Early Life Stages of a Common Broadcast Spawning Coral Associate with Specific Bacterial Communities Despite Lack of Internalized Bacteria

Coral-associated bacteria are critical for the well-being of their host and may play essential roles during ontogeny, as suggested by the vertical transmission of some bacteria in brooding corals. Bacterial acquisition patterns in broadcast spawners remain uncertain, as 16S rRNA gene metabarcoding of coral early life stages suggests the presence of bacterial communities, which have not been detected by microscopic examinations. Here, we combined 16S rRNA gene metabarcoding with fluorescence in situ hybridization (FISH) microscopy to analyze bacterial assemblages in Acropora tenuis egg-sperm bundles, embryos, and larvae following a spawning event. Metabarcoding results indicated that A. tenuis offspring <= 4-day-old were associated with diverse and dynamic bacterial microbiomes, dominated by Rhodobacteraceae, Alteromonadaceae, and Oceanospirillaceae. While FISH analyses confirmed the lack of internalized bacteria in A. tenuis offspring, metabarcoding showed that even the earliest life stages examined (egg-sperm bundles and two-cell stages) were associated with a diverse bacterial community, suggesting the bacteria were confined to the mucus layer. These results can be explained by vertical transmission of certain taxa (mainly Endozoicomonas) in the mucus surrounding the gametes within bundles, or by horizontal bacterial transmission through the release of bacteria by spawning adults into the water column.