Symbiont identity impacts prokaryotic microbiome dynamics during heat stress in a model system for corals

The coral microbiome is highly complex, and interactions between microbiome members have been proposed as an important component of coral thermotolerance. However, establishing causal links among specific microbiome members remains difficult in corals because these communities are diverse and difficult to manipulate experimentally. We used Aiptasia, an emerging coral model system, to test how algal symbiont identity influences the structure and dynamics of the prokaryotic microbiome during heat stress. We generated clonal Aiptasia lines hosting two well-defined strains of photosynthetic algae in the family Symbiodiniaceae. We exposed these animals to an acute thermal stress assay while tracking prokaryotic dynamics with 16S sequencing. Among heat-stressed animals, algal strain was the strongest driver of prokaryotic community composition. We also identified line-associated indicator taxa that may be linked to differences in bleaching resistance. Finally, the more thermally sensitive host-algal association showed greater inter-sample dissimilarity in prokaryotic community structure under heat stress, suggesting that sustained microbiome variability may characterize more stress-sensitive cnidarian holobionts. These results suggest algal symbionts may shape bleaching responses not only through effects on host physiology, but also through their influence on prokaryotic microbiome dynamics.