Recruit symbiosis establishment and Symbiodiniaceae composition influenced by adult corals and reef sediment

For most reef-building corals, the establishment of symbiosis occurs via horizontal transmission, where juvenile coral recruits acquire their algal symbionts (family Symbiodiniaceae) from their surrounding environment post-settlement. This transmission strategy allows corals to interact with a diverse array of symbionts, potentially facilitating adaptation to the newly settled environment. We exposed aposymbiotic Pseudodiploria strigosa recruits from the Flower Garden Banks to natal reef sediment (C−S+), symbiotic adult coral fragments (C+S−), sediment and coral fragments (C+S+), or seawater controls (C−S−) and quantified rates of symbiont uptake and Symbiodiniaceae community composition within each recruit using metabarcoding of the ITS2 locus. The most rapid uptake was observed in C+S+ treatments, and this combination also led to the highest symbiont alpha diversity in recruits. While C−S+ treatments exhibited the next highest uptake rate, only one individual recruit successfully established symbiosis in the C+S− treatment, suggesting that sediment both serves as a direct symbiont source for coral recruits and promotes (or, potentially, mediates) transmission from adult coral colonies. In turn, presence of adult corals facilitated uptake from the sediment, perhaps via chemical signaling. Taken together, our results reinforce the key role of sediment in algal symbiont uptake by P. strigosa recruits and suggest that sediment plays a necessary, but perhaps not sufficient, role in the life cycle of algal Symbiodiniaceae symbionts.