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

Photorespiration in Chlorella pyrenoidosa Chick. was assayed by measuring (18)O-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 (18)O-labeling kinetics of the intermediates, pool sizes, derived rate equations, and nonlinear regression techniques. Glycolate synthesis was higher in high CO(2)-grown cells than in air-grown cells when both were assayed under the same O(2) and CO(2) concentrations. Synthesis of glycolate, for both types of cells, was stimulated by high O(2) levels and inhibited by high CO(2) levels. Glycolate synthesis in 1.5% CO(2)-grown Chlorella, when exposed to a 0.035% CO(2) atmosphere, increased from about 41 to 86 nanomoles per milligram chlorophyll per minute when the O(2) 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 O(2) concentration was lowered to 2% or the CO(2) concentration was raised to 1.5%. Glycolate excretion was also sensitive to O(2) and CO(2) concentrations in 1.5% CO(2)-grown cells and the glycolate that was excreted was (18)O-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 (18)O-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.