Over the past ten years researchers have demonstrated that reef corals might adapt to the negative effects of elevated temperatures by replacing the dinoflagellate symbionts they commonly host with symbionts that better resist thermal stress. However, little research has examined the frequency or reliability of this adaptive mechanism in the wild.
Coral bleaching involves the breakdown in the symbiosis between reef-building coral and their obligate endosymbionts, dinoflagellates in the genus Symbiodinium. While bleaching can be triggered by a range of stimuli, mass bleaching events are most commonly induced by a synergistic effect between temperature and light. Even an increase as small as one degree C above long-term maximum temperatures can trigger bleaching. With projected warming trends due to global climate change, corals will chronically be exposed to temperatures greater than their bleaching thresholds unless they can adapt to increased temperatures.
Many observational and experimental studies suggest that not all coral-Symbiodinium combinations react equally to bleaching-inducing stresses and therefore variability exists upon which natural selection might act. Because many coral species have the potential to host different symbionts within an adult’s lifespan, much attention has focused on the potential for coral reefs to adapt to new climate extremes by changes in the frequency of distinct Symbiodinium lineages.
Symbiodinium Physiological Diversity
The genus Symbiodinium is genetically diverse and is divided into multiple divergent clades, each distinguished by a letter A-H. The first study to demonstrate that this diversity has bleaching-related consequences was Rowan et al.'s work in Caribbean Panama. The authors observed that, even within a single coral colony, regions of the colony hosting Symbiodinium symbionts in clade C might bleach, while those hosting clades A or B might not.
Similar observations have been made in Pacific corals. Before, during and after the 1998 bleaching event, Glynn et al. sampled Pocillopora damicornis in Pacific Panama. They noted that each of the 9 corals sampled that hosted clade C Symbiodinium bleached during the thermal stress while none of the 33 that hosted clade D bleached. Also, the frequency of clade D Symbiodinium increased after the bleaching event.
Experimental work has followed up on these observations. Working with Pocillopora verrucosa from Guam, Rowan placed colonies bearing either clade C or clade D Symbiodnium into tanks held at either a control temperature (28.5° C) or an elevated temperature (31.3° or 32.0° C). Colonies with clade C Symbiodinium in the 32° treatment showed evidence of heat-induced damage to the light reactions of photosynthesis, while those colonies hosting clade D Symbiodinium did not. From these results, Rowan concluded that C and D are adapted to different thermal regimes, and that clade D appears to be a high temperature specialist.
Studies have shown that some corals are capable of switching their dominant symbiont type within a colony's lifetime. For corals that can do it, shifting to a more thermally tolerant symbiont might allow them to survive long-term increases in environmental temperature. However, even within a species, some populations of coral show evidence of shifting symbionts, while others do not.
The increased thermal robustness provided by hosting clade D is not without some cost. Little and co-authors documented that juvenile Acropora tenuis hosting clade D Symbiodinium grew at half the rate of those hosting clade C in the same site. This is the only example of a trade-off for clade D thermal resistance yet published, and it is yet unknown if the growth limitations apply to A. tenuis adults or other species.
This adaptive mechanism exists. Is it important?
These studies strongly suggest both that there exists variability in the thermal tolerance of distinct coral-Symbiodinium symbioses and that in some cases environmental stress can raise the frequency of more tolerant symbioses. Thus, researchers have identified one mechanism by which coral reefs may adapt to climate change. However, without a broader perspective we have little ability to gauge the importance of this mechanism. Does this adaptive process happen frequently in the wild? Can we identify a predictable record of its occurrence?
In one attempt to address these issues, Baker and co-authors compared patterns of Symbiodinium distributions across four regions which they divided into two groups: 1) high-temperature reefs with a history of bleaching during the 1998 El Niño (Kenya, Persian Gulf) and 2) cooler reefs that did not suffer extensive bleaching in 1998. They found that clade D was common on the two high-temperature reefs but absent on the two cooler reefs. Based on the observed patterns, the authors proposed that thermally stressed reefs were adaptively shifting to clade D Symbiodinium and that such adaptive processes were a common feature of thermal stress in reefs around the world.
Though this is an important hypothesis, data to test it are relatively scarce. Here we survey a bleaching susceptible pair of table top Acroporid corals, Acropora hyacinthus and Acropora cytherea, for their Symbiodinium clade at two spatial scales: across the equatorial Pacific Ocean from the Philippines to Palmyra and at contrasting inner reef and outer reef sites. These data confirm Baker et al.’s model on a local scale, but suggest that on a regional scale, the complexities of coral reef adaptation likely go beyond temperature and bleaching history. regular expulsion or digestion of excess symbionts, and other mechanisms.
Between May, 2005 and August, 2006, we sampled sister species of table top Acroporid corals, Acropora hyacinthus and Acropora cytherea in four regions of the Pacific - Palmyra Atoll, the central Philippines, American Samoa, and Fiji. In American Samoa, we explicitly sampled across habitat boundaries in three sites, sampling both from thermally variable back-reef environments and the nearby fore-reef in each site. In Palmyra Atoll, the Philippines, and Fiji we sampled from a range of habitats, including back-reef and fore reef-environments.
Preliminary results suggest that clade D is more prevalent in warmer habitats within Samoa. However, we have not observed a similar trend on a regional scale. We did not find a consistent trend connecting clade D to regions with higher temperature variation, and more extreme heating events across the Pacific. Over the sixteen years since the first molecular study of Symbiodinium, we have gained a working knowledge of how Symbiodinium functional diversity plays out on local scale reefs, but we are still limited in our ability to draw general trends. By focusing on new ways to identify areas with functional diversity and new functionally distinct types we can maximize our understanding of Symbiodinium’s potential to aid coral adaptation to climate change. To do so, we will need environmentally focused reviews of the existing data, and a greater understanding of the functional genetic correlates of high temperature resistance in Symbiodinium.