Zooxanthellae Evolution

Symbiotic algae within the genus Symbiodinium have short generation times relative to their coral hosts, which may allow them to evolve rapidly and adapt to climate change. Will Symbiodinium evolution also confer adaptation to their host?

In the face of unprecedented global change, ecologists are attempting to understand how communities of species, interactions among species, and individual physiologies will respond to rapid environmental changes. Communities of species respond to global change through both ecological (changes in relative abundance) and evolutionary (changes in relative gene frequencies) mechanisms. Coral reef ecosystems are among the most threatened under a changing climate and are rapidly declining due to increasing levels of environmental stress. Many reef species (e.g. corals and other cnidarians, mollusks, sponges, flatworms, foraminiferans) are able to survive in nutrient-poor tropical oceans because they rely on symbioses with endosymbiotic dinoflagellates in the genus Symbiodinium. The intracellular symbionts provide amino acids and simple sugars to the host in exchange for nitrogenous waste products. When local ocean temperatures exceed a threshold level, this symbiosis breaks down, resulting in a loss of algal symbionts or pigmentation, termed “bleaching”. Relatively long generation times of host species make rapid adaptation to a changing climate unlikely, but theory suggests that rapid adaptation to increased temperatures by shorter-lived symbionts may prevent the breakdown of this symbiosis and confer adaptation to the holobiont (host and associated microbes). Our ability to predict the effects of increased sea surface temperature on coral reef communities is limited by incomplete knowledge of the functional response of the different symbiont types and the role that natural selection might play in generating phenotypes that are able to function under thermal stress.