Photosynthesis in the ocean is as important to the planetary climate as that of plants; and is performed by a wide range of cyanobacteria and eukaryotic algae, which possess chloroplasts derived through endosymbioses. Previously, I have used phylogenomics and in vivo localisation to show that chloroplasts of secondary red endosymbiotic origin, which form over 85% of total eukaryotic algal abundance in the modern ocean, are differentiated from plant and other chloroplasts via complex sets of nucleus-encoded and chloroplast-targeted proteins, derived from multiple sources including the endosymbiont, host, and horizontal acquisitions. I will use experimental and computational approaches to identify how the mosaic composition of the secondary red chloroplast proteome underpins its success in the modern ocean. This will include next-generation proteomic (LOPIT) characterisation of dinoflagellate chloroplasts, the least-studied secondary red chloroplast group; phylogenomic and spatial reconstruction of chloroplast proteomes across the algal tree of life, and environmental data from the Tara Oceans expedition; and functional characterisation of key proteins via CRISPR/CAS9 mutagenesis of the model diatom Phaeodactylum. I am particularly interested in characterising novel proteins connected to marine primary production; and temperature adaptations. In preliminary work, I have identified a chloroplast-to-mitochondria metabolite transporter unique to secondary red chloroplasts that underpins photo-acclimation under Fe limitation conditions; and a complete chloroplast glycolysis pathway specific to diatoms, which regulates physiology under conditions associated with high oceanic latitudes (continuous illumination, and low temperature). Future projects may seek to identify chloroplast proteins associated with specific oceanic regions (e.g., the Arctic), and modelling how chloroplast physiology will underpin algal responses to oceanic warming caused by anthropogenic climate change.