Here we performed pulse-chase experiments with 13C-enriched dissolved inorganic carbon, followed by TEM and quantitative NanoSIMS isotopic imaging to visualize photosynthetic C assimilation by individual symbiotic dinoflagellates and subsequent translocation to their Orbulina universa host.NanoSIMS image processing was carried out as described in LeKieffre et al. (2017) and Nomaki et al. (2018). Briefly, TEM images were aligned with corresponding NanoSIMS 12C14N- images (Online Resource 1) using the software Look@NanoSIMS (Polerecky et al. 2012), which allows a user to hand-draw regions of interest (ROIs) corresponding to different organelles (e.g., dinoflagellate starch grains, foraminiferal lipid droplets, and fibrillar bodies). For each type of organelle and each time point, the average 13C-enrichment and its standard deviation were calculated based on 3 replicate foraminifera (except for the 6 h and 30 h time points, where only 2 replicates were available). The ROIs drawn on TEM images were also used to assess the relative abundance (in %) of lipid droplets in the foraminiferal endoplasm and starch grains in the dinoflagellate cytoplasm, respectively. Lipid droplet abundance was determined as the number of pixels occupied by lipid droplets divided by the total number of pixels of foraminiferal endoplasm. Starch grain abundance was determined as the number of pixels of occupied by starch grains divided by the total number of pixels covering dinoflagellate cytoplasm.

Supplement to: LeKieffre, Charlotte; Spero, Howard J; Russell, Ann D; Fehrenbacher, Jennifer; Geslin, Emmanuelle; Meibom, Anders (2018): Assimilation, translocation, and utilization of carbon between photosynthetic symbiotic dinoflagellates and their planktic foraminifera host. Marine Biology, 165(6)