Despite a growing interest in mesophotic coral ecosystems (MCEs), information on the photosynthetic endosymbionts (genus Symbiodinium) associated with scleractinian corals inhabiting deep reef ecosystems is sparse. Here, the deep-water Symbiodinium diversity is assessed from 10 different coral genera at a depth range of 45 to 70 m on the Great Barrier Reef (GBR), Australia. Symbiodinium identity was established using denaturing gradient gel electrophoresis (DGGE) fingerprinting of the internal transcribed spacer region 2 (ITS2) of the ribosomal DNA. Except for the novel Symbiodinium type C131 (found in Porites), all Symbiodinium types have previously been identified in shallow reef corals across the Pacific. Specimens of Seriatopora, Montipora, and Porites harboured similar symbionts as reported in shallow water (e.g. C3n, C3n-hh, C15, and C17), thus adhering to patterns of host-specificity across a wide depth range. However, several other Symbiodinium types were found to transcend previously established patterns of host-specificity at mesophotic depths. For example, 'host-specialist' types C3i and C3k (previously only reported in Acropora spp.) were found here to associate with a range of different genera (Leptoseris, Pachyseris, Fungia, and Echinophyllia). Although limited in sample size, this preliminary survey indicates that mesophotic habitats on the GBR may not represent an isolated community in terms of Symbiodinium diversity, which has significant relevance to their potential to act as refugia. Moreover, the present study identifies the need to examine symbiont diversity across broad environmental ranges (including MCEs) in order to gain an accurate understanding of symbiosis specificity and distribution range of specific coral-Symbiodinium associations.
KEY WORDS: Symbiodinium · Mesophotic · Deep reefs · Coral · ITS2 · DGGE · Great Barrier ReefResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 439: 117-126, 2011 118 zation of Symbiodinium into cladal (e.g. Rowan & Powers 1991, Baker & Rowan 1997 and subcladal types (e.g. van Oppen et al. 2001, LaJeunesse 2002 has greatly enhanced our understanding of the symbiosis. Studies of Symbiodinium diversity on broad latitudinal or longitudinal gradients have shown distinct biogeographical patterns (e.g. LaJeunesse et al. 2003, Silverstein et al. 2011), whereas more local or species-specific studies have highlighted ecological zonation, physiological diversification, and host-specificity of distinct symbionts (e.g. Rowan & Knowlton 1995, LaJeunesse et al. 2003, 2004, Iglesias-Prieto et al. 2004, Sampayo et al. 2007, Frade et al. 2008a.Community-wide shifts in symbiont diversity occur with increasing depth (LaJeunesse 2002, LaJeunesse et al. 2003, 2004, but this observation is likely to be, in part, the result of host community composition changes over depth. Nonetheless, studies focusing on single coral species show a similar pattern, and depth zonation of symbionts has been shown to occur on a cladal...