in T. thiebautii) allow Trichodesmium to regulate buoyancy in the water column. These gas vesicles can withstand high pressure, presumably those up to 100 – 200 m in the water column, allowing Trichodesmium to move vertically through the water column harvesting nutrients. Nitrogen-fixation N2 is the most abundant chemical in the atmosphere. However, diatomic nitrogen is not usable for most biological processes. Nitrogen fixation is the process of converting atmospheric diatomic nitrogen into biologically usable forms of nitrogen such as ammonium and nitrogen oxides. This process requires a substantial amount of energy (in the form of ATP) in order to break the triple bond between the nitrogen atoms. 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. Inhibitor studies even revealed that photosystem II activity is essential for nitrogen fixation in this organism. All this may seem contradictory at first glance, because the enzyme responsible for nitrogen fixation, nitrogenase, is irreversibly inhibited by oxygen. However, Trichodesmium utilises photosynthesis for nitrogen fixation by carrying out the Mehler reaction, during which the oxygen produced by PSII is reduced again after PSI. This regulation of photosynthesis for nitrogen fixation involves rapidly reversible coupling of their light-harvesting antenna, the phycobilisomes, with PSI and PSII. Ecology Trichodesmium is found in oligotrophic waters, often when waters are calm and the mixed layer depth is shallow (around 100 m). Trichodesmium is found primarily in water between 20 and 34 °C and is frequently encountered in tropical and sub-tropical oceans in western boundary currents. Its presence is