Unlocking subcellular imaging of a cnidarian photosymbiont Breviolum minutum , through expansion microscopy

A cnidarian photosymbiont alga, Breviolum minutum, is an emerging model to study symbiosis due its ability to colonise host in absence of light, and amenability to genetic and physiological manipulations. This alga undergoes subcellular reorganisation in response to stress conditions such as elevated temperature and nutrient deprivation. However, subcellular visualisation of this alga is challenging because of its broad spectrum autofluorescence (400-700 nm) and relatively small size (6-8 m). We developed a super resolution imaging, Expansion Microscopy (ExM) workflow - a hydrogel-based technique for mechanical enlargement of cells, that reveals previously inaccessible subcellular features in B. minutum. This ExM workflow presents a set of thermic and enzymatic conditions which enables 4-fold expansion of B. minutum, optical clearing of autofluorescence as well as the removal of its thick cellulose rich cell wall. We implemented a recently described platinum (II)-based tri-functional linker 1, to retain in situ hybridised oligonucleotides targeted to 18S rRNA within ExM hydrogel and exploited its azide reactive group for post-ExM fluorophore labelling (DBCO modification). We observed actin patches (a cytoskeletal feature) and calmodulin (a calcium binding signalling protein) that are not previously visualised in B. minutum. This approach overcomes some of the long-standing problems in visualisation of B. minutum using commonly available reagents and commercially available low-cost ExM compatible chemistries. The broader uptake of this tool for the visualisation of diverse species of photosymbionts will pave the way for fundamental discoveries underpinning cellular reorganisation in formation and breakdown of symbiosis.