Variation in bleaching sensitivity of two coral species across a latitudinal gradient on the Great Barrier Reef: the role of zooxanthellae

Abstract

The ability of corals to cope with environmental change, such as increased temperature, relies on the physiological mechanisms of acclimatisation and long-term genetic adaptation. We experimentally examined the bleaching sensitivity exhibited by 2 species of coral, Pocillopora damicornis and Turbinaria reniformis, at 3 locations across a latitudinal gradient of almost 6 degrees on the Great Barrier Reef (GBR). Target bleaching temperature was reached by using a ramping rate of 0.2°C/h. We found that the bleaching sensitivity and recovery of both species differed between corals with clade D symbionts and those with clade C. However, in P. damicornis bleaching susceptibility corresponded more strongly with latitude than with zooxanthella type and hence, temperature history, suggesting that local adaptation has occurred. The observed bleaching sensitivity was shown by a decrease in photochemical efficiency (F v /F m ) in both species of coral. The rate of recovery in T. reniformis was highest in explants containing clade D symbionts. The occurrence of clade D in the northern section of the GBR may reflect a long-term response to high sea water temperatures, while the presence of clade D in low abundance in T. reniformis at Heralds Prong Reef and Percy Island may be a result of recent bleaching events.

Variation in photosynthesis and respiration in ­geographically distinct populations of two reef‑building coral species

“…Glynn et al 2001, Berkelmans & van Oppen 2006. We hypothesize that the occurrence of Symbiodinium D in Turbinaria reniformis at Lizard Island is an adaptive response to the higher temperature regime at this site compared with Davies Reef (Ulstrup et al 2006a). In contrast, the summer increase in dark respiration, and hence higher metabolic cost, of T. reniformis in the central region could reflect lower tolerance to in creased temperature (Coles & Jokiel 1977), and possibly irradiance, of the association with Symbiodinium C2.…”

“…In contrast, Turbinaria reniformis collected at Davies Reef harboured Symbiodinium C2 and at Lizard Island Symbiodinium D (Fig. 1a) sensu Ulstrup et al (2006a) (Fig. 1b).…”

“…The association between Pocillopora damicornis and Symbiodinium C1 was stable with respect to dark respiration across both spatial and temporal scales under nonbleaching conditions. The specificity of the association may, however, come at a cost during bleaching conditions, as a previous study demonstrated greater bleaching sensitivity of P. damicornis compared to T. reniformis irrespective of location (Ulstrup et al 2006a).…”

“…This suggests that thermal tolerance is controlled by local environmental conditions that corals have adapted to (West & Salm 2003) or that have caused the expression of distinct phenotypes. For instance, Ulstrup et al (2006a) found that Turbinaria reniformis in the central region of the Great Barrier Reef (GBR) harboured predominantly Symbiodinium C, whereas conspecifics in the northern region of the GBR harboured predominantly Symbiodinium D. Although this pattern is specific to the type of coral examined, the occurrence of Symbiodinium D in this instance may provide a selective advantage for T. reniformis in the relatively warmer environment of the northern GBR.…”

“…Partitioning of genetically distinct Symbiodinium communities in conspecific coral populations has been found to correlate with latitude , Rodriguez-Lanetty et al 2001, Savage et al 2002, Ulstrup et al 2006a) as well as with coral bleaching susceptibilities (Glynn et al 2001, Berkelmans & van Oppen 2006, Ulstrup et al 2006a). This suggests that thermal tolerance is controlled by local environmental conditions that corals have adapted to (West & Salm 2003) or that have caused the expression of distinct phenotypes.…”