Marine oscillatoria (Trichodesmium): explanation for aerobic nitrogen fixation without heterocysts.

Nitrogen fixation in marine Oscillatoria appears to be associated with differentiated cells located in the center of the colony. These central cells exhibit reduced pigmentation relative to peripherally located cells and do not incorporate 14CO2 in photosynthesis. Central cells apparently do not produce O2 which would deactivate nitrogenase. When central cells are exposed to O2 via disruption of the colonies, N2 fixation (acetylene reduction) decreases sharply even though individual trichomes remain intact. Disruption of colonies in the absence of O2 does not cause reduced nitrogenase activity. In the sea, turbulence from wave action apparently separates trichomes allowing O2 to enter thus decreasing nitrogenase activity. These observations explain how Oscillatoria is able to fix N2 without heterocysts in an aerobic environment and why it blooms virtually always occur in calm seas.