AUTORADIOGRAPHIC LOCALIZATION OF ¹³N AFTER FIXATION OF ¹³N-LABELED NITROGEN GAS BY A HETEROCYST-FORMING BLUE-GREEN ALGA

¹³N, generated by proton bombardment of ¹³C powder, is rapidly and easily converted to ¹³N-N₂, 0.01 atm pressure, ca. 10 mCi/ml, by automated Dumas combustion. ¹³N fixed (as ¹³N-N₂) 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 N₂ fixation by intact, aerobically grown filaments of Anabaena cylindrica is carried out by the heterocysts. If all of the N₂ fixation takes place in the heterocysts, then the movement of nitrogen along the filaments can be characterized by a constant τ < ca. 5 s (cell^-2).