Species in the cyanobacterial genus Trichodesmium are considered to be the main contributors to microbial dinitrogen (N2) fixation in the ocean. However, Martínez-Pérez et al. now show that the symbiotic cyanobacterium UCYN-A, which associates with eukaryotic algae, also fixes N2 in the surface waters of the tropical North Atlantic. They assessed N2 fixation rates and the diazotrophs (N2-fixing microorganisms) that are responsible for them on a west–east transect. Trichodesmium spp. were mainly present in the western basin, whereas UCYN-A was abundant in the western, eastern and central basins. UCYN-A1 and UCYN-A2 were both present and they were associated with a smaller and larger alga, respectively. Importantly, all UCYN-A1 and UCYN-A2 algal associations could actively fix N2; this may be the first demonstration of UCYN-A2 doing so. Finally, as UCYN-A is abundant in many parts of the ocean and has a much faster growth rate than Trichodesmium spp., UCYN-A–algae associations might be key diazotrophs globally.