Colocalization and potential interactions of <i>Endozoicomonas</i> and chlamydiae in microbial aggregates of the coral <i>Pocillopora acuta</i>

Abstract

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 coral <i>Pocillopora acuta</i>. Combining imaging techniques, laser capture microdissection, and amplicon and metagenome sequencing, we show that (i) CAMAs are located in the tentacle tips and may be intracellular; (ii) CAMAs contain <i>Endozoicomonas</i> (Gammaproteobacteria) and <i>Simkania</i> (Chlamydiota) bacteria; (iii) <i>Endozoicomonas</i> may provide vitamins to its host and use secretion systems and/or pili for colonization and aggregation; (iv) <i>Endozoicomonas</i> and <i>Simkania</i> occur in distinct, but adjacent, CAMAs; and (v) <i>Simkania</i> may receive acetate and heme from neighboring <i>Endozoicomonas</i>. Our study provides detailed insight into coral endosymbionts, thereby improving our understanding of coral physiology and health and providing important knowledge for coral reef conservation in the climate change era.