Abstract
The endosymbiotic communities of corals are major contributors to the evolutionary success of scleractinian corals as the main reef‐building organisms in modern coral reefs. The most studied endosymbiotic taxa associated with scleractinian corals are dinoflagellate family Symbiodiniaceae (previously the genus Symbiodinium ), but corals are truly holobionts with a myriad of other microorganisms living together in what is known as the coral microbiome. The goal of this research was to characterize the genetic variability of the microbiome (based on 16S rRNA) and Symbiodiniaceae (based on ITS2) associated with the eurybathic Caribbean sheet coral Agaricia lamarcki along a depth gradient. This plating‐crustose coral is found along a wide depth distribution from shallow coastal reefs at approximately 10 m down to mesophotic depths at > 75 m in southwest Puerto Rico. Environmental conditions vary across this gradient, leading us to expect significant differences in zooxanthellae clades and microbiome community composition. We collected fragments of 36 A. lamarcki colonies along a shallow‐mesophotic depth gradient from 20 to 70 m at Black Wall in southwest Puerto Rico. PCR and Next‐generation sequencing targeting 16S rRNA and ITS2 genes were performed on DNA extracted from coral biomass containing coral tissue and surface mucus layer. Despite no differences in alpha‐diversity/richness metrics, our study found significant differences in coral‐associated bacterial community composition between shallower reefs (20–30 m) and mesophotic reefs (50–70 m). Contrastingly, we found no significant differences in the Symbiodiniaceae composition of A. lamarcki across the depth gradient, indicating a uniform algal endosymbiont community in this location. Given the increasing loss of shallow water coral reefs, understanding coral adaptation mechanisms to mesophotic environmental conditions becomes ever more important to establish proper management and conservation efforts.