A proton pump enhancing photosynthesis links phagocytosis to marine phytoplankton symbiogenesis

The chloroplasts of most marine eukaryotic phytoplankton are surrounded by intracellular membranes derived from ancestral secondary endosymbiotic events between heterotrophic unicellular eukaryotes and red microalgae. We report that the proton-pumping enzyme V-type H+-ATPase (VHA), ubiquitously used in eukaryotic intercellular digestion, is localized around chloroplasts of centric diatoms where VHA-activity significantly enhances photosynthesis over the full range of oceanic irradiances. Similar results in pennate diatoms, dinoflagellates, and coccolithophorids, but not green or red microalgae, imply a shared VHA-dependent mechanism in secondary endosymbiotic phytoplankton that enhances photosynthesis and conservatively contributes 7% of primary production in the ocean. Analogous VHA-dependent mechanisms in extant photosymbiotic marine invertebrates provide evidence for an adaptive advantage throughout the transition from endosymbiosis to symbiogenesis. ### Competing Interest Statement The authors have declared no competing interest.