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Metabolism and Enzymology

Glycolate Metabolism in Low and High CO₂-Grown Chlorella pyrenoidosa and Pavlova lutheri as Determined by ¹⁸O-Labeling ¹

Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802

Photorespiration in Chlorella pyrenoidosa Chick. was assayed by measuring ¹⁸O-labeled intermediates of the glycolate pathway. Glycolate, glycine, serine, and excreted glycolate were isolated and analyzed on a gas chromatograph/mass spectrometer to determine isotopic enrichment. Rates of glycolate synthesis were determined from ¹⁸O-labeling kinetics of the intermediates, pool sizes, derived rate equations, and nonlinear regression techniques. Glycolate synthesis was higher in high CO₂-grown cells than in air-grown cells when both were assayed under the same O₂ and CO₂ concentrations. Synthesis of glycolate, for both types of cells, was stimulated by high O₂ levels and inhibited by high CO₂ levels. Glycolate synthesis in 1.5% CO₂-grown Chlorella, when exposed to a 0.035% CO₂ atmosphere, increased from about 41 to 86 nanomoles per milligram chlorophyll per minute when the O₂ concentration was increased from 21% to 40%. Glycolate synthesis in air-grown cells increased from 2 to 6 nanomoles per milligram chlorophyll per minute under the same gas levels. Synthesis was undetectable when either the O₂ concentration was lowered to 2% or the CO₂ concentration was raised to 1.5%. Glycolate excretion was also sensitive to O₂ and CO₂ concentrations in 1.5% CO₂-grown cells and the glycolate that was excreted was ¹⁸O-labeled. Air-grown cells did not excrete glycolate under any experimental condition. Indirect evidence indicated that glycolate may be excreted as a lactone in Chlorella. Photorespiratory ¹⁸O-labeling kinetics were determined for Pavlova lutheri, which unlike Chlorella and higher plants did not directly synthesize glycine and serine from glycolate. This alga did excrete a significant proportion of newly synthesized glycolate into the media.

³ Present address: Commission on Life Sciences, National Research Council, Rm. 343, 2101 Constitution Avenue, NW, Washington DC 20418.

¹ Supported in part by Grant No. 2466 from the U.S. Department of Agriculture Competitive Grants, The Pennsylvania State University, and through the Ben and Helen D. Memorial Fund awarded to E. J. de V.