Trichodesmium is unusual in that it is one of the few filamentous cyanobacteria in the open ocean. In coastal areas, in mats on sediments, and in estuaries, filamentous cyanobacteria are not so rare, but in the large areas of the open ocean, Trichodesmium is the primary free-living filamentous cyanobacterium. It is also the most abundant nitrogen-fixing cyanobacterium in the open ocean that we know of.

Nitrogen fixation in cyanobacteria: Many different species of cyanobacteria can fix nitrogen, including unicellular, filamentous nonheterocyst-forming and filamentous, heterocyst-forming.

Nitrogenae, the enzyme that catalyzes reduction of nitrogen gas in the atmosphere to biologically available ammonium, is extremely sensitive to oxygen.

Since cyanobacteria evolve oxygen during daylight hours through photosynthesis, cyanobacteria have devised ways to separate nitrogen fixation from photosynthesis.