Site of nitrogenase activity in the blue-green alga Anabaena sp. L-31.

Abstract

MANY blue-green algae fix nitrogen, assimilate carbon dioxide and evolve oxygen, and as algal nitrogenase is inhibited1–3 by high oxygen pressure, enhanced nitrogen fixation accompanying photosynthesis is surprising. Heterocysts do not contain4 or have comparatively less amounts4–7 of photosystem II (PS II) pigments, which are responsible for the evolution of oxygen. This tends to favour the suggestion of Fay et al.8 that these cells are the sites of nitrogenase activity. Until now, however, attempts at obtaining unequivocal evidence for heterocysts as principal loci for nitrogenase activity have yielded conflicting results. Stewart et al.7 first demonstrated nitrogenase activity in heterocysts incubated aerobically, a finding confirmed by Wolk and Wojciuch9 and Van Gorkom and Donze10. By contrast, Smith and Evans3,11 and Kurz and La Rue12 reported results favouring vegetative cells as the major site of nitrogenase activity. Other evidence2,13 showed high nitrogenase activity in cell-free preparations of Anabaena cylindrica and the non-heterocystous alga Plectonema boryanum strain 594.