Manipulation of the Symbiodiniaceae microbiome influences symbioses and reproductive ecology in the host, Exaiptasia diaphana, over generations

Little is known of the role and function that Symbiodiniaceae-associated microbiota contribute to cnidarian symbioses. To identify how the bacterial and fungal communities associated with Symbiodiniaceae may affect the cnidarian holobiont Exaiptasia diaphana (Aiptasia), we treated cultures of the symbiodiniacean Breviolum minutum (SSB01) with antibacterial and antifungal compounds. We then inoculated asexually-derived aposymbiotic juvenile Aiptasia (G1 generation) with exposed B. minutum cultures and recorded holobiont characteristics over 30 days. We identified significant differences in the bacterial and fungal communities of larger G1 juveniles, when compared to smaller G1s, with both size classes asexually produced from the same G0 parents, suggesting a maturation of the associated microbial communities with development. Additionally, we observed distinct treatment differences in the onset of Symbiodiniaceae symbioses between these two G1 size classes. Control B. minutum cultures exhibited a significantly greater infection rate in larger G1 anemones when compared with cultures exposed to either antibacterial and antifungal compounds, whereas the opposite trend was documented in the smaller G1 anemones. While no differences were observed between algal photochemical parameters, or the growth and behavior of G1 juveniles, we observed a significant variation in the production of G2 juveniles between treatments. Overall, we document strong ecological implications of manipulating Symbiodiniaceae microbiota, not for the algae themselves, but for the cnidarian holobiont, as well as for maturation of Aiptasia individuals.