Abstract
Diazotrophic cyanobacteria of genus Trichodesmium are important nitrogen fertilizer in tropical and subtropical ocean. As nitrogenase is labile to O2, filamentous diazotrophic cyanobacteria develop heterocysts, which lack O2-evolving photosystem (PSII), for N2-fixation. Non-heterocystous or unicellular species fix N2 during the night. Trichodesmium do not develop heterocysts, but fix N2 only in the day. We found that nitrogenase was completely inactivated in the dark, and activated upon the shift from dark to light. We have proposed the hypothesis that activation and/or de novo synthesis of nitrogenase, which has been inactivated by O2, may be achieved by the light-dependent mechanism(s). Recently USA researchers reported that PSII activity in individual cells was oscillated temporally, and that nitrogenase was detected only in limited cells. The possible protection mechanisms of nitrogenase from O2 in Trichodesmium will be discussed. There is no information about hydrogen production or hydrogenase in Trichodesmium at present.
