Reef-building Corals interact intricately with a variety of microorganisms, including bacteria and symbiotic algae from the Symbiodiniaceae family. The context of the environment has varied degrees of influence on these coral-associated communities. By changing the amount of light available to symbiotic algae, inducing the coral's stress response, or acting as a reservoir for both harmful and necessary bacterial and algal symbionts, sedimentation can shape a coral's microbial community. We used 16S rDNA and ITS-2 amplicon sequencing to characterise the bacterial and algal communities linked to the massive scleractinian coral Porites lobata across pairs of sites along a naturally occurring sedimentation gradient in Fouha Bay, southern Guam, in order to investigate the impact of sedimentation on a coral's microbiome. By taking samples from the perimeter and core of colonies as well as the neighbouring sediment, we also examine the impact of proximity to sediment on the coral colony scale. The Cladocopium C15 algal symbiont colonies of P. lobata frequently housed various genotypes within a single colony and were connected with a variety of genotypes. However, there was no evidence of structure among the various Cladocopium genotypes based on colony position or location along the sedimentation gradient. Although the diversity of bacterial communities varied across the sedimentation gradient, some uncommon taxa were more numerous at some sites than others. In coral colonies, Planococcaceae is more prevalent both at the colony's periphery and in its heart, which is closer to the river mouth. Near the river mouth, Peredibacter likewise exhibits significant abundance, but only in the silt and the colony's periphery. We discover that sediment, as opposed to a coral's location along the sedimentation gradient, has a greater impact on structuring bacterial populations at the colony scale. In comparison to the core communities, the edges are more similar to the sediment in appearance and are also richer in similar pathways, such as those engaged in nitrogen fixation. Additionally, we discover that Endozoicomonas predominates in centre samples as opposed to edge samples, confirming a role for this taxon in restricting bacterial diversity and organising bacterial communities in coral colonies. Together, these findings demonstrate the varied effects that sedimentation can have on distinct coral colony microhabitat sections
