A re-examination of the problem of photochemical nitrogen reduction by blue-green algae

Summary Although it was possible in the light in the absence of carbon dioxide to obtain a ratio of nitrogen fixed to oxygen evolved in nitrogen-starved cells of A. cylindrica near to 1:1.5, that quoted by other workers, ratios varying between 1:0.9 and 1:3.0 were also obtained. The amount of oxygen evolved under the same conditions by normal cells in the presence of pyruvate was increased considerably. Since the addition of pyruvate also resulted in increased carbon dioxide output in the dark with the same algal material, oxygen output in the light was attributed to the production of factors necessary for carbon assimilation.Addition of pyruvate to nitrogen-starved and normal cells in the light resulted in similar rates of oxygen evolution after an initially higher rate in the starved cells. The ratio of overall nitrogen fixed to oxygen evolved, was 1:6.6 for the starved cells and 1:6.4 for the normal cells, showing that the presence of an added substrate increased oxygen output relative to nitrogen uptake.¹⁴CO₂ was recovered from sodium pyruvate-1-¹⁴C in flasks incubated in the dark, showing that, at least in the dark, pyruvate was decarboxylated.The interpretation of these results is that endogenous and exogenous substrates available to cells of A. cylindrica become decarboxylated and that, in the light, carbon dioxide produced may be assimilated photochemically with accompanying oxygen evolution. This interpretation has been discussed in relation to reports of photochemical nitrogen reduction in blue-green algae.