J. Phycol. 44, 1126–1135 (2008) Ó 2008 Phycological Society of America DOI: 10.1111/j.1529-8817.2008.00567.x CORRESPONDENCE BETWEEN COLD TOLERANCE AND TEMPERATE BIOGEOGRAPHY IN A WESTERN ATLANTIC SYMBIODINIUM (DINOPHYTA) LINEAGE1 Daniel J. Thornhill2 Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA Department of Biological Sciences, 101 Rouse Life Sciences Building, Auburn University, Auburn, Alabama 36849, USA Dustin W. Kemp Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA Brigitte U. Bruns Department of Plant Biology, University of Georgia, Athens, Georgia 30602, USA William K. Fitt Odum School of Ecology, University of Georgia, Athens, Georgia 30602, USA and Gregory W. Schmidt Department of Plant Biology, University of Georgia, Athens, Georgia 30602, USA Many corals form obligate symbioses with photo- climates correspond significantly with the photosyn- synthetic dinoflagellates of the genus Symbiodinium thetic cold tolerance of these symbiotic algae. Freudenthal (1962). These symbionts vary geno- Key index words: Astrangia poculata; biogeogra- typically, with their geographical distribution and phy; cold temperature stress; coral; molecular abundance dependent upon host specificity and tol- diversity; Oculina arbuscula; Symbiodinium; zoo- erance to temperature and light variation. Despite xanthellae the importance of these mutualistic relationships, the physiology and ecology of Symbiodinium spp. Abbreviations: ITS2, internal transcribed spacer remain poorly characterized. Here, we report that region 2; PAM, pulse amplitude modulated rDNA internal transcribed spacer region 2 (ITS2) defined Symbiodinium type B2 associates with the cnidarian hosts Astrangia poculata and Oculina arbus- cula from northerly habitats of the western Atlantic. The structural and trophic foundations for coral Using pulse-amplitude-modulated (PAM) fluoro- reef ecosystems are dependent upon the obligate metry, we compared maximum photochemical effi- mutualisms between reef-building corals and their ciency of PSII of type B2 to that of common dinoflagellate algal symbionts (genus Symbiodinium). tropical Symbiodinium lineages (types A3, B1, and Optimally, intracellular Symbiodinium provides the C2) under cold-stress conditions. Symbiont cultures coral host with 90% or more of its energy needs were gradually cooled from 26°C to 10°C to simu- through photosynthesis (Muscatine 1990). In the late seasonal temperature declines. Cold stress past few decades, however, episodes of increased decreased the maximum photochemical efficiency ocean temperatures have led to disruptions of these of PSII and likely the photosynthetic potential for important symbiotic associations as a result of ther- all Symbiodinium clades tested. Cultures were then mal stress (Hoegh-Guldberg 1999, Hughes et al. maintained at 10°C for a 2-week period and gradu- 2003). Therefore, considerable attention has been ally returned to initial conditions. Subsequent to low given to the possibility that thermal acclimatization temperature stress, only type B2 displayed rapid of corals may occur via acquisition of heat-tolerant and full recovery of PSII photochemical efficiency, dinoflagellate symbiont complements (e.g., Bud- whereas other symbiont phylotypes remained demeier and Fautin 1993, Baker et al. 2004, Berkel- nonfunctional. These findings indicate that the mans and van Oppen 2006, Goulet 2006, Thornhill distribution and abundance of Symbiodinium spp., et al. 2006a,b). An alternative conjecture is that and by extension their cnidarian hosts, in temperate corals might undergo selective adaptation including latitudinal shifts in distribution. This possibility would require host and symbiont resilience to 1Received 20 August 2007. Accepted 10 March 2008. considerable seasonal changes in temperature and 2Author for correspondence: e-mail [email protected]. 1126 COLD-TOLERANT SYMBIODINIUM 1127 diurnal light exposure compared to the more con- Carrington 2007). Seasonal fluxes in temperature, stant environment in the tropics (Muller-Parker and light, and nutrients are considerably more pro- Davy 2001). nounced in temperate versus tropical marine habi- Symbiodinium is a genetically diverse group, con- tats (e.g., Muller-Parker and Davy 2001). Despite sisting of eight divergent lineages or clades (desig- this, zooxanthellae densities in symbiotic anemones, nated A through H; Pochon et al. 2006) and for example, are apparently more stable and less numerous more taxonomically specific types seasonal in temperate than in tropical hosts (Muller- (reviewed in Baker 2003, Coffroth and Santos 2005, Parker and Davy 2001 and references within), raising Stat et al. 2006). However, relatively little is known questions about how these temperate symbioses about the physiological properties that distinguish function physiologically. the various types. Differential tolerance of some Tropical and subtropical coral reefs contain com- symbiont species to high temperature stress has plex communities of numerous Symbiodinium species been the subject of many recent reports (e.g., War- (e.g., Rowan and Knowlton 1995, van Oppen et al. ner et al. 1996, 1999, Rowan 2004, Tchernov et al. 2001, LaJeunesse 2002, LaJeunesse et al. 2003, 2004, Berkelmans and van Oppen 2006). Exposure Thornhill et al. 2006a,b). The diversity of Symbiodini- to high temperature stress is now recognized to gen- um types from temperate habitats is apparently erally culminate in chronic photoinhibition in the lower than that occurring in the tropics (LaJeunesse symbionts (Warner et al. 1999). Such thermally and Trench 2000, LaJeunesse 2001, Rodriguez- induced perturbations are likely triggered by Lanetty et al. 2001, 2003, Savage et al. 2002, Visram decreased photochemical and nonphotochemical et al. 2006). However, symbiotic diversity in many quenching of PSII excitation energy, which can be temperate regions and the physiological properties exacerbated by disabled repair of photodamaged of these symbionts remain largely unexplored. Here, PSII in some symbiont types (Warner et al. 1999). we examine the biogeography and photosynthetic Symbiotic dinoflagellates exposed to cold tempera- physiology of Symbiodinium spp. associated with two ture stress may also undergo chronic photoinhibi- scleractinian hosts from the temperate western tion (Saxby et al. 2003), corresponding to decreased Atlantic. The identities of Symbiodinium types from photosynthetic membrane fluidity, as has been doc- the corals A. poculata and O. arbuscula were charac- umented for many vascular plant species (Smillie terized using denaturing gradient gel electrophore- et al. 1988, Aro et al. 1990, Greer 1990, Krause sis (DGGE) and sequencing of the ITS2 region of 1992). Furthermore, low temperature stress might the rDNA. disrupt the host–symbiont association by mecha- It has been hypothesized that temperate zooxan- nisms akin to high-temperature-induced coral thellae should be able to withstand greater ranges bleaching (Steen and Muscatine 1987, Hoegh- of temperatures than their tropical counterparts Guldberg et al. 2005). (Muller-Parker and Davy 2001). Therefore, we tested The distribution of coral reefs appears to be pri- this hypothesis using PAM fluorometry to character- marily limited by a lower thermal threshold of ize the maximum quantum efficiency of PSII 18°C, light limitation, and decreased aragonite (Fv ⁄ Fm) and relative electron transport rates (ETR) saturation state in high-nutrient temperate waters of cultured temperate and tropical Symbiodinium (Dana 1843, Vaughan 1919, Birkeland 1988, Cross- types under cold-stress conditions. Such investiga- land 1988, Kleypas et al. 1999), although biotic tions of the relationship between Symbiodinium physi- interactions have also been implicated (Johannes ology and biogeography provide further insights et al. 1983, Miller and Hay 1996). While diversity of into the ecology and evolution of this important symbiotic cnidarians is low outside the tropics, dinoflagellate group. several species of symbiotic corals, anemones, and other cnidarians occur in temperate habitats MATERIALS AND METHODS (Schuhmacher and Zibrowius 1985). Similar to trop- Sample collection and Symbiodinium identification. In 2005, ical corals, temperate cnidarians hosting Symbi- the symbiotic hosts A. poculata (= A. danae,=A. astreiformis, odinium benefit from enhanced calcification and Peters et al. 1988) and O. arbuscula were surveyed from three growth rates (Jacques and Pilson 1980, Jacques et al. coastal locations along the eastern seaboard of the U.S., 1983, Miller 1995, Dimond and Carrington 2007), including a limestone and live-bottom reef near Gray’s Reef reduced nitrogen loss (Szmant-Froelich and Pilson National Marine Sanctuary (J Reef, Georgia, USA; 31.601° N, 80.791° W); an open bay site in Fort Wetherill State Park, 1984), and translocation of photosynthetically fixed Narragansett Bay (Rhode Island, USA; 41.478° N, 71.357° W); carbon (Szmant-Froelich 1981, Schiller 1993, Miller and a tidal river site in the lower Pettaquamscutt River, 1995). However, unlike the obligate associations Narragansett Bay (Rhode Island, USA; 41.449° N, 71.449° W; occurring in most tropical hosts, temperate symbi- Table 1). A map of the estimated distributions of these otic cnidarians typically associate facultatively with scleractinian species and sampling locations for this study is Symbiodinium and often survive without symbionts provided in Figure 1. Temperatures at these noncoral reef sites are always <18°C during winter months