Rates of glycolate synthesis and metabolism during photosynthesis of Euglena and microalgae grown on low CO2.

The rate of glycolate excretion in Euglena gracilis Z and some microalgae grown at the atmospheric level of CO2 was determined using amino-oxyacetate (AOA). The extracellular O2 concentration was kept at 240 μM by bubbling the incubation medium with air. Glycolate, the main excretion product, was excreted by Euglena at 6 μmol·h(-1)·(mg chlorophyll (Chl))(-1). Excretion depended on the presence of AOA, and was saturated at 1 mM AOA. A substituted oxime formed from glyoxylate and AOA was also excreted. Bicarbonate added at 0.1 mM did not prevent the excretion of glycolate. The excretion of glycolate increased with higher O2 concentrations in the medium, and was competitively inhibited by much higher concentrations of bicarbonate. Aminooxyacetate also caused excretion of glycolate from the green algae, Chlorella pyrenoidosa, Scenedesmus obliquus and Chlamydomonas reinhardtii grown on air, at the rates of 2-7 μmol·h(-1)·(mg Chl)(-1) in the presence of 0.2-0.6 mM dissolved inorganic carbon, but the cyanobacterium, Anacystis nidulans, grown in the same way did not excrete glycolate. The efficiency of the CO2-concentrating mechanism to suppress glycolate formation is discussed on the basis of the magnitude of glycolate formation in these low-CO2-grown cells.