Symbiotic Foreclosure: coral bleaching predictions and a potential acclimation mechanism

NOAA—the National Oceanic and Atmospheric Administration—issued a press release on September 22nd declaring coral bleaching likely in the Caribbean.  NOAA reports that:

With temperatures above-average all year, NOAA’s models show a strong potential for bleaching in the southern and southeastern Caribbean through October that could be as severe as in 2005 when over 80 percent of corals bleached and over 40 percent died at many sites across the Caribbean.

Comparing future bleaching outcomes to 2005 is an especially bad sign. 1998 and 2005 were the most thermally destructive years for coral reefs since record keeping began in 1880, damaging reefs in the Indian Ocean, Western Pacific and the Caribbean1. Reefs in the Indian Ocean and in parts of the Pacific have begun to recover after the particularly well-known 1998 bleaching event, yet the situation in the Caribbean has worsened due to regional mass bleaching, and the general lowering of reef resilience through sedimentation, nutrient pollution, and over-exploitation1.


Shallow corals are capable of feeding heterotrophically, that is, snagging bits of organic matter and plankton out of the watercolumn, but they overwhelming make their living by playing host to photosynthetic, intracellular dinoflagellate algae, known collectively as zooxanthellae (Symbiodinium spp.). These algal symbionts can be biochemically compromised by heat stress and can then be expelled by the host, an event known as bleaching; the coral will inevitably starve unless it can recruit more symbionts.  Bleaching can be caused by other factors as well2, e.g. heat/cold shock, heavy metals, prolonged darkness, among others; however, thermal stress appears to be a main factor, especially in the regional-scale bleaching that NOAA is predicting. The connection between increased seawater temperature and bleaching has been confirmed by laboratory studies and observed in the field3, particularly after El Nino/Southern Oscillation climatic events.  A recent article published in the New York Times generated some discussion on the NOAA-administered Coral List in the past couple of days in regards to designating 2010 as an El Niño or La Niña year.  We’ve been in a La Niña (characterized by cool temperatures in the Pacific) since May, but earlier in 2010, an El Niño (characterized by warm temperatures) was present.

The planet, including the ocean, is warming4.  One possible mechanism for corals to adapt to hotter climate regimes is the idea of ‘symbiont shuffing.’  Not all types of zooxanthellae are created equal. Some groups—namely type D—are better at managing higher temperatures without expulsion from the host coral; more importantly, shuffling to type D has resulted in increased thermal tolerance in at least one species of scleractinian (stony, reef-building coral).  But don’t excited yet:  we do not know if this symbiont switching is widespread, if particular coral species are capable of this at all, or if this phenomenon may incur some sort of physiological cost.  A study in PLoS ONE5 on the Great Barrier Reef worked to clarify this last tenet.  Jones and Berkelmans found in lab experiments that Acropora millepora corals grew 29% slower with type D symbionts, than those with C2 symbionts—a thermally sensitive type.  In the field, this skeletal growth difference was 38%.  If reefs acclimate via this mechanism, growth rates would be depressed if this trend was widespread among reef-building species.  How quickly certain types of corals grow is an extremely important facet of recovery after a disturbance; the authors point out that without timely coral regrowth, phase shifts can occur to an algal-dominated states within coral reef ecosystems.  This has happened at locales in the Caribbean, albeit for different reasons (the dieoff of a dominant grazer).  So although potentially beneficial in terms of adapting to increased temperatures, a switch to type D does not seem to be a boon to coral physiology, at least in terms of growth. A bleaching event during this study dropped growth rates across all symbiont types by more than 50% for a year and a half, showing (not surprisingly) that loss of their zooxanthellae has a larger impact than shuffling zooxthanellae types. Thermal stress coupled with the physiological cost associated with this acclimation mechanism are expected to interact, possibly further compromising reef resilience in a warmer world. Something that I’d like to add is that this shuffling is not known to ameliorate the effects of ocean acidification, which refers to the alteration of seawater chemistry (really a change in the carbon cycle itself) in a way that causes coral calcification to become more energetically difficult.

CO2 levels are expected to exceed 500 ppmv by 21006.  This level of change has not been seen in at least the past 420 thousand years, perhaps longer6.  The long-term fate of these extremely biodiverse oceanic centres is yet unknown. These animals are enormously resilient in one form or another on evolutionary and geological timescales, but the sheer rate of this environmental change is huge (2-3 orders of magnitude faster than most changes in the past 420 kyr6) and we may see drastic changes in the reef ecosystem functionality well within this century.

This post was chosen as an Editor's Selection for ResearchBlogging.org

Image 1: The Batman Comic Generator (this particular representation was inspired by A Replicated Typo‘s icon at Research Blogging).  Image 2:  Bleaching fire coral at Flower Gardens (Gulf of Mexico). NOAA-FGBNMS).

References:

1.  Wilkinson C (ed) (2008)  Status of coral reefs of the world: 2008. Global Coral Reef Monitoring Network and Rainforest Research Centre, Townsville, Australia, 296 p.
2.  Douglas AE (2003). Coral bleaching–how and why? Marine pollution bulletin, 46 (4), 385-92 PMID: 12705909
3.  Hoegh-Guldberg O (1999) Climate change, coral bleaching, and the future of the world’s coral reefs. Marine and Freshwater Research 50:839-866
4.  Solomon SD et al. (eds) (2007) Climate change 2007:  The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge UK and New York USA.
5. Jones, A., & Berkelmans, R. (2010). Potential Costs of Acclimatization to a Warmer Climate: Growth of a Reef Coral with Heat Tolerant vs. Sensitive Symbiont Types PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010437
6. Hoegh-Guldberg O, Mumby PJ, Hooten AJ, Steneck RS, Greenfield P, Gomez E, Harvell CD, Sale PF, Edwards AJ, Caldeira K, Knowlton N, Eakin CM, Iglesias-Prieto R, Muthiga N, Bradbury RH, Dubi A, & Hatziolos ME (2007). Coral reefs under rapid climate change and ocean acidification. Science (New York, N.Y.), 318 (5857), 1737-42 PMID: 18079392