The blue-green alga fixed elementary nitrogen to a limited extent in the dark in a strictly inorganic medium. Growth and nitrogen fixation continued in the dark when a suitable organic substrate was present in the medium. Among the organic substrates tested, sucrose (0.01 M) was the most readily utilized and was outstanding in supporting nitrogen fixation in the dark. The alga was adapted to heterotrophic conditions after continuous incubation and repeated subcultivation in the dark. Inter-relations between carbon assimilation and nitrogen fixation under heterotrophic conditions were observed. Sucrose assimilation proceeded more vigorously in the light and resulted in a fourfold increase in the rate of growth and nitrogen fixation. Sucrose assimilation was increased in the light in the absence of carbon dioxide from the gas phase, but nitrogen fixation was greatest when the alga was supplied with sucrose and carbon dioxide. Carbon dioxide was inhibitory to sucrose assimilation and slightly to nitrogen fixation in the dark.


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