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

Abstract

Photorespiration in Chlorella pyrenoidosa Chick. was assayed by measuring '10-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 180-labeling kinetics of the intermediates, pool sizes, derived rate equations, and nonlinear regression techniques. Glycolate synthesis was higher in high C02-grown cells than in air-grown cells when both were assayed under the same 02 and CO2 concentrations. Synthesis of glycolate, for both types of cells, was sfimulated by high 02 levels and inhibited by high CO2 levels. Glycolate synthesis in 1.5% C02-grown Chlorella, when exposed to a 0.035% CO2 atmosphere, increased from about 41 to 86 nanomoles per milligram chlorophyll per minute when the 02 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 02 concentration was lowered to 2% or the CO2 concentration was raised to 1.5%. Glycolate excretion was also sensitive to 02 and CO2 concentrations in 1.5% C02-grown cells and the glycolate that was excreted was '10-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 Iutheri, 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.The existence and the amount of photorespiration in freshwater algae is uncertain (8 to photorespire when transferred to low C02 levels and to do so in a manner similar to terrestrial C3 plants (33). They have high C02 compensation points (14), evolve large amounts of C02 into C02-free air during illumination (14), and are photosynthetically inhibited by 21%°02(18, 25). They also excrete large amounts of glycolate (4, 7, 35). However, photorespira-I tion appears to be suppressed and glycolate excretion is negligible if the cells are grown under air-levels of 02 and C02 (3,4,14,25).It is thought that algae grown under air-levels of C02 actively pump in dissolved inorganic carbon, raising the intracellular level of inorganic carbon (2). This would presumably suppress the oxygenase function ofribulose bisphosphate carboxylase/oxygenase, thus lowering the rate of glycolate synthesis. However, it is not entirely clear ifglycolate synthesis is completely suppressed. It has been reported that if cells are treated with a glycolate metabolism inhibitor they excrete almost as much glycolate as if they had been grown under elevated C02 (31).The main objective of our research was to quantify and then compare the photorespiratory rate ofair-grown and 1.5% C02-in-air-grown Chlorella pyr...

Global change and the future of harmful algal blooms in the ocean

“…A major cellular source of glycolate is photorespiration (Spencer & Togasaki 1981); however, it might be expected that this source would be reduced when ambient CO 2 :O 2 ratios are high, and so this does not appear to be consistent with the observed trends in lipophilic toxin synthesis under acidified conditions. However, another study with a freshwater green alga showed that glycolate production rates were 15 to 20 times higher in elevated CO 2 -grown cells relative to air-grown cells (de Veau & Burris 1989).…”

Algal growth enhancement by bacteria: Is consumption of photosynthetic oxygen involved?

“…[25]). Moreover, the synthesis of glycolate, one of the photorespiratory intermediates, is stimulated by high DO levels and inhibited by high CO, levels both in high CO,-grown and in air-grown cells of Chlorella pyrenoidosa [26].…”