Microbiome Structuring Within a Coral Colony and Along a Sedimentation Gradient

Abstract

Reef-building corals form complex relationships with a wide range of microbial partners, including symbiotic algae in the family Symbiodiniaceae and various bacteria. These coral-associated communities can be shaped to varying degrees by environmental context. Sedimentation can structure a coral’s microbial community by altering light availability for symbiotic algae, triggering the coral’s stress response, or serving as a reservoir for both pathogenic and essential bacterial and algal symbionts. To examine the influence of sedimentation on a coral’s microbiome, we used 16S rDNA and ITS-2 amplicon sequencing to characterize the bacterial and algal communities associated with the massive scleractinian coral Porites lobata across pairs of sites along a naturally occurring sedimentation gradient in Fouha Bay, southern Guam. Additionally, we investigate the influence of proximity to sediment on the coral colony scale, by sampling from the edge and center of colonies as well as the nearby sediment. The P. lobata colonies associated with several different genotypes of Cladocopium C15 algal symbionts and often harbored different genotypes within a single colony. However, the different Cladocopium genotypes showed no structuring according to colony position or location along the sedimentation gradient. Bacterial communities were largely consistent across the sedimentation gradient, however, some rarer taxa were differentially abundant across sites. Planococcaceae shows higher abundance closer to the river mouth in coral colonies in both the edge and center of colonies. Peredibacter also shows high abundance near the river mouth but only in sediment and the edges of the colony. We find sediment plays a larger role structuring bacterial communities at the colony scale compared to a coral’s position along the sedimentation gradient. Edge communities look more similar to the sediment compared to the center communities and are also enriched in similar pathways such as those involved in nitrogen fixation. We also find center samples to be dominated by Endozoicomonas compared to the edge, supporting a role for this taxon in structuring bacterial communities and limiting bacterial diversity in coral colonies. Together these results show the differential impact sedimentation can have between sections of the coral colony microhabitat.