The aim of this work is to document the complex nutritional strategy developed by kleptoplastic intertidal foraminifera. We study the mixotrophic ability of a common intertidal foraminifer, E. williamsoni, by (1) investigating the phylogenetic identity of the foraminiferal kleptoplasts, (2) following their oxygenic photosynthetic capacity, and (3) observing the modification in cellular ultrastructural features in response to photoautotrophic conditions. This was achieved by coupling molecular phylogenetic analyses and TEM observations with non-destructive measurements of kleptoplast O2 production over a 15-day experimental study. Results show that the studied E. williamsoni actively selected kleptoplasts mainly from pennate diatoms and had the ability to produce oxygen, up to 13.4 nmol O2 cell−1 d−1, from low to relatively high irradiance over at least 15 days. Ultrastructural features and photophysiological data showed significant differences over time, the number of lipid droplets, residual bodies and the dark respiration increased; whereas, the number of kleptoplasts decreased accompanied by a minor decrease of the photosynthetic rate. These observations suggest that in E. williamsoni kleptoplasts might provide extra carbon storage through lipid droplets synthesis and highlight the complexity of E. williamsoni feeding strategy and the necessity of further dedicated studies regarding mechanisms developed by kleptoplastidic foraminifera for carbon partitioning and storage.