Endozoicomonas-chlamydiae interactions in cell-associated microbial aggregates of the coralPocillopora acuta

Corals are associated with a variety of bacteria, which occur in the surface mucus layer, gastrovascular cavity, skeleton, and tissues. Some tissue-associated bacteria form clusters, termed cell-associated microbial aggregates (CAMAs), which are poorly studied. Here, we provide a comprehensive characterization of CAMAs in the coralPocillopora acuta. Combining imaging techniques, laser capture microdissection, and amplicon and metagenome sequencing we show that CAMAs: (i) are located in the tentacle tips and may be intracellular; (ii) containEndozoicomonas, Kistimonas(both Gammaproteobacteria), andSimkania(Chlamydiota) bacteria; (iii)Endozoicomonasmay provide vitamins to its host and use secretion systems and/or pili for colonization and aggregation; (iv)EndozoicomonasandSimkaniaoccur in distinct, but adjacent, CAMAs; (v)Simkaniamay rely on acetate and heme provided by neighboringEndozoicomonas. Our study provides detailed insight into coral endosymbionts, which will guide the assessment of their suitability for probiotic approaches to mitigate coral bleaching.