OrbiSIMS Imaging Identifies Molecular Constituents of the Perialgal Vacuole Membrane of with Symbiotic .

The protist (mostly single-celled organisms), , forms an intracellular symbiotic relationship with the single-celled algae, , where provides nutrients (i.e., Ca, Mg, and K), carbon dioxide for photosynthesis and protection from viruses, while provides oxygen, carbon fixation, and nutrients. Key to this successful relationship is the perialgal vacuole (PV) membrane, which surrounds and protects it from digestion by . The membrane is fragile and difficult to analyze using conventional methods therefore very little is known about the molecular composition. We used the OrbiSIMS, a new high-resolution mass spectrometer with subcellular resolution imaging, to study the compartmentalization of endosymbionts and elucidate biomolecular interactions between the host and endosymbiont. Ions from the region of interest, close to , and specific to the target samples containing PVs were found based on the chemical mapping and masses of the ions. We show chemical localizations of oligosaccharides in close proximity of endosymbionts in . These oligosaccharides are detected in host-endosymbiont samples containing PV membrane-bound algae and absent in free-living algae and digestive vacuole (DV) membrane-bound algae in .