Global Free‐Living Symbiodiniaceae Biodiversity Mirrors Local Environments

ABSTRACT Aim For free‐living Symbiodiniaceae, we aim to synthesise current knowledge, identify gaps in our understanding of biogeography and conduct the first quantitative genetic analysis of biogeography at a global scale. Location Global. Taxon Free‐living dinoflagellates of the Family Symbiodiniaceae. Methods Publicly available sequences were used to characterise the free‐living Symbiodiniaceae community in the environment. Using the genetic ITS2 marker combined with the DADA2 pipeline, amplicon sequence variants (ASVs) were used to assess Symbiodiniaceae diversity, abundance and distribution patterns from local to global scales. Results Relative abundances, community composition and sequence diversity differed significantly between the wider Caribbean and Indo‐Pacific, within the Indo‐Pacific, and across our three study regions: Great Barrier Reef (GBR), Flower Garden Banks National Marine Sanctuary (FGBNMS) and Moorea in French Polynesia. Symbiodiniaceae community assemblage was most different between the GBR and FGBNMS, with a dominance of Cladocopium in the GBR and Breviolum in FGBNMS. There was also significant variability within these regions, as shown through a beta dispersion test. The highest diversity indices were found in the GBR, followed by Moorea and FGBNMS. Main Conclusions We found free‐living Symbiodiniaceae biogeography differed significantly between and within locations at the global scale. Importantly, these global patterns in diversity mirrored local‐scale patterns. These free‐living biogeographical patterns also followed in hospite symbiont community patterns. With most studies of free‐living diversity from the Indo‐Pacific, we highlight the need for expanded sampling efforts in unexplored regions such as the Indian Ocean. Given the potentially significant role of free‐living Symbiodiniaceae in coral acclimation to climate change, identifying and protecting these taxa should be a conservation priority.