Ontogeny and Dynamics of Cnidarian-Algal Symbioses
Mary Alice Coffroth Principal Investigator
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Symbioses between cnidarians and dinoflagellates in the genus Symbiodinium are widespread in the marine environment. Their importance to reef-building corals and reef nutrient cycles is well documented, but surprisingly little is known about the ontogeny of the symbiosis and the demographics of zooxanthellae populations within their hosts. This project will examine the dynamics of the symbiosis in gorgonian corals that produce azooxanthellate planula larvae, from initial uptake of zooxanthellae through to the host adult colony and host population. Data on genetic structure of symbiont communities and in situ field studies will be used to elucidate the mechanisms by which zooxanthella populations within a host are established and maintained. Diversity of zooxanthellae within and between gorgonian hosts will be examined to establish whether there are taxa specific to given habitats or given host species. This will identify the level of Symbiodinium-host specificity.<br/><br/>Molecular techniques (DNA fingerprinting, microsatellite polymorphic loci and restriction fragment length polymorphism in small subunit ribosomal genes) will be used to characterize algal genotypes in the initial zooxanthella colonization of planulae. Variation in zooxanthella populations within single host colonies and within and among host populations and species will also be characterized in this manner. In many gorgonian species multiple zooxanthella taxa initiate the symbiosis while only one dominant taxon is present in the adult symbiosis. Both in situ and lab controlled colonizations of azooxanthellate planulae of the gorgonians Plexaura kuna, Pseudoplexauraporosa, Pseudopterogorgia efisabethae and Briareum asbestinum will be followed to elucidate the mechanism by which these associations change over the course of host development. The zooxanthella populations in adult gorgonians will also be characterized to determine the diversity of the colonizing algal populations, whether changes in zooxanthella populations can be induced and whether zooxanthellae within a given host reflect host-algal specificity, environmental selection among zooxanthellae or simply a stochastic, founder effect.<br/><br/>Understanding the dynamics of zooxanthella populations has important ramifications for studying the symbiosis and understanding how the symbiosis responds to environmental changes. It is now recognized that zooxanthellae are a diverse group. If this diversity is widespread within single host colonies or if the association is in a state of continual flux, then changes in functional and physiologic traits may reflect changes in the within host algal community structure. An understanding of these processes is essential to understanding the symbiosis. For instance, physiologic adaptations to conditions such as temperature and light may in fact be mediated by the demographics of the algal symbionts. In this case it will be critical to examine each component of the symbiosis and consider the population dynamics and ecological responses of both the coral and the algae to understand and conserve reef ecosystems. The first step, however, is to quantify this diversity and address how this diversity is established.