by Dawn Barlow
Over just the past few decades ocean temperature has contributed to significant losses in global coral cover, and the extent to which corals can undergo physiological acclimatization or genetic adaptation to thermal changes remains uncertain. However, this information will be crucial for the effectiveness of conservation strategies and accuracy of projections of reef futures. This study conducted by Howells et al. (2013) investigates the potential for corals to acclimatize to temperatures that exceed historical thermal regimes. This is done by investigating several parameters—bleaching, mortality, Symbiodinium type fidelity, and reproductive timing—in coral colonies that have been transplanted between warm central regions and cool southern regions of the Great Barrier Reef for a period of 14 months.
This particular study investigates the thermal tolerance of both the coral hosts as well as their symbiotic partners, dinoflagellates known as Symbiodinium. Bleaching of corals takes place when the capacity of the photoprotective and antioxidant mechanisms in Symbiodinium cells to prevent and remove damaging reactive oxygen molecules from leaking into the coral tissue is exceeded, and dissociation takes place between the coral and Symbiodinium caused by oxidative stress. Both coral and photosynthetic Symbiodinium acclimatize seasonally to changing temperatures using various mechanisms. But this study discusses the fact that acclimatization is ultimately limited by the genetic make-up of both partners in the symbiosis, because once the capacity for acclimatization is exceeded, any more thermal tolerance requires adaptation through selection of tolerant genetic variants. The authors found that there was significant bleaching and mortality in all of the transplanted corals, in both regions. Additionally, there was a shift of Symbiodinium types in the transplanted coral in the warmer region, but this did not prove to be more photosynthetically efficient.
Howells et al. collected and reciprocally transplanted fragments of coral colonies between two inshore reef sites in the central and southern regions of the Great Barrier Reef. The study species of coral used for this study was Acropora millepora, as it is common and sensitive to bleaching. Symbiodinium types hosted by the different A. millepora populations differed between the two sites, with heat-tolerant Symbiodinium type D at the central site and heat-sensitive Symbiodinium type C2 at the southern site. The coral fragments were transplanted at the beginning of autumn and monitored at 1- to 4-month intervals over a period of 14 months, when coral condition, Symbiodinium type, photochemical efficiency, reproductive timing, and growth rate were all recorded.
At the southern site, the coral fragments transplanted from the central site remained healthy as temperatures began to decrease for the first several months while staying in the thermal range of the central region and even as it grew colder than the typical winter minimum. However, after the main cold period in the year a cold-water bleaching was observed, where 50% of the transplanted coral fragments suffered partial- or whole-colony mortality, and the coral fragments from the central region that were still alive had bleached. Bleaching was sustained even after seawater temperatures began to warm again. At the central region, the transplanted corals remained healthy as the seawater temperatures cooled. But then when temperatures began to rise again and exceeded the typical summer maximum for the southern region, the transplanted corals all bleached, and shortly after that 50% were dead and those that were alive had very little surviving tissue. The authors found that for the most part the Symbiodinium type remained constant in coral fragments for the duration of the study. However, in the southern corals that were transplanted to the central region, they observed that after bleaching there was a shift from Symbiodinium type C2 to type D. There was no difference in photosynthetic efficiency between the types—pigment was equally low in all bleached fragments whether they were associated with type D, C2, or a mix of the two. For both coral genotype transplants, coral growth was substantially slower regardless of whether or not the fragments showed visual signs of temperature stress. Some of the transplanted corals spawned up to a month earlier than their counterparts at their native sites.
Howells et al.’s reciprocal-transplant study demonstrated the poor ability A. millepora-Symbiodinium symbioses to acclimatize to temperature changes that exceeded the upper and lower limits of their local thermal regimes. This shows that the thermal tolerance of this particular species is likely determined primarily by genetic adaptations to local thermal regimes. The Symbiodinium types associated with the corals in the different regions studied probably had a strong influence on the thermal tolerance of the corals. Though there was a possibility that corals could withstand bleaching by changing Symbiodinium partners, at least in the short term, results from this study showed that changing Symbiodinium partners was not sufficient in enabling the host corals to cope with prolonged and extreme changes in temperature.
While these results highlight and confirm the importance of conserving coral populations, they also demonstrate that coral restoration through transplants is unlikely to be successful if the temperatures of the donor and recipient reefs are not correctly matched up. While it had been previously thought that it could be feasible to transplant corals from warmer regions to colder waters, this study demonstrates that corals also have a lower thermal limit. The authors suggest that future research is needed to investigate whether success of coral transplantation could be improved by using individual samples from multiple donor sites across ecological and thermal gradients.
Howells, E. J., Berkelmans, R., van Oppen, M. J., Willis, B. L., Bay, L. K. Historical thermal regimes define limits to coral acclimatization. Ecology 94.5, 1078–1088. http://www.researchgate.net/publication/249964630_Historical_thermal_regimes_define_limits_to_coral_acclimatization/file/e0b4952df008c0e9e4.pdf