Abstract
The first step of N2-fixation is catalyzed by the enzyme complex, nitrogenase. As nitrogenase is highly sensitive to O2, aerobic N2-fixation of cyanophytes usually occurs during the night or in the specialized cells for N2-fixation(heterocysts). However, N2-fixation of non-heterocystous diazotrophs of genus Trichodesmium is light-dependent. Inter-cellular localization of N2-fixing system in Trichodesmium has been studied but the results are still in controversy; some reports suggested the presence of N2-fixing cells (diazocytes). In-situ immuno-cytochemical analysis of Trichodesmium sp. NIBB1067 revealed that (1) nitrogenase was expressed in almost all cells grown under N2-fixing conditions, (2) there was no correlation between the abundance of diazocyte-like cells and the potential activity of nitrogenase, and (3) diazocyte-like cells were also found in cells grown with combined nitrogen. The factors that may caused the inconsistent results and the possible mechanism(s) to protect N2-fixing system from O2 in Trichodesmium will be discussed.
