Emerson enhancement of carbon fixation but not of acetylene reduction (nitrogenase activity) in Anabaena cylindrica.

Enhancement of carbon fixation was demonstrated in the bluegreen alga, Anabaena cylindrica, grown in either aerobic or microaerobic conditions. Under identical conditions no enhancement of acetylene reduction was observed. Light absorbed by photosystem I supported relatively more acetylene reduction than carbon fixation. No competition between the two processes was observed under light-limiting conditions. The findings suggest that carbon fixation and acetylene reduction may depend on different pools of reductant and ATP. When aerobically grown cells were placed in the dark or at limiting light intensities, acetylene reduction was higher in air than under argon. In contrast, carbon fixation was lower in air than in argon.