Synthesis, Excretion, and Metabolism of Glycolate under Highly Photorespiratory Conditions in Euglena gracilis Z

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Abstract
Glycolate was excreted from the 5% CO2-grown cells of Euglena gracilis Z when placed in an atmosphere of 100% O2 under illumination at 20,000 lux. The amount of excreted glycolate reached 30% of the dry weight of the cells during incubation for 12 hours. The content of paramylon, the reserve polysaccharide of E. gracilis, was decreased during the glycolate excretion, and of the depleted paramylon carbon, two-thirds was excreted to the outside of cells and the remaining metabolized to other compounds, both as glycolate. The paramylon carbon entered Calvin cycle probably as triose phosphate or 3-phosphoglycerate, but not as CO2 after the complete oxidation through the tricarboxylic acid cycle. The glycolate pathway was partially operative and the activity of the pathway was much less than the rate of the synthesis of glycolate in the cells under 100% O2 and 20,000 lux; this led the cells to excrete glycolate outside the cells. Exogenous glycolate was metabolized only to CO2 but not to glycine and serine. The physiologic role of the glycolate metabolism and excretion under such conditions is discussed.