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 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