NITROGENASE CONFINED TO RANDOMLY DISTRIBUTED TRICHOMES IN THE MARINE CYANOBACTERIUM TRICHODESMIUM THIEBAUTII1

ABSTRACT Nitrogenase reductase (Fe‐protein) was detected in the marine planktonic cyanobacterium Trichodesmium. The molecular weight was about 38 kD, as shown by western blotting using anti ‐Rhodospirillum rubrum nitrogenase reductase antiserum. The enzyme was confined to a limited number (ca. 10–40%) of randomly distributed trichomes in the Trichodesmium colonies, as shown by immunogold localization and transmission electron microscopy. Associated microorganisms had little or no nitrogenase. Nitrogenase showed a diel cycle in localization: present throughout the cytoplasm of cells in N 2 ‐fixing (daytime) colonies but at the periphery of non‐N 2 ‐fixing (nighttime) colonies. This structural arrangement of N 2 ‐fixing trichomes and nitrogenase is novel and different from the previously held paradigm for this and other diazotrophic cyanobacteria.