Regulatory effects of ammonia on carbon metabolism in Chlorella pyrenoidosa during photosynthesis and respiration

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Addition of ammonia to Chlorella pyrenoidosa, respiring in the dark following a period of photosynthesis, causes a stimulation of the flow of carbon into the synthesis of amino acids similar to that observed upon addition of ammonia during photosynthesis. In both cases, this stimulation is due not only to the increased availability of NH 4 + for reductive amination of α-ketoglutarate to glutamate but is also due to stimulation of the rate of conversion of phosphoenolpyruvate to pyruvate. Addition of NH 4 + in the dark causes a large increase in the formation of 6-phosphogluconate, beyond the increase in 6-phosphogluconate already seen when the light is turned off. When the light is turned off, the level of starch begins to decrease, and the rate of this decrease is not changed by the subsequent addition of ammonia. In contrast, the level of sucrose becomes nearly constant when the light is turned off, but begins immediately to decline when ammonia is added. As observed before, the level of ATP drops temporarily when the light is turned off and then rises to a steady state similar to that seen in the light. Upon the addition of ammonia, a similar transient drop and re-establishment in the level of ATP is seen. These and other reported results are discussed with respect to sites and mechanisms of light-dark metabolic regulation leading to increased flow of carbon from carbohydrate reserves into mitochondrial metabolism in the dark, and the sites and mechanisms by which ammonia affects the rate of this flow.