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Microcalorimetric Measurements of the Heat Evolution and Its Sensitivity to Dinitrophenol during Different Stages of Synchronous Cultures of Scenedesmus

^a Botanical Institute, University of Stockholm, S-104 05 Stockholm, Sweden

The applicability of the LKB flow microcalorimeter in a microalgal system was tested. Respiration and its sensitivity to 2,4-dinitrophenol during different stages of a synchronous culture of Scenedesmus obtusiusculus were studied. In cells harvested during the first 8 hours of illumination, heat evolution was 10 to 15 microwatts per 10 million cells and then rose to a peak of about 80 microwatts per 10 million when the cells were harvested in the beginning of the dark period, just before the onset of the divisions. Calculated per milligram dry weight, the highest values were about 140 microwatts, also obtained early in the dark period, after which the values dropped. They were roughly 90 microwatts per milligram at the onset of light and a little below 70 microwatts per milligram after 8 hours of illumination. Thus, the synthesis of new cell material does not give rise to actively respiring mitochondria during the first half of the light period, after which the situation is reversed. Low concentrations of dinitrophenol increased heat evolution, and high concentrations were inhibitory. Cells from the early stages of development were more sensitive to dinitrophenol than older cells. Young cells gave a transient peak of heat evolution even when the steady state was inhibited. Coupled oxidative phosphorylation seems necessary for substrate mobilization and transport to the mitochondria of the young cells. Possibly, young cells use mainly fatty acids and old cells carbohydrates as their main substrate for respiration.