Bacterial Assemblages Associated with Two of the Most Common and Phylogenetically Divergent Reef-Building Corals in the Caribbean, Montastraea faveolata and Porites astreoides

This study investigates the bacterial assemblages found within the surface mucus layer (SML) of two common Caribbean coral species: Montastraea faveolata and Porites astreoides. Because coral-associated microbes (the holobiont) are essential for host health, defense, and potentially for adaptation to environmental change, understanding their baseline structure and stability is critical. The researchers sampled healthy corals across four sites in the Caribbean—ranging from St. Thomas to Belize—to determine if these corals harbor unique bacterial signatures and how those signatures shift across geographic and anthropogenic gradients.

The team employed both denaturing gradient gel electrophoresis (DGGE) fingerprinting and 454 bar-coded pyrosequencing to profile the 16S rRNA genes of bacteria associated with the coral SML. By collecting 52 total colonies across different levels of proximity to the mainland (2.5 km to 20 km), the authors were able to assess both host-specific bacterial signatures and the influence of environmental conditions on those communities. This approach allowed the team to compare the taxonomic diversity and specificity of the microbiota between these two phylogenetically distinct corals.

The researchers found that M. faveolata and P. astreoides host distinct, host-specific bacterial assemblages. P. astreoides displays relatively low microbial diversity, being heavily dominated (>75%) by the genus Endozoicomonas. In contrast, M. faveolata harbors a more diverse bacterial community, including higher abundances of potential nitrogen-fixing bacteria like those in the orders Rhizobiales and Burkholderiales, especially at the more pristine sampling site in Belize. The study also observed that, across both species, sites closer to the mainland (which may be subject to higher anthropogenic stress, such as runoff and sedimentation) showed shifts toward higher relative abundances of bacteria often associated with disease. This highlights the sensitivity of the coral microbiome to its surroundings, suggesting that these bacterial profiles could eventually serve as bioindicators for reef health and early signs of environmental distress.