Intracellular bacteria are common and taxonomically diverse in cultured and in hospite algal endosymbionts of coral reefs - The ISME Journal
Corals house a variety of microorganisms which they depend on for their survival, including endosymbiotic dinoflagellates (Symbiodiniaceae) and bacteria. While cnidarian–microorganism interactions are widely studied, Symbiodiniaceae–bacteria interactions are only just beginning to receive...
www.nature.com
Just published this year. So endosymbiotic Symbiodiniaceae dinoflagellates living inside the cells of corals and stuff... have bacteria living inside their own cells.
Long story short, according to the study it is not 100% clear what roles these bacteria may play, but could potentially be important in scavenging reactive oxygen species produced by photosynthetic activities of the dinoflagellates, and are therefore potential candidates for probiotics that could be supplied to corals to protect against/help fight off coral bleaching.