Spatiotemporal profiling of a dominant coral's photo-endosymbiotic assemblage indicates that acclimation is supported by phenotypic plasticity of single cells

Abstract

Coral-associated dinoflagellates (Symbiodiniaceae) are photosynthetic endosymbionts that influence coral acclimation and adaptation, as indicated by photo-physiological plasticity (phenotypic variance) in response to environmental change. Symbiont shuffling (shifts in endosymbiont community composition), changes in endosymbiont cell density, and phenotypic plasticity have all been proposed as mechanisms to adjust to environmental change. However, few studies have been able to partition which of the three strategies were responsible for observed phenotypic variance. Therefore, we quantified the biodiversity, cell density, and phenotypic variance of single cells for Acropora pulchra-associated Symbiodiniaceae assemblages. Using a combination of metabarcoding and flow cytometry, we simultaneously characterized Symbiodiniaceae assemblages at the community (biodiversity), population (cell density), and individual level (phenotype) under natural environmental conditions to determine whether phenotypic variation of Symbiodiniaceae communities is concomitant with either symbiont shuffling, changes in cell density, or phenotypic plasticity. Symbiodiniaceae assemblages displayed season-specific phenotypic variance, while biodiversity was geographically structured and cell density showed limited data structure. Based on these patterns, we reveal that phenotypic plasticity of individual Symbiodiniaceae cells is the source of a phenotypic variation, thus indicating that phenotypic plasticity is a mechanism for rapid acclimation to mild environmental change.