Studies on nitrogen fixation by blue-green algae

Summary

• 1. The unicellular blue-green alga Chlorogloea fritschii Mitra has been isolated in pure bacteria-free culture.

• 2. Evidence showing that this alga is able to fix elementary nitrogen has been obtained by determinations by the micro-Kjeldahl method of increases in total combined nitrogen in culture and also by demonstration of the uptake of elementary nitrogen in a closed culture system by measurement of nitrogen/argon ratios, with a mass-spectrometer.

• 3. Studies have been made of the effects of temperature, light intensity, hydrogen ion concentration and sodium chloride concentration on growth and nitrogen fixation by the alga. At the optimum temperature of 35°C cell numbers doubled in approximately 3 days during the exponential phase of growth. A considerable degree of synchronization of cell division was achieved by using combinations of light and temperature treatments.

• 4. Evidence has been obtained of a close, connexion between nitrogen fixation and photosynthesis in C. fritschii. Oxygen was found to be produced by illuminated suspensions of the alga when elementary nitrogen was supplied as the only exogenous hydrogen-acceptor. No appreciable amount of nitrogen was fixed by the alga in the dark under circumstances in which considerable assimilation of nitrate-nitrogen took place. Fixation was found to be closely dependent on light intensity both in short-term and long-term experiments.