Flexibility and Specificity in Coral-Algal Symbiosis: Diversity, Ecology, and Biogeography ofSymbiodinium

Abstract

Reef corals (and other marine invertebrates and protists) are hosts to a group of exceptionally diverse dinoflagellate symbionts in the genus Symbiodinium. These symbionts are critical components of coral reef ecosystems whose loss during stress-related "bleaching" events can lead to mass mortality of coral hosts and associated collapse of reef ecosystems. Molecular studies have shown these partnerships to be more flexible than previously thought, with different hosts and symbionts showing varying degrees of specificity in their associations. Further studies are beginning to reveal the systematic, ecological, and biogeographic underpinnings of this flexibility. Unusual symbionts normally found only in larval stages, marginal environments, uncommon host taxa, or at latitudinal extremes may prove critical in understanding the long-term resilience of coral reef ecosystems to environmental perturbation. The persistence of bleaching-resistant symbiont types in affected ecosystems, and the possibility of recombination among different partners following bleaching, may lead to significant shifts in symbiont community structure and elevations of future bleaching thresholds. Monitoring symbiont communities worldwide is essential to understanding the long-term response of reefs to global climate change because it will help resolve current controversy over the timescales over which symbiont change might occur. Symbiont diversity should be explicitly incorporated into the design of coral reef Marine Protected Areas (MPAs) where resistance or resilience to bleaching is a consideration.

Free-Living Tube Worm Endosymbionts Found at Deep-Sea Vents

“…Symbioses, notably those that are facultative, clearly have an impact on and may be a driving force of local microbial diversity in varied ecosystems (2,10). Indeed, the bacterial symbionts of the shrimp Rimicaris exoculata make up a major component of the surrounding microbial community at hydrothermal vents in the Atlantic Ocean (25).…”

Novel algal symbiont (Symbiodinium spp.) diversity in reef corals of Western Australia

“…Scleractinian corals typically associate with Symbiodinium in 4 sub-generic clades, A, B, C, and D (reviewed in Baker 2003), while symbionts in clades F and G are only rarely harbored by stony corals (Rodriguez-Lanetty et al 2004, LaJeunesse et al 2010a). Numerous sub-clade 'types' have also been identified within each Symbiodinium clade, most frequently using the internal transcribed spacer-2 (ITS-2) region of a multi-copy ribosomal DNA (rDNA) gene family (e.g.…”

“…In a given coral host, local environmental conditions (e.g. temperature, irradiance), symbiont availability, biogeography, and/or stress history (reviewed in Baker 2003) can all influence which Symbiodinium taxa numerically dominate a colony. Corals living at latitudinal extremes are particularly interesting in this respect, because they experience intense seasonal environmental changes which may promote diverse symbiont communities.…”

The distribution of Symbiodinium diversity within individual host foraminifera

“…Ecological theory suggests that competition between multiple symbiont lineages destabilizes mutualism by selecting for more virulent, less cooperative strains; this is disadvantageous to the host (Frank 1996). The adaptive bleaching hypothesis and symbiont shuffling together (Buddemeier and Fautin 1993;Baker 2003;Fautin and Buddemeier 2004) provide a possible explanatory counterbalancing benefit to this cost. A heterogeneous mix of symbiont types may offer the host a more flexible response to stress (Rowan 1998).…”

“…Symbiodinium displays wide genetic diversity, both within its many hosts and across multiple spatial scales (Baker 2003;Coffroth and Santos 2005;Stat et al 2006). This diversity groups into eight clades, lettered A through H, and within each of these clades further genetic diversity represents ecologically distinct lineages of Symbiodinium, hereafter referred to as ''types'' (reviewed in Coffroth and Santos 2005).…”