Cyanobacteria fuel ocean ecosystems through their ability to convert solar radiation and carbon dioxide into organic carbon. This photosynthesis sustains food webs, and the byproduct oxygen keeps our planet habitable. However, the concept of marine cyanobacteria being purely photoautotrophic has recently been questioned. Previous research has shown that many cyanobacteria utilize organic compounds1,2, and possess genes that enable the heterotrophic assimilation of organic carbon3. Organisms that combine both modes, photosynthetic carbon fixation and heterotrophic assimilation of organic carbon, are referred to as mixotrophs — a metabolic strategy thought to be crucial in sustaining oligotrophic marine ecosystems4. The assimilation of organic molecules could provide a way to obtain essential resources in low-nutrient environments, confer an advantage against competitors or serve as a way to save energy that would otherwise be used to synthesize these compounds with standard metabolic pathways2.