STIMULATORY AND INHIBITORY EFFECT OF LIGHT ON THE NITRATE ASSIMILATION BY CHLORELLA

Abstract

1. Using Chlorella ellipsoidea as material it was demonstrated that the process of assimilation of nitrate or nitrite was affected by light in quite different ways according as the cells had been placed under dark-adapted or light-adapted condition. The cells cultured with nitrate as the nitrogen source accumulated a large amount of nitrite on long continued illumination, and the nitrite thus accumulated rapidly disappeared when light was turned off (inhibitory effect of light). When the cells were kept in the dark for a sufficient length of time, re-accumulation of nitrite took place. However, this nitrite decreased markedly when the cells (dark-adapted cell) were illuminated (stimulatory effect of light). The rate of formation of amino acids decreased when nitrite was accumulated, and increased when the level of nitrite dropped, indicating that the change of nitrite level was closely related to the amino acid and follow-up protein syntheses.
2. The addition of keto acids to the light-adapted cells decreased the level of nitrite, while practically no effect of the addition was observed with the dark-adapted cells. This observation together with the fact that the amino acids content of light-adapted cells increased markedly when the cells were kept in the dark for a short period of time (30 minutes) led us to the inference that in the light-adapted cells keto acids form the limiting factor for the nitrate assimilation and that the keto acids are accumulated in the dark conceivably owing to the enhanced operation of the Krebs cycle in the darkness.
3. Various experimental results were explained by assuming two factors controlling the process of amino acids synthesis from nitrate. One factor is a photogenic reducing agent(s), R, which is required for the reduction of nitrate and the intermediates between nitrate and amino acids, and the other is keto acids, which react with hydroxylamine or ammonia to form amino acids. The light-adapted cells contain a large amount of R but only a small amount of keto acids, while dark-adapted cells contain a small amount of R but a large amount of keto acids. Along the line of this reasoning, it was postulated that the assimilation of nitrate or nitrite by algal cells would proceed most efficiently when the cells are subject to an intermittent illumination of appropriate intervals.