Marine nitrogen-fixing cyanobacteria play a central role in the open-ocean microbial community by providing fixed nitrogen (N) to the ocean from atmospheric dinitrogen (N2) gas. Once thought to be dominated by one genus of cyanobacteria, Trichodesmium, it is now clear that marine N2-fixing cyanobacteria in the open ocean are more diverse, include several previously unknown symbionts, and are geographically more widespread than expected. The next challenge is to understand the ecological implications of this genetic and phenotypic diversity for global oceanic N cycling. One intriguing aspect of the cyanobacterial N2 fixers ecology is the range of cellular interactions they engage in, either with cells of their own species or with photosynthetic protists. From organelle-like integration with the host cell to a free-living existence, N2-fixing cyanobacteria represent the range of types of interactions that occur among microbes in the open ocean. Here, we review what is known about the cellular interactions carried out by marine N2-fixing cyanobacteria and where future work can help. Discoveries related to the functional roles of these specialized cells in food webs and the microbial community will improve how we interpret their distribution and abundance patterns and contributions to global N and carbon (C) cycles.