Abstract
BackgroundReef-building corals live in symbiosis with a diverse range of dinoflagellate algae (genus Symbiodinium) that differentially influence the fitness of the coral holobiont. The comparative role of symbiont type in holobiont fitness in relation to host genotype or the environment, however, is largely unknown. We addressed this knowledge gap by manipulating host-symbiont combinations and comparing growth, survival and thermal tolerance among the resultant holobionts in different environments.Methodology/Principal FindingsOffspring of the coral, Acropora millepora, from two thermally contrasting locations, were experimentally infected with one of six Symbiodinium types, which spanned three phylogenetic clades (A, C and D), and then outplanted to the two parental field locations (central and southern inshore Great Barrier Reef, Australia). Growth and survival of juvenile corals were monitored for 31–35 weeks, after which their thermo-tolerance was experimentally assessed. Our results showed that: (1) Symbiodinium type was the most important predictor of holobiont fitness, as measured by growth, survival, and thermo-tolerance; (2) growth and survival, but not heat-tolerance, were also affected by local environmental conditions; and (3) host population had little to no effect on holobiont fitness. Furthermore, coral-algal associations were established with symbiont types belonging to clades A, C and D, but three out of four symbiont types belonging to clade C failed to establish a symbiosis. Associations with clade A had the lowest fitness and were unstable in the field. Lastly, Symbiodinium types C1 and D were found to be relatively thermo-tolerant, with type D conferring the highest tolerance in A. millepora.Conclusions/SignificanceThese results highlight the complex interactions that occur between the coral host, the algal symbiont, and the environment to shape the fitness of the coral holobiont. An improved understanding of the factors affecting coral holobiont fitness will assist in predicting the responses of corals to global climate change.
Highlights
The obligate symbiosis between reef-building corals and unicellular algae of the genus Symbiodinium, commonly referred to as zooxanthellae, is a key feature of tropical coral reefs
Establishment and stability of symbioses with the different algal partners Newly settled, azooxanthellate polyps of A. millepora, which had been offered six different symbiont types, successfully established symbioses with Symbiodinium types C1, D and C2*/A in juvenile cohorts originating from both the Magnetic Island and Keppel Islands populations
Single Strand Conformation Polymorphism (SSCP) analyses of ethanol preserved squash preparations showed that juveniles exposed to a mixture of Symbiodinium C2* and A established symbioses with Symbiodinium A symbionts only
Summary
The obligate symbiosis between reef-building corals and unicellular algae of the genus Symbiodinium, commonly referred to as zooxanthellae, is a key feature of tropical coral reefs. The coral-zooxanthellae symbiosis is very sensitive to increases in temperature, and changes of as little as 1uC above the average summer maximum can lead to a breakdown of the symbiosis. This breakdown results in expulsion and/or degradation of the algal partner causing the phenomenon known as coral bleaching (reviewed by Coles and Brown [3]). Reef-building corals live in symbiosis with a diverse range of dinoflagellate algae (genus Symbiodinium) that differentially influence the fitness of the coral holobiont. The comparative role of symbiont type in holobiont fitness in relation to host genotype or the environment, is largely unknown. We addressed this knowledge gap by manipulating host-symbiont combinations and comparing growth, survival and thermal tolerance among the resultant holobionts in different environments
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