Advancing Symbiodiniaceae Functional Ecology Through a Trait-Based Framework.

Symbiodiniacean dinoflagellates are fundamental to the functioning of coral reefs, underpinning primary production, nutrient cycling, and calcification through intimate intracellular symbioses with corals and other marine invertebrates. The identity and functional traits of these endosymbionts strongly influence host physiology, particularly thermal tolerance and stress resilience. Despite their ecological importance, Symbiodiniaceae have not yet been characterized within a formal functional ecology framework. Trait-based functional ecology enables standardized comparative analysis using metrics including functional diversity (richness, evenness, and divergence) and redundancy, which are critical for assessing ecosystem stability and vulnerability. Progress on this front requires elucidating clearly defined traits for Symbiodiniaceae. Here, we propose a standardized functional trait framework for these organisms. We identify key conceptual and methodological barriers that have hindered the integration of Symbiodiniaceae into formal descriptions of functional ecology, including unresolved species boundaries, limited trait standardization, and the context-dependent expression of traits in hospite versus in vitro. Building on principles from trait-based ecology, supported by empirical data and experimental measurements, we define and propose a set of 19 functional traits categorized into nine core functions: photosynthesis, photoprotection, cellular growth, population growth, energy reserves and composition, symbiotic relationship, nitrogen assimilation, ecological plasticity, and thermal tolerance. These traits capture fundamental dimensions of algal symbiont performance, including resource acquisition, stress tolerance, metabolic allocation, and host interaction, providing a foundation for calculating functional diversity metrics. Integrating Symbiodiniaceae into a functional trait framework will improve our capacity to assess functional redundancy, vulnerability, and resilience of coral reefs, ultimately strengthening forecasts of reef persistence under ongoing climate change.