Towards a trait-based understanding of Symbiodiniaceae nutrient acquisition strategies

Symbiodiniaceae is a diverse group of dinoflagellates that form symbioses with marine invertebrates, provisioning energy and nutrients for their hosts. Symbiont diversity is a well-known predictor of host fitness and stress tolerance. Yet, we have a limited understanding of the mechanisms by which in hospite symbiont communities are structured. Therefore, we hypothesized that phylogenetic differences in nitrogen assimilation may affect symbiont dominance, as has been demonstrated in phytoplankton communities. Here, we quantified species-specific rates of nitrate assimilation using stable isotope labeling, and investigated key traits that have been adopted in phytoplankton ecology to explain the fundamental concept of nitrogen acquisition strategies and size-related trade-offs in Symbiodiniaceae. Traits related to structure and function were measured to look for convergent ecological strategies. Despite the limited sample size, we could distinguish two groups among the five species of Symbiodiniaceae with features described by the phytoplankton's competition theories for resources: the affinity-adapted species which were associated with larger size, higher nitrogen content and nitrate affinity; and the velocity-adapted species which were small size, efficient in maximum nitrate assimilation, specific carbon assimilation rate, and growth. Our work supports the relevance of a functional trait-based approach to describe Symbiodinaceae diversity. The two contrasting nitrogen acquisition strategies identified may be fundamental to explain the composition and dynamics of Symbiodinaceae in hospite, stressing the importance of bottom-up mechanisms in shaping symbiont composition.

Automated summary

This study investigated the nitrogen assimilation rates of different species within Symbiodiniaceae and identified two groups with contrasting nitrogen acquisition strategies: the affinity-adapted species associated with larger size and higher nitrogen content, and the velocity-adapted species characterized by small size and efficient nitrate assimilation. These findings support the relevance of functional trait-based approaches in describing Symbiodiniaceae diversity and highlight the importance of bottom-up mechanisms in shaping symbiont composition.