Benjamin C. C. Hume, Christian R. Voolstra, Chatchanit Arif, Cecilia D’Angelo, John A. Burt, Gal Eyal, Yossi Loya, and Jörg Wiedenmann. Ancestral genetic diversity associated with the rapid spread of stress-tolerant coral symbionts in response to Holocene climate change. PNAS 2016. doi: 10.1073/pnas.1601910113.
Coral reefs supports complex ecosystems that reside in the nooks and crannies of their calcium carbonate skeletons. In addition to playing host to a diversity of marine organisms, corals harbor microbial symbionts that provide essential services such as food and defense that sustain the living coral animal. In particular, microalgae symbionts play the essential role of fixing carbon dioxide to produce the sugar that feeds and sustains their coral host.
Although coral commonly occupy warm tropical waters, an increase of a degree or two Celsius can be devastating. Warming leads to a phenomenon known as bleaching, in which pigmented microalgal sybmionts are smoked out of their coral homes, resulting in a characteristic loss of color and death-by-starvation of the coral. Due to global warming these bleaching events have become increasingly common, leaving scientists who care about coral reefs scrambling to understand what factors lead to reef resilience in times of warming.
The waters of the Persian Gulf (see map below) are on record as the warmest in the world, reaching summer maxima of 35 degrees Celsius (95 degrees Farenheit!), a temperature that would mean sure death for coral inhabiting other habitats. Moreover, the shift from a cool climate to present day temperatures occurred only 4,000 years ago, a mere 2,000 years after the present day boundaries of the Persian Gulf are thought to have been established. Hence the trajectory of Persian Gulf Coral is analogous to present day projections of global climate change, making the Persian Gulf a convenient “natural laboratory” in which to study the origin of extreme heat tolerance in coral.
Recent work by a multinational group of scientists lead by Benjamin C. Hume of the University of Southampton suggested tolerance of Persian Gulf coral might owe to its heat tolerant microalgal sybmiont Symbiodinium thermophilum. However, the origins of this superalgae remained unclear. To get at this question, Hume and colleagues surveyed reef sites within the Persian Gulf in addition to sites in the Red Sea and Gulf of Oman that are geographically isolated from the Persian Gulf but occur at the same latitudes. Their findings are reported in a recent issue of the Proceedings of the National Academy of Sciences.
Consistent with previous studies Hume and colleagues found that microalgae of Persian Gulf coral derived from a largely homogenous population of S. thermophilum. To a lesser extent they found examples of S. thermophilum among coral from the Gulf of Oman and Red Sea. However, these populations showed far greater genetic diversity than those from the Persian Gulf. Based on their finding of genetically disparate S. thermophilum in Red Sea and Gulf of Oman cites, Hume and colleagues argue that in the early days of the Persian Gulf, coral were offered a spectrum of S. thermophilum from which to pick and choose. However, in light of warming, only the most heat tolerant strains survived, leading to the present day homogeneity. An alternative explanation is that a population of S. thermophilum became trapped in the Persian Gulf and rapidly co-evolved with its coral partner-in-crime to become tolerant to extreme heat. However, this scenario is less likely given the relatively rapid shift in temperatures compared to generally much longer time scales required for evolutionary adaptation.
Hume and colleagues conclude that maintenance of a diverse pool of S. thermophilum might well have been the savior of the Persian Gulf coral. In light of rapidly warming seawater temperatures, they argue that preservation of latent genetic diversity of symbionts will provide coral a much needed leg up in adapting to near-term temperature increases. Whether chance encounters with supersymbionts will be enough to save the coral remains uncertain. Warming is not the only symptom of climate change. Rising sea levels that threaten to drown coral, and ocean acidification that dissolves their skeletons are some of the other hardships coral will face in the coming years—all the more reason to keep up the science.