Abstract
Early life stages of most coral species acquire microalgal endosymbionts (Symbiodiniaceae) from the environment, but whether exogenous symbiont uptake is possible in the adult life stage is unclear. Deep sequencing of the Symbiodiniaceae ITS2 genetic marker has revealed novel symbionts in adult corals following bleaching; however these strains may have already been present at densities below detection limits. To test whether acquisition of symbionts from the environment occurs, we subjected adult fragments of corals (six species in four families) to a chemical bleaching treatment (menthol and DCMU). The treatment reduced the native microalgal symbiont abundance to below 2% of their starting densities. The bleached corals were then inoculated with a cultured Cladocopium C1acro strain. Genotyping of the Symbiodiniaceae communities before bleaching and after reinoculation showed that fragments of all six coral species acquired the Cladocopium C1acro strain used for inoculation. Our results provide strong evidence for the uptake of Symbiodiniaceae from the environment by adult corals. We also demonstrate the feasibility of chemical bleaching followed by reinoculation to manipulate the Symbiodiniaceae communities of adult corals, providing an innovative approach to establish new symbioses between adult corals and heat-evolved microalgal symbionts, which could prove highly relevant to coral reef restoration efforts.
Highlights
Scleractinian corals prosper in oligotrophic waters by forming mutualistic relationships with microalgae (Symbiodiniaceae) that translocate photosynthate to their host [1]
Whilst environmental uptake of exogenous Symbiodiniaceae has been demonstrated in adult sea anemones [13] and octocorals [14], experimental evidence of switching in adult corals remains unconvincing with only a transient symbiosis being reported in adult colonies of Porites divarcata following bleaching [15]
We used chemical bleaching to remove >98% of the native symbiont cells from adult fragments of six coral species spanning four families (i.e. Diploastrea heliopora (Diploastraeidae), Dipsastraea pallida (Merulinidae), Echinopora lamellosa (Merulinidae), Platygyra daedalea (Merulinidae), Porites lobata (Poritidae) and Stylophora pistillata (Pocilloporidae)), which were successfully reinfected with a cultured Symbiodiniaceae strain
Summary
Scleractinian corals prosper in oligotrophic waters by forming mutualistic relationships with microalgae (Symbiodiniaceae) that translocate photosynthate to their host [1]. Changes in the relative abundance of Symbiodiniaceae species (shuffling) [10] and the acquisition of new symbionts from the environment (switching) [11] in adulthood are potential mechanisms for corals to adapt to increases in sea surface temperatures [4,5,6, 8,9,10,11]. We used chemical bleaching to remove >98% of the native symbiont cells from adult fragments of six coral species spanning four families (i.e. Diploastrea heliopora (Diploastraeidae), Dipsastraea pallida (Merulinidae), Echinopora lamellosa (Merulinidae), Platygyra daedalea (Merulinidae), Porites lobata (Poritidae) and Stylophora pistillata (Pocilloporidae)), which were successfully reinfected with a cultured Symbiodiniaceae strain.
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