Figure 20.19 Can Corals Reacquire Dinoflagellate Endosymbionts Lost to Bleaching?
Corals are important components of marine ecosystems, yet they are under threat due to various anthropogenic activities (e.g., agricultural run-off) and climate change. The health of corals depends on their symbiotic relationship with photosynthetic dinoflagellates in the genus Symbiodinium. Coral bleaching is a result of the breakdown of this symbiosis. In this elegant study, Cynthia Lewis and Mary Coffroth induced bleaching in an octocoral (Briareum sp.) and then studied the recovery from bleaching. Specifically, they were interested in whether recovering corals take up new endosymbionts (i.e., from an exogenous source) or simply are repopulated with Symbiodonium populations that remained in the host coral at low levels. To test the hypothesis that corals can take up new symbionts from their environment, the researchers first examined natural levels of symbiont. The coral colonies were bleached by exposure to complete darkness for 12 weeks, and symbiont levels in the bleached coral colonies were measured before returning them to the light. For six weeks all of the bleached colonies were left in the light, but half (i.e., the experimental group) were exposed to a novel Symbiodinium strain B211. After three and six weeks the researchers measured symbiont levels in the experimental and control (i.e., no exposure to Symbiodinium strain B211) coral colonies. Results showed that the coral colonies in the experimental group contained Symbiodinium strain B211. This finding allowed the researchers to conclude that, following bleaching, Briareum sp. corals are indeed able to take up symbiotic Symbiodinium from their environment. Interestingly, the researchers also documented that there appears to be a preference for certain Symbiodinium strains. Three colonies that took up the novel B211 strain were later found to have replaced it with one of their original Symbiodonium strains. Furthermore, the researchers also documented shifts in the relative abundance of symbiont strains, with one of the original strains occurring in a much higher percentage of coral colonies following bleaching. Although this study was limited to laboratory coral populations, recently Jones et al. (2008) have studied the response to coral bleaching in natural populations. They also documented a shift in the type of Symbiodinium strain, in this case correlated with differential tolerance of the strains to heat stress. It is possible that such symbiont flexibility will facilitate the maintenance and recovery of coral reef ecosystems in the future, as climate change increases the temperature of the oceans.
Original Paper
Lewis, C. L., and M. A. Coffroth. 2004. The Acquisition of Exogenous Algal Symbionts by an Octocoral After Bleaching. Science 304: 1490–1492.
http://dx.doi.org/10.1126/science.1097323
Links
Jones, A. M., R Berkelmans, M. J. H van Oppen, J. C. Mieog, and W. Sinclair. 2008. A community change in the algal endosymbionts of a scleractinian coral following a natural bleaching event: field evidence of acclimatization. Proceedings of the Royal Society B 275: 1359–1365.
http://dx.doi.org/10.1098/rspb.2008.0069
Henkel, T. P. 2010. Coral Reefs. Nature Education Knowledge 1(11): 5.
http://www.nature.com/scitable/knowledge/library/coral-reefs-15786954
Gaskill, M. 2010. Coral bleaching goes from bad to worse. Nature News.
http://dx.doi.org/10.1038/news.2010.621
Stanford University: Coral Bleaching
http://www.stanford.edu/group/microdocs/coralbleaching.html
Kimball’s Biology Pages: Symbiosis
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Symbiosis.html
R. Berkelmans, and M. J. H. van Oppen. 2006. The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change. Proceedings of the Royal Society B 273: 2305–2312.
http://dx.doi.org/10.1098/rspb.2006.3567
Abrego, D., K. E. Ulstrup, B. L. Willis, and M. J. H van Oppen. 2008. Species–specific interactions between algal endosymbionts and coral hosts define their bleaching response to heat and light stress. Proceedings of the Royal Society B 275: 2273–2282.
http://dx.doi.org/10.1098/rspb.2008.0180
LaJeunesse, T. C., et al. 2010. Host–symbiont recombination versus natural selection in the response of coral–dinoflagellate symbioses to environmental disturbance. Proceedings of the Royal Society B 277: 2925–2934.
http://dx.doi.org/10.1098/rspb.2010.0385