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Metacommunity ecology of Symbiodiniaceae hosted by the coral Galaxea fascicularis

Patricia H. Wepfer*, Yuichi Nakajima, Francis K. C. Hui, Satoshi Mitarai, Evan P. Economo

*Corresponding author:

ABSTRACT: Coral-algae symbiosis represent the trophic and structural fundament of coral reef ecosystems. However, despite global threats to coral reefs and the dependence of coral health and stress resistance upon such mutualisms, little is known about the community ecology of endosymbiotic Symbiodiniaceae. Concepts and methods from metacommunity ecology may help to understand the assembly and stability of symbiont communities and the mutualisms they comprise. In this study, we sampled colonies of the symbiont-generalist coral Galaxea fascicularis in southwestern Japan and assessed the effects of environmental and host factors on Symbiodiniaceae community composition, while simultaneously exploring residual correlations among symbiont types that may reflect non-random assembly processes such as species interactions. We metabarcoded the Symbiodiniaceae ribosomal internal transcribed spacer 2 (ITS2) region and characterized the endosymbiotic community using two different OTU identity cut-offs, and analyzed them with generalized dissimilarity modeling and joint species distribution modeling. We found that Symbiodiniaceae form discrete communities characterized by the dominance of ITS2-types C1, C21a, or D1, that are each associated with a different suite of co-occurring background types and tend to exclude each other in an endosymbiotic community. The communities showed modest responses to temperature, water depth, host genotype, polyp size, and bleaching status, and within the ITS2-types there was local sequence variation. After accounting for the effects of those variables, there remained residual correlations in community composition, pointing to the possibility that Symbiodiniaceae community assembly in corals may be structured by interspecific competitive or facilitating interactions rather than only exogenous variables.