Photoacclimation mechanisms of corallimorpharians on coral reefs: Photosynthetic parameters of zooxanthellae and host cellular responses to variation in irradiance Baraka Kuguru a,b , Yair Achituv a , David F. Gruber c , Dan Tchernov b,d,e, ⁎ a The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52900, Israel b The Interuniversity Institute for Marine Sciences in Eilat, P.O. Box 469, Eilat 88103, Israel c Department of Natural Sciences, City University of New York, Baruch College, P.O. Box A-0506, 17 Lexington Avenue, New York, New York 10010, United States d Department of Evolution, Systematics and Ecology, Hebrew University of Jerusalem, Edmund Safra Campus, Givat Ram, Jerusalem 91904, Israel e Marine Biology Department, The Leon H. Charney School of Marine Sciences, University of Haifa, Mount Carmel, Haifa 31905, Israel abstract article info Article history: Received 17 March 2010 Received in revised form 7 July 2010 Accepted 8 July 2010 Keywords: Corallimorpharians Microhabitat Photoacclimation Photosynthesis Ultraviolet radiation Zooxanthellae Rhodactis rhodostoma and Discosoma unguja are the most common corallimorpharians on coral reefs in the northern Red Sea, where individuals of R. rhodostoma form large aggregations on intertidal reef flats and those of D. unguja occupy holes and crevices on the reef slope. Aside from these contrasting patterns of microhabitat, little is known concerning their mechanisms of photoacclimation to environmental conditions. We demonstrate here that different mechanisms of photoacclimation operate in both species and that these differences explain, in part, the contrasting patterns of distribution and abundance of these common corallimorpharians. Experimental exposure of the species' respective polyps to the synergistic effects of ultraviolet and photosynthetically active radiation revealed that endosymbiotic zooxanthellae protected the host R. rhodostoma from photooxidation damage. Zooxanthellae do so by reducing their chlorophyll pigment and cellular abundance, as well as by adjusting their efficiency of light absorption and utilization according to the level of irradiance. The host photoprotects its endosymbionts from harmful ultraviolet radiation (UVR) by synthesizing enzymatic antioxidants against oxygen radicals. In contrast, individuals of D. unguja utilize a behavioral mechanism of photoacclimation in which they physically migrate away from exposed areas and towards shaded habitats and thus avoid the damaging biological effects of UVR. We conclude that a combination of physiological and behavioral mechanisms appear to control microhabitat segregation between these corallimorpharian species on tropical reefs. These various mechanisms of local adaptation to environmental conditions may be largely responsible for the wide distributional ranges of some corallimorpharians, and may enable these common reef organisms to tolerate environments that are highly variable, both spatially and temporally. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Corallimorpharians are non-calcifying, evolutionarily important relatives of stony corals (Medina et al., 2006), although the exact relationship between Corallimorpharia and the Scleractinia remains under debate (Fukami et al, 2008). Increased understanding of the ecophysiology of corallimorpharians can provide insights into the evolution of corals from Mesozoic to recent forms (Stanley and Fautin, 2001) and their ability to survive drastic climatic changes. Coral reefs are among the most vital and biologically diverse ecosystems on the planet. Despite their great value, both ecological and socio-economical, however, coral reefs are severely threatened by anthropogenic global climate change (IPCC, 2001; IPCC, 2007). The steady rise in atmospheric CO 2 has led to higher sea surface temperatures (SST) (Hoegh-Guldberg, 1999; Hoegh-Guldberg et al., 2002, 2007) and lower pH levels. Increasing atmospheric CO 2 has been postulated to deplete the ozone layer (Austin et al., 1992), leading to an increase of ultraviolet radiation (UVR) on the oceans' surfaces (Harley et al., 2006). Understanding the protective mechan- isms used by marine organisms to mitigate the damage caused by UVR is particularly urgent today, as the thinning of atmospheric ozone by greenhouse gases has magnified the intensity of UVR reaching the sea surface in some areas (McKenzie et al., 1998). In clear tropical seawater, UVR penetrates to ecologically important depths (Gleason and Wellington, 1993). UVR radiation breaks down dissolved organic carbon (Hader et al., 2007), which is responsible for short-wavelength absorption in the water column. In addition, oceanic warming and acidification results in faster degradation of dissolved and particulate organic carbon (DOC, POC), thereby enhancing the penetration of UVR into the water column (Sinha and Hader, 2002). Short-term increases in UVR intensity under calm, clear water conditions may expose Journal of Experimental Marine Biology and Ecology 394 (2010) 53–62 ⁎ Corresponding author. Marine Biology Department, The Leon H. Charney School of Marine Sciences, University of Haifa, Mount Carmel, Haifa 31905, Israel. Tel.: + 972 4 8288790; fax: + 972 4 8282515. E-mail address: dtchernov@univ.haifa.ac.il (D. Tchernov). 0022-0981/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2010.07.007 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe