Berg, J. T.; Fifer, J.; Davies, S.; Bentlage, B.

Near-shore coral reefs in southern Guam (Mariana Islands) experience severe sedimentation, in particular during the wet season when rainfall and erosion are high. We sampled fragments of the reef-forming coral Porites lobata from opposite ends of a sedimentation gradient in Fouha Bay, southern Guam, during dry and wet seasons. Using DNA metabarcoding, we characterized the diversity and composition of P. lobata-associated Symbiodiniaceae and bacterial microbiome communities. As in many species of Porites, Symbiodiniaceae communities of P. lobata were dominated by variants of Cladocopium C15 with sites showing differences in Symbiodiniaceae communities attributable to variation in these Cladocopium C15 variants. Bacterial microbiomes of P. lobata were dominated by Endozoicomonadaceae, a family of putative coral bacterial endosymbionts involved in nutrient cycling. Site and seasonal differences in bacterial diversity and community composition were apparent. In close proximity to the mouth of the river draining into Fouha Bay, bacterial diversity was highest during the wet season when sedimentation is generally severe. Microbiome reorganization in response to sedimentation may explain this result, but we also found overrepresentation of bacteria associated with terrestrial origin close to the river mouth and/or during the wet season. Together these patterns highlight that coral Symbiodiniaceae and bacterial communities are both spatially and temporally structured in this disturbed system.