Planktonic marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans. Their unusual adaptations, from the molecular to the macroscopic level, contribute to their ecological success and biogeochemical importance. Trichodesmium fixes nitrogen gas (N-2) under fully aerobic conditions while photosynthetically evolving oxygen. its temporal pattern of N-2 fixation results from an endogenous daily cycle that confines N-2 fixation to daylight hours. Trichodesmium colonies provide a unique pelagic habitat that supports a complex assemblage of consortial organisms. These colonies often represent a large fraction of the plant biomass in tropical, oligotrophic waters and contribute substantially to primary production. N-2 fixation by Trichodesmium is likely a major input to the marine and global nitrogen cycle.