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"Trichodesmium" is the major diazotroph in marine pelagic systems and is an important source of "new" nitrogen in the nutrient poor waters it inhabits. It has been estimated that the global input of nitrogen fixation by "Trichodesmium" is approximately 60-80 Tg (megatonnes or 1012 grams) N per year. Nitrogen fixation in "Trichodesmium" is unique among diazotrophs because the process occurs concurrently with oxygen production (via photosynthesis). In other cyanobacteria, N2 and CO2 reduction are separated either in space (using heterocysts to protect the sensitive nitrogenase enzyme from oxygen) or time. However, "Trichodesmium" lacks heterocysts and nitrogen fixation peaks during daylight hours (following a diel flux initiated in the morning, reaching a maximum fixation rate midday, and ceasing activity at night). Since the first realization of this enigma, "Trichodesmium" has been the focus of many studies to try and discover how nitrogen fixation is able to occur in the presence of oxygen production without any apparent structure separating the two processes.