Many corals host thermally resistant symbionts in high-temperature habitat
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Physiologically distinct lines of dinoflagellate symbionts, Symbiodinium spp., may confer distinct thermal tolerance thresholds on their host corals. Therefore, if a coral can alternately host distinct symbionts, changes in their Symbiodinium communities might allow corals to better tolerate increasing environmental temperatures. However, researchers are currently debating how commonly coral species can host different symbiont types. We sequenced chloroplast 23 s rDNA from the Symbiodinium communities of nine reef-building coral species across two thermally distinct lagoon pools separated by ~500 m. The hotter of these pools reaches 35°C in the summer months, while the other pool’s maximum temperature is 1.5°C cooler. Across 217 samples from nine species, we found a single haplotype in both Symbiodinium clades A and D, but four haplotypes in Symbiodinium clade C. Eight of nine species hosted a putatively thermally resistant member of clade D Symbiodinium at least once, one of which hosted this clade D symbiont exclusively. Of the remaining seven that hosted multiple Symbiodinium types, six species showed higher proportions of the clade D symbiont in the hotter pool. Average percentage rise in the frequency of the clade D symbiont from the hotter to cooler pool was 52% across these six species. Even though corals hosted members of both the genetically divergent clades D and C Symbiodinium, some showed patterns of host–symbiont specificity within clade C. Both Acropora species that hosted clade C exclusively hosted a member of sub-clade C2, while all three Pocillopora species hosted a member of sub-clade C1 (sensu van Oppen et al. 2001). Our results suggest that coral–algal symbioses often conform to particular temperature environments through changes in the identity of the algal symbiont.
KeywordsSymbiodinium Host–symbiont specificity Acropora Pocillopora Pavona Platygyra Millepora Clade D water temperature
The authors would like to acknowledge the assistance of C. Birkeland, Dan Barshis, and Cheryl Squair at University of Hawaii; Peter Craig and Fale Tuilagi (US National Park Service, American Samoa); Seabird McKeon at the University of Florida; and Kirsten Oleson and Jason Ladner of Stanford University for their support in American Samoa; and the National Park of American Samoa. We would also like to thank the NSF Predoctoral fellowship program, the Woods Institute for the Environment, NOAA and the National Science Foundation for funding.
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