplitude modulated fluorometry Christine H. L. Scho ¨ nberg 1,4 , Ryota Suwa 2 , Michio Hidaka 2 & William Kok Weng Loh 1,3 1 Centre for Marine Studies, The University of Queensland, St Lucia, Qld, Australia 2 Department of Marine and Environmental Science, Graduate School of Engineering and Science, University of the Ryukyus, Nishihara, Okinawa, Japan 3 Marine Biology Australia, St Lucia, Qld, Australia 4 Present address: Carl von Ossietzky University Oldenburg, Faculty 5, Institute of Biology and Environmental Sciences, Department of Animal Bio- diversity and Evolution, Oldenburg, Germany Problem Recent studies on coral reefs are largely concerned with anthropogenically caused damage and effects of climate change. Global warming has resulted in a mounting num- ber of serious bleaching events (e.g. Loya et al. 2001) and is believed to cause lasting damage if not near-extinction of the coral reef environments we presently know (e.g. Keywords Acropora; Cliona; photosynthesis; pulse amplitude modulated fluorescence; stress; Symbiodinium; symbiosis. Correspondence Christine H. L. Scho ¨ nberg, Department of Animal Biodiversity and Evolution, Institute of Biology and Environmental Sciences, Faculty 5, Carl von Ossietzky University Oldenburg, 26111 Oldenburg, Germany. E-mail: christine.schoenberg@uni-oldenburg.de Accepted: 15 November 2007 doi:10.1111/j.1439-0485.2007.00216.x Abstract Photochemical efficiency (F v ⁄ F m ) was compared between a common symbiotic bioeroding sponge, Cliona cf. orientalis, and a common reef-builder, Acropora pa- lifera using pulse amplitude modulated fluorescence (PAM). The study was con- ducted on Sesoko Island, Okinawa, where reefs were severely damaged during previous bleaching episodes. Sponge and coral dinoflagellate symbionts were trea- ted with heat and light in a tank experiment, both in hospite (=still within their host) and isolated from their hosts. We found significant differences for photo- chemical efficiency of holobionts (=host and symbiont together) compared to the isolate symbionts and over time. All symbionts suffered in isolation and displayed stronger reactions to the treatments, and there was evidence for increasing dam- age despite returning to control conditions. However, because of large variability of the bi-symbiont coral samples and restrictions of the experimental design, our main results remained inconclusive, with no significant differences between sponge and coral samples and between the different stress treatments. Judging the results based on the uniform trends in the subsets of data, the G-clade sponge symbionts appeared to be more stress tolerant than the C- and D-clade coral symbionts, with no treatment effects in hospite and less damage in isolation com- pared to the coral symbionts, but this is an unconfirmed assumption. Isolated sponge symbionts were very resistant against heat stress, but may have suffered from light stress. In hospite, the latter risk can be countered by the sponge’s 3- dimensional morphology, the endolithic life style that affords shading, and by behavioural adaptation, i.e. the ability to move symbionts away from the source of stress. Overall, C. cf. orientalis symbionts displayed a more stable photochemi- cal efficiency during and after stress than those of A. palifera. Results of this study suggest that with climate change C. cf. orientalis might have a better survival potential than A. palifera, but further investigations are necessary. Marine Ecology. ISSN 0173-9565 Marine Ecology 29 (2008) 247–258 ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd 247