OrbiSIMS imaging identifies molecular constituents of the perialgal vacuole membrane of Paramecium bursaria with symbiotic Chlorella variabilis

The protist (mostly single-celled organisms), Paramecium bursaria, forms an intracellular symbiotic relationship with the single-celled algae, Chlorella variabilis; where P. bursaria provides nutrients (i.e. Ca2+, Mg2+ and K+), carbon dioxide for photosynthesis and protection from viruses, whilst C. variabilis provides oxygen, carbon fixation, and nutrients. Key to this successful relationship is the perialgal vacuole (PV) membrane, which surrounds C. variabilis and protects it from digestion by P. bursaria. The membrane is fragile and difficult to analyse using conventional methods therefore very little is known about the molecular composition. We used the OrbiSIMS, a new high-resolution mass spectrometer with sub-cellular 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 C. variabilis, and specific to the target samples containing PVs were found based on the chemical...