eLife assessment
The manuscript makes a fundamental contribution to our understanding of sugar release by symbiotic dinoflagellates, and is of broad interest for the fields of ecology, marine biology, and cell biology. The experiments, which combine algal culture with targeted metabolomics, transcriptomics and the application of inhibitors, provide substantial, though not entirely complete evidence for an acidic environment mimicking conditions reported for the intracellular organelle that hosts the symbiotic algae, leading to upregulation of algal cellulases, which in turn degrade the algal cell wall and thereby releasing glucose and galactose that can be used as a source of food by the coral host. This is a new idea and could significantly contribute to our understanding of photosymbiosis.