The onset of microbial associations in the coral Pocillopora meandrina

Associations between healthy adult reef-building corals and bacteria and archaea have been observed in many coral species, but the initiation of their association is not understood. We investigated the onset of association between microorganisms and Pocillopora meandrina, a coral that vertically seeds its eggs with symbiotic dinoflagellates before spawning. We compared the bacterial communities associated with prespawned oocyte bundles, spawned eggs, and week old planulae using multivariate analyses of terminal restriction fragment length polymorphisms of SSU rRNA genes, which revealed that the composition of bacteria differed between these life stages. Additionally, planulae raised in ambient seawater and seawater filtered to reduce the microbial cell density harbored dissimilar bacterial communities, though SSU rRNA gene clone libraries showed that planulae raised in both treatments were primarily associated with different members of the Roseobacter clade of Alphaproteobacteria. Fluorescent in situ hybridization with an oligonucleotide probe suite targeting all bacteria and one oligonucleotide probe targeting members of the Roseobacter clade was used to localize the bacterial cells. Only planulae greater than 3 days old were observed to contain internalized bacterial cells, and members of the Roseobacter clade were detected in high abundance within planula tissues exposed to the ambient seawater treatment. We conclude that the onset of association between microorganisms and the coral P. meandrina appears to occur through horizontal uptake by planulae older than 79 h, and that uptake is preferential to members of the Roseobacter clade and potentially sensitive to the ambient seawater microbial community.