Autoradiographic Localization of 13n after Fixation of 13n-labeled Nitrogen Gas by a Heterocyst-forming Blue-green Alga

(13)N, generated by proton bombardment of (13)C powder, is rapidly and easily converted to (13)N-N(2), 0.01 atm pressure, ca. 10 mCi/ml, by automated Dumas combustion. (13)N fixed (as (13)N-N(2)) by algal filaments was localized by an autoradiographic technique which permits track autoradiography with isotopes having short half-lives. Our findings show directly that a minimum of about 25% of the N(2) fixation by intact, aerobically grown filaments of Anabaena cylindrica is carried out by the heterocysts. If all of the N(2) fixation takes place in the heterocysts, then the movement of nitrogen along the filaments can be characterized by a constant tau < ca. 5 s (cell(-2)).