Reef Corals: Symbiotic Dinoflagellate/Host Combinations and their Physiological Response to Environmental Change

University Of Georgia Research Foundation Inc, Athens GA

Investigators

Abstract

The growth, calcification, nutrition, reproduction, health and, in many cases, morphological form of reef corals and related cnidarians is the result of their mutualism with symbiotic dinoflagellates of the genus Symbiodinium, often referred to as "zooxanthellae". Several different species of Symbiodinium, as well as dozens of genetically different types within at least five molecular "clades" have been described within the past 30 years, with at least a quarter of coral species able to harbor more than one type of Symbiodinium within their gastrodermal cells. New molecular techniques have enabled coral reef biologists to make rapid progress in distinguishing genomic similarities and differences among Symbiodinium symbionts, but research on differences in algal physiology and its repercussions on host coral physiology (photosynthesis, growth rates, reproduction, and general health of the host) lags far behind. Without such information, it is impossible to project what benefits to the symbiosis (or coral host) come from having which specific combinations of Symbiodinium. Coral reefs comprise one of the world's most diverse ecosystems, both in terms of number of species and complexity of interactions among genera. Yet, coral reefs worldwide are deteriorating: the coral colonies that provide reef framework and habitat to multitudes of fish and invertebrates are decreasing in percent cover, experiencing unprecedented incidents of disease, and showing obvious signs of stress largely in response to ocean warming. Especially alarming is the increased occurrence of coral bleaching in which symbionts are lost as the principal contributors of carbon (from photosynthesis) for host growth, reproduction and development. Upon severe bleaching, when the white skeleton shows through the animal tissue as symbiont densities decline, coral tissue biomass and energy stores decrease, host growth ceases, reproduction is impaired, and portions or entire colony may die. It is also thought that stressed (i.e. bleached) corals are more susceptible to disease. Understanding of the causative factors for the decline of corals and the processes that might promote their recovery is very poor. Particularly little is known the of the complexity and specificity of the integration of the different types of Symbiodinium with their hosts, especially which combinations of symbionts lead to greater host growth, reproduction and survival and how such optimal combinations are established and maintained. This research is designed to resolve the complexities of the coral host/symbiont relationship, in terms of documenting the ability of different symbionts: (1) to provide nutrition to their host (photosynthesis of symbiont and translocation of photosynthate from alga to host), (2) to influence growth of the host, (3) to stably inhabit (=infect) different hosts, (4) to provide protection from ultra-violet light by the production of various 'sun screens', (5) to survive periods of higher-than-normal temperatures, (6) to photoadapt to different light conditions. The overall goal of the experiments is to be able to model seasonal and long-term changes in symbiont-host combinations, especially in regard to changes in the intact system in response to environmental change that might lead to greater or diminished physiological performance (including survival) of the intact association. The data will allow us to predict which associations are capable of surviving conditions expected in the world's tropical oceans over the next 50-100 years of global warming, and whether "switching" to algal types with higher tolerance levels might be a viable outcome.